Power Supply Voltage and Consumption

[Garahbara]

Guys,

have done some research, and searching other threads/archives etc, but can't really find a definitive answer.

My system involves multiple PICAXE-20M (10+ of them), in which each processor powers 1 or 2 LEDs, and each processor communicates with each other via pulsein/out. The system currently has 5 processors up and running, but will eventually be expanded to 10+ or more. It is a signalling system for a model railway. Each signal has it's own dedicated PICAXE-20M, using an LDR (readadc) for train detection.

Currently running it all off a switchable supply at 4.5V (switchable from 1.5V DC - 12V DC) using a common supply bus. Input pin voltages (via "pulsein"), of course, will not exceed the supply voltage (via "pulseout").

From what I can determine, using a 6V DC supply should not be a problem. Just confirming that this correct, and 6V DC is not too much voltage?

I need the switchable supply for something else, and have dug out of my junk draw a 240V AC "power pack" (battery eliminator. i.e. power supply for a Discman CD player/cordless phone/battery recharger thingy) that supplies 6V DC @ 300MA.

Should this power supply be OK for power my PICAXE system? And how many processors will this 300MA supply support?


Thanks guys.

 

[goom]

6V will probably fry all of your PICAXES. Under low load, I suspect that your "6V DC" supply will be even more than 6V, and just fry them that much quicker.
You may want to consider using, say, a 9V supply and a 5V regulator. An LM7805 regulator is very cheap, and just need a couple of capacitors as per the datasheet.
PICAXE power consumption is quite low, so total current will be dominated by the LED's.

 

[boriz]

Yep. An unregulated 6v 300mA plugtop transformer (wallwart) will produce at least 7 or 8 volts unloaded. Max allowable voltage supply for Picaxe is 5.5v and although you might find it working above this, it probably won’t for long.

For prototyping, I use 4 AA’s. If I’m using 4 normal alkaline batteries @ 6v, I use a series rectifier diode to drop the voltage to a safe level. Sometimes I use 4 rechargeables @ 4.8v and remove the diode.

When I need good stabilisation or accurate ADC, then I use a 5v regulator (which needs to be supplied with over 7v, because you need at least a 2v overhead).

 

[Garahbara]

Thanks guys.

Older posts on here (years) seem to say tht 6V DC is OK (ish), but new ones, and new data sheets say 5.5V DC max. I have no idea if the plugtop transformer is regulated or not, but will give it a miss and find something else.

Hunting through my junk shed, I've got an old PC Power Supply in an old pooter. Would something off that be suitable?

Boriz,

The LDR (for readadc) is powered from the same bus as the processor itself. (with 100K resistor of course, as per manual). I only need to detect a "difference", not an absolute/fixed level. I have a calibrating alogythm in the code so it will detect a certain percentage difference, (as the train passes over the LDR) therefore catering for most levels of ambient light.

 

[Andrew Cowan]

While a PC power supply will be able to supply 5V, it can supply it at up to 25A or so.

You will draw around 0.6A, meaning that while a PC power supply would work, it would also be overkill. A significant danger is that if any poor/lose/incorrect connections are made, you could have 25A going through trancks, PICs, LEDs etc. 25A will vaporise most electronic components.

If you had a 1A max power supply, it may still blow components, but it is much less likly to do serious damage to PCBs etc. I'd carry on looking.

A

 

[Hubsy]

I would never run any chips on an unregulated supply, too much noise.

A power supply from a computer will give you 5vdc, bit overkill. As long as the power supply will power up without the motherboard being connected.

You could used a 7805 regulator power from your layout 12vdc, will need decent heat sink.

I use 5vdc 3amp ready made industrial power supplies on my layout, expensive but very good regulation.

Cheap option would be a regulated plug in power supply, do you have a company called Maplins down under. Only a few quid for a basic supply.

hubsy

 

[hippy]

The 6V supply issue has been a source of confusion over the years, not just here but elsewhere, and there is a lot of conflicting information around. Clarifying the issue in all the PICAXE datasheets is a task which has been scheduled. The overwhelming majority have already been updated.

In the good old days, PICmicro's ( which are what the PICAXE is built upon ) tolerated a supply of 6V and a maximum voltage of 7V5.

The PICmicro evolved, different chip fabrication was used, and the upper operating voltage dropped to 5V5 with the absolute maximum voltage lowered to 6V5; 6V was below that so some people kept using 6V reporting they sffered no detrimental consequences in doing so.

Taking all that Microchip say on operating voltage; PICmicro's should not be operated above 5V5, and must not be operated above 6V5 for reliable full-life operation. The recommendation is therefore to use a supply voltage which does not exceed 5V5. Operation above 5V5 is not guaranteed and may damage the chip.

To determine what sort of supply current is needed it is necessary to determine what the maximum current is that will be taken. A PICAXE by itself may require say 5mA, two LED's may require 10mA each, so a PICAXE plus two LED's would have a total requirement of 25mA, ten of those being 250mA. It's then worth adding a bit more to cater for surges which happen when pins change and during programming. When dealing with relatively low currents to start with, doubling isn't a bad rule of thumb; say 500mA.

If the circuit uses less, then a lower current supply will work and be acceptable; so the key is to determine what current a circuit draws.

A PC PSU can deliver many amps, far more than most circuits will need, so is ideal on that front, but the downside is that it will deliver as much as it can in a fault condition. Accidentally short +V to 0V on a PICAXE board and standard batteries probably run down rather quickly, rechargeables could explode, a PC PSU may keep delivering 20A or more for a long while, melting wires and even burning tracks off boards. Even with fuses in place catastrophic damage can be done while the fuse is still thinking about disconnecting the power.

A good PSU design will limit voltage and limit current to a level where damage is less likely to occur. A 5V 100mA regulator per PICAXE/LED combo could be a good configuration, and a PICAXE-side PSU fault on one board should hopefully not affect any others. As more boards are powered by fewer regulators, the risk of greater damage in a fault condition rises.

A PC PSU is appealing on the grounds that it can deliver almost anything a circuit could take, but the counter is that a circuit doesn't usually want to receive what the PSU can deliver if a fault condition occurs.

 

[Garahbara]

Hubsy,

You have a layout as well? TOOT!!!

As you would probably have experienced, you need a different power supply for everything. You've got your standard variable 12V DC, of course. I run DCC so I've also got a 8 AMP 15V AC supply to run that. I've got a laptop computer plugged into the DCC system, so that has it's own power supply as well. Then I've got the solenoid turnout motors (Peco) that run off 17V AC (and a CDU). Then there's the layout lighting, (houses, street lights etc) that run off fixed 12V DC. And I've now got my signalling system that runs off PICAXE-20M processors requiring 4.5V DC.

**counts them up** That makes 6 different power supply voltages etc, just to run a train set!!!

I've got all sorts of junk power packs and power supplies from old mobile phone chargers etc, that I'll utilise if I can before adding the expense of individual "new" power supplies. I'll check out what I can get out of my old computer power supply first, though. I can always fuse it with a 1 amp fuse of some sort, I suppose. But I'll need to determine the quality of the 5V DC a computer power supply will give as well.

Any further thoughts and suggestions from you guys most welcome.

 

[Andrew Cowan]

A PC will give a high quality 5VDC.

Do a search on google for 'PC power supply conversion' for some tips on getting it to run without a PC. Some need a minimum load.

A

 

[boriz]

A 7805 driven from 7.5v would be ideal, but 12v will be fine. Just keep an eye on it's temperature. Rule of thumb: If it's too hot to hold in your fingers, then it needs better heatsinking.

 

[tiscando]

Switching regulators are more efficient and don't require large heatsinks compared to linear regulators.

 

[Andrew Cowan]

However, they cost more and have higher external component count (eg inductors). However, they are also very fast acting, and good at smoothing power lines.

A