Off topic transistor question.

[Darren Blake]

Not AV related, but hopefully someone on here can help with a basic transistor question.

I have just built a circuit which requires up to 0.8W to be dissipated across a normal NPN transistor. Problem is that the Ptotal limit on the transistor I have chosen is 0.8W

To get around this I wired two transistors in parallel (base to base, collector to collector, emitter to emitter). The theory being that they would automatically carry half the power each.

Problem is that one transistor still seems to be carrying all the power (one gets really hot and the other stays cold to the touch).

I have since read that transistors in parallel can be prone to this if not properly balanced. The solution appears to be to add a low resistance (0.1 Ohms) between each emitter and ground. This forces the transistors to "current share".

I have pieced this much together from various pages on the web, but not found an article specifically on it. Please could someone who knows what they are talking about confirm or deny?

Cheers, and sorry for the off-topic post!

[RichardH]

Yes, adding a low value resistor is the thing to do.

The Naim phono boards do something like this - they have 5 transistors in parallel, and each transistor has a 15 Ohm resistor after the emitter, in line with the circuit rather than down to ground, though. I think this is more to address any noise problems rather than to dissipate the power, though.

I can't offer any advice about the value of the resistor to use for your case - this circuit uses low power transistors, so the value calculation might be different.

HTH

[Darren Blake]

Thanks for the reply! Both of the examples I found on the web referred to 0.1 ohm current sharing resistors, so I'll start experimenting with those unless someone pops up with the proper calculation.

So the transistor part of the circuit will basically be like this:

[Chris Muriel]

Yep - standard way of doing this.
Same applies on many highish current DC power supplies.
I have a 12 Volt, 15 amp Farnell one which uses 8 x 2N3055 and also uses 0.1 ohm sharing resistors.

Chris Muriel, Manchester.

[Darren Blake]

Cheers guys.

Here's a curveball, though:

The smallest metal film resistor I can find is 1 ohm. Will this be too large? I can get a hee-uge wirewound 3W 0.1 ohm job but thats overkill.

Does anyone know what the formula for calulating the sharing resistor's value is, or is it an arbitrary low value?

*Oh* and this isn't a high current application. Its for an LED dimming circuit. MAx collector current will only be 0.24 amps. The transistor is small but has the gain characteristics I want, so I'd rather use several than try to find a bigger one to handle the power.

[SeaneyC]

Use a darlington pair? It's been 3 years since i did this theory, but it worked well on a programmamable light display i designed a while back for A-Level project. Use a small tranny with high gain and a big tranny with very low gain to get the results you want.

[Darren Blake]

Originally posted by SeaneyC
Use a darlington pair? It's been 3 years since i did this theory, but it worked well on a programmamable light display i designed a while back for A-Level project. Use a small tranny with high gain and a big tranny with very low gain to get the results you want.

Hmmm. I'm not 100% sure that this will give me the response I want from the transistor (base:collector current).

I'll build a test cct with two or three trannys arranged as above at the weekend. Hopefully that'll do the trick. If not I'll have to investigate other routes...