Cool PC mod

[craig008]

i decided one that on as it looks as tho it has the same properties as TIP31a which should allow for expamsion without going over the top + my local shop has them in stock
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[xnederlandx]

TIP31A - I'll put it in, It will be done in ~8 hrs, as I need to go to school. Oh and don't forget to post some pics when your done!

I didn't put a resistor between picaxe & transistor, so that it would just switch on fully.

 

[boriz]

As a follower, the transistor will never actually switch on fully because of the Vbe drop. The PICAXE drives the base to +5v (or whatever the +ve rail voltage is at the time), and the emitter is about 0.7v below that. So there can never be less than 0.7v between the collector and the emitter. The power lost to heat because of this drop, for a current of 250mA, is about 175mW (Vce*Ic, in this case 0.7*0.250)

‘Ptot’ (Total power dissipation) is another factor that may need taking into account sometimes. But usually only when the transistor is used in its linear mode*. It is quoted in Watts and again should be a multiple of your anticipated power dissipation to leave a good overhead. The power dissipated by the transistor is the emitter current (Ic) * the collector emitter voltage (Vce). When the transistor is used as a switch and will normally have either a small Vce (when on) or a small Ic (when off), the dissipation is usually so low as to not be a consideration. (BTW, this is why PWM or ‘switching’ is so much more efficient than linear methods. Very little power is lost as heat in the transistor). When your transistor gets too hot, or the magic smoke is released, it usually means that you have exceeded the transistors rated Ptot.

*Linear mode means when Vce varies in an analogue manner (as in an amplifier), so that both Ic and Vbe are simultaneously significant. Since, in your circuit, the Vce never gets down to Vce(sat) (usually around 0.2v), the transistor is never saturated and is actually working in linear mode, though only just.

NOTE. When I said that I would choose the BC639, it was just because I happen to have lots handy (they are cheap) and it will do the job. But when my prototype was working, I would be tempted to get a better transistor for the finished product. Something newer, with a little more overhead on the stats.

Actually, for this application, I would definitely use a MOSFET with the source to 0v and the LEDs between +v and the drain. MOSFETs are excellent in switching apps like this, and draw almost zero current from the PICAXE, and would dissipate less power as heat.

 

[craig008]

wow now this is starting to get confusing, i got some people telling me i should use a transistor and some telling me i should use a mosfet,

xnederlandx thanx for ur circuit diagrams, i will get a couple of proto boards and make it up but please do not get too hasty with the next circuit diagram until we have decided whether to use mosfet of transistor.

boriz just out of interest will the temp from the transistor make much difference to my case temp, this is starting to become and issue as i found it to be 39 degress C in there before i installed another fan but it is still at 30 degress C now and this is unloaded.
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[Dippy]

grapes, I respectfully suggest you get a book on basic transistor descriptions and circuits.

There are a few websites that go through the basics too.

"wow now this is starting to get confusing, i got some people telling me i should use a transistor and some telling me i should use a mosfet"
What does MOSFET mean? How about the ...FET bit.. Field Effect Transistor.
N channel MOSFETs can be used a low sided switch similar to an NPN bipolar.
P channel ... high sided similar to PNP - as in xnederlandsx circuit.
MOSFETs have many advantages in simple circuits like this, though for HF stuff you usually have to pay a little more attention to the gate driving. And also pay attention to ON resistance and references to Logic Level.

It's a piece of cake but you must read up on the basics, unless you want people to do the circuit for you and then you'll never truly understand what's going on. Honestly, it's worth reading about - then you won't have to rely on Forums

 

[craig008]

yeh have been reading about this stuff but i am not very good at book learing, i learn best when it is explained and i can ask questions which is why i use forums and people i know and mosfet stands for metal oxide semiconductor field effect transistor. the best wahy i ahve found to learn as well is to do it and learn from mistakes and other people, thats is why parents are so protective and all that, they dont want you to make the same mistakes they did so by asking questions i find what not to do and what to do.

books are all very nice and hand y but you can not ask them questions or propose a circuit design to them or a programme it tells you where may be going wrong, all my brothers a re book worms and if you gave them a screw driver they wouldn't know what end to use even tho one calls himself a an aircraft technician - i am a sure they are supposed to be able to fault find and repair problems arnt they??

 

[craig008]

just been looking at the data sheet for ULN 2803 and there does not seem to be any particular reason for conecting it back t the picaxe, unless there is something i have been missing while rding about all this, it just seems a bit of a waste of picaxe pin that may get used at a later date
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[BeanieBots]

just been looking at the data sheet for ULN 2803 and there does not seem to be any particular reason for conecting it back t the picaxe, unless there is something i have been missing while rding about all this, it just seems a bit of a waste of picaxe pin that may get used at a later date

What?
I'm not understanding your comment at all.
What do you mean by "connecting it back"?
Which datasheet said to "connect it back"?

 

[Dippy]

I was guessing (stress guessing) that grapes is referring to the connection on pin10 of the ULN?? But I'm struggling here and need a long rest.

 

[craig008]

well dippy take ur long and deserved rest, and yes i ment the connectio to pin ten, it just seems a bit odd that all the pacaxe boards as far as i can see have it going to graound
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[BeanieBots]

Pin 10 to ground?
The "free wheeling diode" connection?
Not sure I see the point of using a ULN2803 if pin 10 is grounded
What do you think might happen to the outputs?

 

[craig008]

sorry i made an error, i meant the the green wire that comes from pin 10 to pin 13 on the picaxe which is connected to +v on the axe020 board, as for what will happen well it will not supply any V for the outputs when switched so most likely cause, if it works at all an instabilty on the outputs with some magic smoke.

i have ordered a cd with whole load of sruff on it so i can search for what i am after more easily, it should be make this a while lot easier
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[BeanieBots]

Oh, the GREEN wire. That makes it so much clearer.
The GREEN wire that you've used for positive supply. Not the most obvious choice, but then why go with crowd?

Those diodes are to "catch" back emf spikes when switching inductive loads.
They should be connected to the power rail of the device being switched.
Pulling them down to 0v would be the same as pulling the load down via a diode.
Remember, the ULN2803 can only SINK current. It cannot source current. It is not possible for it to "source volts" to a load.
Pulling pin 10 down to 0v would be the same as switching ON all the outputs.

 

[boriz]

wow now this is starting to get confusing, i got some people telling me i should use a transistor and some telling me i should use a mosfet

I'm not telling you anything of the sort. I am just trying to answer your question about how to interpret BJT data.

Unless you live next to an amazingly well stocked transistor shop and have an unlimited budget, then transistor choice is usually a compromise. For example, if I were to prototype this project for my own use, I would use an STP55NF06L N-type power MOSFET. This transistor is rated at 55Amps (max). Overkill? Maybe. But I like this transistor because of it’s low gate threshold voltage (useful for logic and microcontroller voltages) and it’s low ON resistance (<0.018 Ohms) and it’s package (TO-220). Also, It’s the only MOSFET I currently stock. I purchased a bunch of them because I knew they could be used for almost any power switching I could need as a hobbyist.

 

[craig008]

right i see, so i should look at the fact that i will never need to switch the power that a mosfet can and so it would work out better, not just for cost but for general use to use a transistor
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[xnederlandx]

Ok, I will wait untill you have decided what transistor you will use.

 

[craig008]

now need to worry about it too much, got a quick prototypre running use a tip31c transistor and seems to work a charm. i will post some pictures later but have posted the sofware so far
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