PWM remote control of LED lights

craigcurtin

Senior Member
Guys,

I am looking at putting a number of strings of 12v LED lights throughout our house.

The ones i have tested with up until now have been controller by a RF Remote PWM box - however each box is only rated to 8AMPs at 12V

As i intend to have 6 strings of the LEDS in a number of rooms (each string draws 6AMPs) i would have to have an individual PWM unit and remote control for each string.

ideally what i am looking for is a single remote control that would dim all 6 LED strings together (so a max of 36AMPs at 12V)

The boxes i am talking about are these

http://www.ebay.com.au/itm/PWM-DC-12V-8A-RF-Remote-control-LED-Dimmer-/250964810854?pt=LH_DefaultDomain_0&hash=item3a6eab1c66#ht_2840wt_1163

When i open one up there does not seem to be anyway of setting a frequency or the code they respond to - i have two of them at the moment and they are both set to different IDs (sticker on the box and remote gives a 4 digit ID number)

When i have queried the sellers they can not quite come to grips with what i want to achieve.

Wondering if there is a way to use the PWM output from a picaxe and step it up to 12v and many amps ?

Craig
 

AllyCat

Senior Member
Hi Craig,

Wondering if there is a way to use the PWM output from a picaxe and step it up to 12v and many amps ?
Well it's certainly possible but you will need to design/build (and pay for) quite a lot of hardware, so I'd be looking at alternative solutions first.

When i open one up there does not seem to be anyway of setting a frequency or the code they respond to - i have two of them at the moment and they are both set to different IDs (sticker on the box and remote gives a 4 digit ID number)
You could well be correct (e.g. the ID code might be in an EEPROM) but a photo of the internals could be "interesting" even if we can't see anything helpful. Similarly for the transmitter.

If nothing can be done with the receivers, what about modifying the remote control(s)? You might be able to add a PICaxe which makes it/them sequentially transmit codes for all the receivers. However, this obviously wouldn't give "instantaneous" control of all the sets of LEDs together.

EDIT: But do you really need a PICaxe at all? From your description it seems that all you need is 5 x Power FET switches (one for each chain) to use in addition to one of your existing "receiver" boxes, "slaved" off its output.

Cheers, Alan.
 
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craigcurtin

Senior Member
Hi Craig,

Well it's certainly possible but you will need to design/build (and pay for) quite a lot of hardware, so I'd be looking at alternative solutions first.

You could well be correct (e.g. the ID code might be in an EEPROM) but a photo of the internals could be "interesting" even if we can't see anything helpful. Similarly for the transmitter.

If nothing can be done with the receivers, what about modifying the remote control(s)? You might be able to add a PICaxe which makes it/them sequentially transmit codes for all the receivers. However, this obviously wouldn't give "instantaneous" control of all the sets of LEDs together.

EDIT: But do you really need a PICaxe at all? From your description it seems that all you need is 5 x Power FET switches (one for each chain) to use in addition to one of your existing "receiver" boxes, "slaved" off its output.

Cheers, Alan.
Hey Alan - pretty much a newbie here on electronics - just thought it would be good to do some more things with PWM on a picaxe - i am fine on the programming side just not the physical hardware side.

Could you explain more about the PowerFET - what that is and how it would work ?

Craig
 

craigcurtin

Senior Member
Hey Alan - pretty much a newbie here on electronics - just thought it would be good to do some more things with PWM on a picaxe - i am fine on the programming side just not the physical hardware side.

Could you explain more about the PowerFET - what that is and how it would work ?

Craig

OK just did a bit of reading - essentially a Power MosFet seems to be an amplifier/relay that can switch at high speed ?

So if i get what you are saying is use one of my current PWM RF controllers - but the output of that (which is a PWM signal at 12v) would feed into 6 suitable Mosfets in parallel - each of these would have a 12v power supply direct to them and their outputs would go to the individual LED strings ?

That sounds perfect if i am correct.

So all i would need to do is find the correct MOSFET that accepts 12v PWM in - and can output a max current of (say 10AMP - to be safe) is that correct ?

I will do some searching but if you could suggest a product that would be brilliant

regards

Craig
 

craigcurtin

Senior Member
OK just did a bit of reading - essentially a Power MosFet seems to be an amplifier/relay that can switch at high speed ?

So if i get what you are saying is use one of my current PWM RF controllers - but the output of that (which is a PWM signal at 12v) would feed into 6 suitable Mosfets in parallel - each of these would have a 12v power supply direct to them and their outputs would go to the individual LED strings ?

That sounds perfect if i am correct.

So all i would need to do is find the correct MOSFET that accepts 12v PWM in - and can output a max current of (say 10AMP - to be safe) is that correct ?

I will do some searching but if you could suggest a product that would be brilliant

regards

Craig
Alan,

OK have found this on Sparkfun and sounds like exactly what i need and well within its capabilites - although it sounds like i would need some heatsinks on it.

http://www.sparkfun.com/products/10213

I believe i would also need a Diode across the LEDs - is that correct ?

Craig
 

AllyCat

Senior Member
Hi Craig,

Yes that looks the kind of device I had in mind.

However, now that you know what to look for, you should look inside your existing box again and try to identify the circuit they're using. For example there might (should) be some filter components to prevent radio interference (which may have the negative effect of slowing risetimes, etc.). A 'scope could be very useful.

6 amps, let alone 36 amps, is a very signigficant current and there are various aspects which should be discussed before rushing in with a "design". Yes, the FETs will require some form of heatsink, a diode is normally only required for "inductive" components (relays, motors, etc.) but just a long cable might have significant inductance at these currents.

Cheers, Alan.
 

nick12ab

Senior Member
Have you considered using one or both of these options:
  • Use two or three LED strips in series with higher voltage in order to reduce current.
  • Instead of dimming using a MOSFET, use a digital pot as a feedback resistor in the switching regulator which would eliminate the need for driving MOSFETs. However it will require digipots that allows high voltages through it.
 

gbrusseau

Senior Member
Is each transmitter mated to the receiver ID, or can you change the transmitter ID to match any receiver? Since the LED strings are located thoughout the house, it would be a wiring nightmare to control all the strings from one location. What I'm getting at is that one transmitter may be able to control all the receivers if say a PICAXE could very quickly change the transmitter ID in succession and transmit the command to each receiver almost simultaneously.
 

craigcurtin

Senior Member
Is each transmitter mated to the receiver ID, or can you change the transmitter ID to match any receiver? Since the LED strings are located thoughout the house, it would be a wiring nightmare to control all the strings from one location. What I'm getting at is that one transmitter may be able to control all the receivers if say a PICAXE could very quickly change the transmitter ID in succession and transmit the command to each receiver almost simultaneously.
Nope there is no option on either the transmitter or receiver to change IDs - i have been emailing a number of the vendors of the products - all of whom are either chinese or indian and none of them speak english worth a damn when asking these types of questions.

In the first instance the room i am trying to light up is a single room so getting to each of the wires etc is easy.

As i extend this through the house it will be harder.

I think i am going to go with the first suggestion and use some Power Mosfets and see if i can get this to work this way to begin with

Craig
 

hippy

Senior Member
Nope there is no option on either the transmitter or receiver to change IDs
As suggested earlier, crack open the equipment and determine what's inside. It may be possible for a PICAXE and an RF transmitter to fake a whole set of transmitters and then all you need to do is install receiver boxes and LED's.

That worked for me with RF controlled plug-in mains switches, could potentially control any number from a single PICAXE, and could work here.
 

westaust55

Moderator
@craigcurtin,

While not a solution to your project might I suggest that rather then using the "Reply with Quote" button at the bottom right of the screen which first puts then entire text of the earlier post above your answer/comment, try using the "Reply to Thread" button at the bottom left of the thread screen.
That will save us having to wade through two lots of words at every post.

To address another member specifically just prefix their member name with @
If we try to keep the "Reply with Quote" to more limited use and then cut out all but the relevant sentence(s) being answered it will make for a tidier thread.
“Reply with Quote” and then just extracting the relevant part is exactly what hippy has done in the post (#11) above.
From my own experience, I know that this policy can be hard to achieve on an iPhone and similar (using a small screen to cut and paste) but there is no real excuse on a PC.

Note that you are not alone in the use of the "Reply with Quote" and same comment is directed as much to the many.
[/end soapbox]
 
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craigcurtin

Senior Member
As suggested earlier, crack open the equipment and determine what's inside. It may be possible for a PICAXE and an RF transmitter to fake a whole set of transmitters and then all you need to do is install receiver boxes and LED's.

That worked for me with RF controlled plug-in mains switches, could potentially control any number from a single PICAXE, and could work here.
Thanks Hippy - i will do that this evening and post up some pics

@Westaus55 - sorry - i will try and be a better Netizen in the future

regards

Craig
 

inglewoodpete

Senior Member
... - all of whom are either chinese or indian and none of them speak english worth a damn when asking these types of questions.
Ah, but don't forget that you saved a fortune by buying these things from far away countries :)

Be really careful when running your wiring in the ceiling. While it is low voltage, the currents can be dangerous. Fire is a real risk. Especially when you combine ceiling heat in an Australian summer (it gets to 60C up there), aging cellulose ceiling insulation and rodent's pee, when they decide to mark their territory on your termination points!
 

craigcurtin

Senior Member
Ah, but don't forget that you saved a fortune by buying these things from far away countries :)

Be really careful when running your wiring in the ceiling. While it is low voltage, the currents can be dangerous. Fire is a real risk. Especially when you combine ceiling heat in an Australian summer (it gets to 60C up there), aging cellulose ceiling insulation and rodent's pee, when they decide to mark their territory on your termination points!
All Good points thanks Pete.

Although as each run of lights would be individually cabled for a 12v transformer we are only talking 1.2AMPs per string so not particularly up there. having said that the cabling will be run in electrical conduit throughout the ceiling space and strung across the uprights rather than resting on the ceiling and buried under insulation

You are right about saving a fortune by buying through EBAY - however when i search for RF 12V PWM - it seems these are the only game in town.

Craig
 

craigcurtin

Senior Member
Eclectic,

Yes the ideas of Hippys does seem to hold merit - the only issue is identifying the actual unit - it seems to be very generic - i have included some photos.

The other EBAY option i have also looked at as it is IR instead of RF for control so possibly more options with the Picaxe and its native IR support.

Here are some shots of the inside of the unit.

I have ordered a couple of Mosfets and i am going to try these to see if they have the capabilities i want

Will report back
 

Attachments

hippy

Senior Member
Here are some shots of the inside of the unit.
Is it possible to read the markings on the 14-pin IC (U4) at middle left ?

I would guess that's the micro and the 8-pin below (U3) an I2C Eeprom. 0V goes to leg 4 of U4 so it may not be a generic Microchip but could be and their RF "Keeloq" controllers may have a different pinout to 14M2 etc.

That Live IN on the left looks mightily close to other tracking and it looks like a transformerless supply so don't open the unit when plugged in, and if one wanted to look at things with a scope or even meter one would have to take care - disconnecting all mains and injecting 0V / +V would be the safest way.

If it's programmed with an ID in Eeprom it might be possible to reprogram the Eeprom to put different codes into that so multiple dimmers can be controlled by the same keyfob.

What does the useful looking 5V / DATA / GND connect to ? Is that the RF receive module ?

If you have a dumb 433MHz RF receiver it would be interesting to connect to a supply and look at its data out with a scope when pressing the keyfob buttons. What's inside a keyfob could also give useful clues as to what the RF protocol may be, which micro that uses.

It all looks promising if you have the time and tools to investigate further.
 

boriz

Senior Member
Q3 looks like the powermosfet. If you can identify the pinout, then it should be no trouble to replace it with a beefier model, or a few separate mosfets with the Source and Gates tied as per existing mosfet, with the Drains separate for different chains.
 

craigcurtin

Senior Member
More info

Is it possible to read the markings on the 14-pin IC (U4) at middle left ?.
nope devoid of markings - i have opened up two and they are the same

I would guess that's the micro and the 8-pin below (U3) an I2C Eeprom. 0V goes to leg 4 of U4 so it may not be a generic Microchip but could be and their RF "Keeloq" controllers may have a different pinout to 14M2 etc.

That Live IN on the left looks mightily close to other tracking and it looks like a transformerless supply so don't open the unit when plugged in, and if one wanted to look at things with a scope or even meter one would have to take care - disconnecting all mains and injecting 0V / +V would be the safest way.
Nope this is 12v in and 12v out - so no great worries about electrocution - this is driven by an external switch mode PS

What does the useful looking 5V / DATA / GND connect to ? Is that the RF receive module ?

.
Yes it is the RF receive module - stamped on the back of that is CZS-3 SZSAW

Sorry but i do not have access to Scope etc.

When i search on the Net for the receiver i get this (and many others !)

http://szyishi.en.alibaba.com/product/517956128-200054722/YS_CZS3_Superregeneration_Small_receiver_head_5_V_remote_module.html

And when searching on the legend on the mainboard zj-rft01 - just all the chinese manufacturers - no data sheets etc

Craig
 

craigcurtin

Senior Member
Q3 looks like the powermosfet. If you can identify the pinout, then it should be no trouble to replace it with a beefier model, or a few separate mosfets with the Source and Gates tied as per existing mosfet, with the Drains separate for different chains.
I like this idea

When i look at the back - the positive incoming 12v - is taken right across to the back of the board and exits to the outgoing 12V - a couple of taps of that presumably to power different chips on the way through

The negative goes straight to that Mosfet and then off the board - trying to read that mosfet it looks like

THis one

http://html.alldatasheet.com/html-pdf/205115/ETC2/P3055LDG/97/1/P3055LDG.html


Craig
 

boriz

Senior Member
All that follows assumes you have identified Q3 correctly, you can solder fairly well and can measure continuity with a meter. Try this at your own risk. I accept no responsibility for anything, ever.

The 'OUT' on that board should go DIRECTLY (zero resistance) to the Drain of Q3. So If you can verify that one of the other pins is connected DIRECTLY to ground (0v or -ve), then that should be the Source pin, and the other is the Gate. Compare this pinout to the datasheet. If they are the same, then I'm picturing something like this:

Solder a beefy wire (Like mains cable) to the power input -ve, this will be the External Ground wire. Solder another beefy wire to the power input +ve, this will be the External 12v Power wire. Solder a connection wire (carries little current, does not need to be beefy, but can be if it's easier) to the Gate pin of Q3, this will be the External Signal wire.

Now, you have one internal Mosfet for one LED chain. And for every extra LED chain, connect another Mosfet thus: Source to External Ground wire, Gate to External Signal wire, Drain to LED chain -ve, LED chain +ve to External 12v Power wire.

This, if done correctly should be sufficient to drive a few chains before the combined Gate capacitance's are high enough to be a problem.

Without proper test equipment, it will be harder than it should be and you'll need to be very careful to check all connections thoroughly before applying power, AND be able to spot trouble fast enough to remove power before doing serious damage.

Also, each additional Mosfet + chain should be added separately after testing the previous one for heating. IE: Add one Mosfet+chain, verify all connections, turn it onto half or three quarters brightness and leave it on for an hour while testing the temperature of ALL the Mosfets. (I just use my finger). Rule of thumb is, if you can't hold your finger on it, it's too hot.

Each additional Mosfet could effect the performance of ALL the mosfets. When you have added too many, you'll know 'coz your finger will burn :)

This is probably exactly what I would do (being the cheapest and easiest option). However, I have several oscilloscopes and various other test equipment. And I'm confident modifying this sort of circuitry. And if it all goes pear shaped, I'm usually comforted by the fact that I didn't spend much :)

*EDIT.

And make sure your PSU is up to the job!
 
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craigcurtin

Senior Member
thanks for the clear description

All that follows assumes you have identified Q3 correctly, you can solder fairly well and can measure continuity with a meter. Try this at your own risk. I accept no responsibility for anything, ever.

The 'OUT' on that board should go DIRECTLY (zero resistance) to the Drain of Q3. So If you can verify that one of the other pins is connected DIRECTLY to ground (0v or -ve), then that should be the Source pin, and the other is the Gate. Compare this pinout to the datasheet. If they are the same, then I'm picturing something like this:

Solder a beefy wire (Like mains cable) to the power input -ve, this will be the External Ground wire. Solder another beefy wire to the power input +ve, this will be the External 12v Power wire. Solder a connection wire (carries little current, does not need to be beefy, but can be if it's easier) to the Gate pin of Q3, this will be the External Signal wire.

Now, you have one internal Mosfet for one LED chain. And for every extra LED chain, connect another Mosfet thus: Source to External Ground wire, Gate to External Signal wire, Drain to LED chain -ve, LED chain +ve to External 12v Power wire.

This, if done correctly should be sufficient to drive a few chains before the combined Gate capacitance's are high enough to be a problem.

Without proper test equipment, it will be harder than it should be and you'll need to be very careful to check all connections thoroughly before applying power, AND be able to spot trouble fast enough to remove power before doing serious damage.

Also, each additional Mosfet + chain should be added separately after testing the previous one for heating. IE: Add one Mosfet+chain, verify all connections, turn it onto half or three quarters brightness and leave it on for an hour while testing the temperature of ALL the Mosfets. (I just use my finger). Rule of thumb is, if you can't hold your finger on it, it's too hot.

Each additional Mosfet could effect the performance of ALL the mosfets. When you have added too many, you'll know 'coz your finger will burn :)

This is probably exactly what I would do (being the cheapest and easiest option). However, I have several oscilloscopes and various other test equipment. And I'm confident modifying this sort of circuitry. And if it all goes pear shaped, I'm usually comforted by the fact that I didn't spend much :)

*EDIT.

And make sure your PSU is up to the job!
Boris,

This sounds like a good plan to follow thanks - i will give it a go as you have outlined

I was planning on using multiple 20AMP 12V power supplies - essentially the first room i intend to experiment with will have 6 strings of LEDS - running as 3 pairs of 2 - i would therefore dedicated a pair of the Mosfets and a power suppliy to each pair of LEDs runs.

I will as you say build it up - just waiting for the mosfets etc to arrive and will report back on progress. I was planning on putting a heatsink on each of the mosfets

Reading what you have written i am essentially bypasing the onboard MOSFET for my external ones - is that correct - or am i in fact using the output of the onboard MOSFET to drive the "signal" wire of the external MOSFETS ?

Craig

Craig
 

boriz

Senior Member
You are not bypassing the internal Mosfet, it will still work as before. You are obtaining/tapping the Gate drive signal of the internal Mosfet and taking it outside the box and using it to drive additional Gates in parallel.

The Paralleled Gates each have a capacitance which needs to be charged in order for the Mosfet to fully switch on. This capacitance takes time to charge and the time will increase as the capacitance increases (more Gates=more Capacitance). The most obvious effect of this slower switch-on should be additional heating in all the paralleled Mosfets (including the internal one), and that's what you have to keep checking for. That's the limitation of this 'hack'.

A heatsink is often a good idea, but remember, you have no 'scope so you are judging the performance of this hack by monitoring of temperature change alone (a blunt instrument indeed). A heatsink will skew the results. And the 'finger test', in this context, is intended for a naked transistor.

Once you have determined that it all works ok, then put the heatsinks on if you think it's necessary. Transistors last longer when running cooler.
 

craigcurtin

Senior Member
Ok thanks for that - makes is cleaer

This should make it a bit clearer: https://www.circuitlab.com/circuit/nsmd3p/dimmer1/

(Note. It's just a hasty sketch, not intended as full circuit.)
Would it be valid to in fact use the output of the onboard mosfet as the switch to the external mosfets and have them wired directly to the power supply ?

I.e. the only load on the internal mosfet is the switching output to trigger the external Mosfets ?

This would then mean i would not need to do any mods inside the box and on the board but just take the outgoing signal and feed that to the trigger of the external mosfets

(Sorry this may seem like a dumb question - but i have only just started learning about MOSFETS this week - so thanks for your patience)

I think the main difference between what i am asking and what you have suggested is that i will be using the onboard mosfet to trigger the external mosfets rather than in parallel with the onboard and have the Microprocessor trying to drive multiple Mosfets. So hopefully i have worded my question the right way - the upside of this is that there is no soldering on the board - i will be making the +ve and -ve direct to the power supply and the external mosfets and just coming from the PWM output of the RF dimmer (after it has gone through the onboard Mosfet)

Craig
 
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boriz

Senior Member
Perfectly reasonable question, and that solution was proposed early in this thread. It can be done, but you will need to add a resistor, I'll sketch a circuit later when I have time.
 

boriz

Senior Member
Ok. Here is the proposed alternative idea, using the existing output: https://www.circuitlab.com/circuit/hserq8/dimmer-2/ (Note. It's just a hasty sketch, not intended as full circuit.)

Firstly, the external transistors are different. They are P-channel Mosfets, not N-channel Mosfets. These are generally thinner on the ground and more expensive.

Secondly, the first 'default' string has been completely replaced by an external resistor. So the dimmer output does not itself drive an LED string. It just provides the signal to drive the external Mosfets. So no need to connect anything to Q3s Gate.

I have little experience with P-channel Mosfets and cannot advise you about what type to use. Something similar to the suggested N-channel, but it's opposite will probably be fine. Also, the resistor (R1) will need to be able dissipate about 1.5 Watts (in heat) so it must be a power resistor. Use a 5 watt type to be on the safe side. It may even benefit from heatsinking.

This alternative idea has it's drawbacks. Not least of which is wasted power in R1. But the benefit is that more additional Mosfet+strings can be driven. As before, add one at a time and leave it running while checking for temperature changes. I'm guessing you could easily drive 6 chains this way.
 
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AllyCat

Senior Member
Hi,

Yes, I think this is coming towards a good approach. Bear in mind that the original drive circuit was intended to feed one FET and now you want to drive (the capacitance of) half a dozen (or more?) FETs and (presumably) tens of metres of cable.

However, I would modify the output circuit of the original box to be a decent "cable/FET driver" and then use a "standard" n-channel FET close to each PSU/LED chain. Note that most of these device configurations invert the voltage signal so you might need to insert another small signal transistor before the "driver" stage to get the correct polarity/phase of dimming.

Like boriz I've no experience of P-channel FETs, but perhaps replacing the FET currently in the box with one might be sufficient (with a pull-down resistor instead of pull-up). Or maybe a pnp transistor suitably configured, or perhaps a "push-pull" driver pair (FET or Bipolar) if you have lots of cable length to drive.

Cheers, Alan.
 

boriz

Senior Member
Trying to keep it simple as possible. 100R should be good enough to quickly turn off the Mosfets and the internal N-Mosfet should have no problem turning them on quickly. In fact, R1 could even stand to be lower value, though I'm not convinced it will be necessary.
 

craigcurtin

Senior Member
OK - i am starting to get a little lost here !! - Remember i am a babe in the woods !

Hi,

Yes, I think this is coming towards a good approach. Bear in mind that the original drive circuit was intended to feed one FET and now you want to drive (the capacitance of) half a dozen (or more?) FETs and (presumably) tens of metres of cable.

However, I would modify the output circuit of the original box to be a decent "cable/FET driver" and then use a "standard" n-channel FET close to each PSU/LED chain. Note that most of these device configurations invert the voltage signal so you might need to insert another small signal transistor before the "driver" stage to get the correct polarity/phase of dimming.

Like boriz I've no experience of P-channel FETs, but perhaps replacing the FET currently in the box with one might be sufficient (with a pull-down resistor instead of pull-up). Or maybe a pnp transistor suitably configured, or perhaps a "push-pull" driver pair (FET or Bipolar) if you have lots of cable length to drive.

Cheers, Alan.
@Alan

If i read what you are saying correctly - i think you mean

1) Bypass the existing MOSFET on the board - which is presumably a N-Channel as this is the most common - and put a single p channel MOSFET in its place ?
2) The output of this P-Channel mosfet would then be used to drive the switching input of my external MOSFETS (which could be the more common N-Channel)

Can you please explain the purpose of the resistor ?

Would this be a suitable MOSFET ?

http://www.jaycar.com.au/products_uploaded/ZT-2467.pdf

@Boriz - thanks for hanging in there and taking the time to explain all of this

regards

Craig
 

boriz

Senior Member
It's basic electronic theory. A resistor turns a current into a voltage or a voltage into a current. The resistor, added to the internal current source makes a voltage source, required to drive the external voltage driven Gates. Any resistor value would do the same, but in this case the combined capacitance of the downstream Gates requires a fairly heavy handed approach and a fairly low value resistor.

The internal Mosfet pulls the external Gates down to ground, switching the external transistors on. The resistor pulls the Gates up to 12v, turning them off. This will happen hundreds (maybe even thousands) of times a second. The low value of the resistor ensures a rapid switch off when the internal Mosfet 'relaxes' and ceases to conduct current. A rapid switch off is important to keep the length of time the external Mosfets are in their linear region (or amplifying region) as short as possible to avoid heating losses.

That Mosfet looks ok to me and may be worth a punt, but I'd probably get advice from someone more experienced using Power P-Mosfets before spending any money.
 
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craigcurtin

Senior Member
It's basic electronic theory. A resistor turns a current into a voltage or a voltage into a current. The resistor, added to the internal current source makes a voltage source, required to drive the external voltage driven Gates. Any resistor value would do the same, but in this case the combined capacitance of the downstream Gates requires a fairly heavy handed approach and a fairly low value resistor.

The internal Mosfet pulls the external Gates down to ground, switching the external transistors on. The resistor pulls the Gates up to 12v, turning them off. This will happen hundreds (maybe even thousands) of times a second. The low value of the resistor ensures a rapid switch off when the internal Mosfet 'relaxes' and ceases to conduct current. A rapid switch off is important to keep the length of time the external Mosfets are in their linear region (or amplifying region) as short as possible to avoid heating losses.

That Mosfet looks ok to me and may be worth a punt, but I'd probably get advice from someone more experienced using Power P-Mosfets before spending any money.
Ok thanks for this.

That Mosfet only costs $5 and i can pick one up at lunch tomorrow so i will give it a go and see what i can work out.

If i was to use a module such as this for the external MOSFETS http://www.jaycar.com.au/productView.asp?ID=XC4244

it appears that it has a built in pull down resistor (presumably a P-Mosfet actually does the opposite of an N Mosfet and pull the Gate up from ground ?)

Craig
 

boriz

Senior Member
That's just a N-Mosfet on a little PCB. I thought you were going with the P-Mosfet solution?

"That Mosfet only costs $5 and i can pick one up at lunch tomorrow so i will give it a go and see what i can work out." - Don't forget the 5 Watt 100 Ohm resistor.

"presumably a P-Mosfet actually does the opposite of an N Mosfet and pull the Gate up from ground ?" - You mean 'pull the Drain up from ground' ? Yes.

The Gate voltage is referenced to the Source voltage.

With an N-Mosfet, the Source is connected to ground and the gate is made positive with respect to the Source to turn the transistor on.

With a P-Mosfet, the Source is connected to +v (in this case +12v), and the Gate is made negative with respect to the Source to turn it on.

Here's a simple illustration of using a P-Mosfet to switch power on and off to a load:

tran59.gif

In this illustration, Vdd can be seen as your +12v and RL can be seen as your string of LEDs. Von is provided by your dimmer output and Voff is provided by the 100R resistor.
 
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AllyCat

Senior Member
Hi,

Yes, to use the original box unmodified you must use a P-Mosfet (with source to the +12 volt rail) and a low value resistor between gate and source. The PDF you linked looks OK. It/they might work moderately well, but it's not an "ideal" arrangement.

Unfortuanately we really need to know more about the drive waveform applied to the gate of the internal FET, in particular the frequency and amplitude. An oscilloscope will give you that information, but we might be able to get some useful information if you only have a multimeter.

The frequency is important because there will be a certain amount of power dissipation associated with each "switching edge". At 50 Hz there will be 100 edges, but at 50kHz (a not impossible frequency) 100,000 edges, i.e. a thousand times more dissipation (heat).

The gate voltage is also important because it basically needs to be close to zero volts to turn the FET OFF, and perhaps 5 volts to turn it ON, but 10+ volts can be better to ensure that the FET is thoroughly saturated to give the minimum voltage drop (and thus lowest dissipation). So your gate might be driven from a "TTL" driver/ic from 0v to +5v, or it might get a full swing of 0 to +12 volts.

From the above, and as Boriz said, the FET may have a considerable power dissipation whilst the gate voltage is between about 1 and 4 volts, because current is flowing whilst there is a significant voltage drop across the FET (P = I x V). So we need to minimise the switching time betwen these drive levels. A typical gate capacitance seems to be about 1nF and a cable may be 100pF/metre, so you might have a total load of around 10nF (e.g. 6 x FETs and 40m cable). To swing the gate though 3 volts in say 1u second (or less if running at high frequency) needs a significant current, hence the need for a low driver resistor value.

But I'm afraid it's rather more complex than that. A transistor/FET driver can pull current (i.e. from the gate capacitance) until the driver is saturated (with less than a volt across itself). However, a resistor will only give a reasonable current whilst there is plenty of volts (e.g. 12 v) across it, but rather low current (and thus slow switching) with only a few volts across it. Therefore, you ideally want the "driver" transistor/FET to pull the output device to OFF (zero gate-source voltage) and the resistor to pull the output to ON, because it will pull the gate to 4 volts fairly quickly and the slower rise to 12volts then just "gilds the lily" by giving the lowest saturation voltage for most of the ON time. Thus an "unmodified" box won't give an optimum drive arrangement for P-MOSFET power devices.

I hope that I haven't confused you too much. I won't say more until you can say what (if any) measurement equipment you have. That will probably define the best approach to follow.

Cheers, Alan.
 

boriz

Senior Member
All true. But I'm attempting to make this as easy as possible for Craig. This hack will work. It's is not the most electrically efficient solution, but definitely the simplest. And the 'add one and test temperatures, add another and test temperatures...' approach is going to be the easiest test method without instruments and hard data. He said already he has no 'scope. And I said already that the resistor could be made smaller if necessary to reduce any switching losses.
 

eclectic

Moderator

boriz

Senior Member
"could Craig's project be achieved using" Yes. And many other solutions too. But since Craig has little experience and little equipment. I think simpler is better.
 
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