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Old 28th Nov 2020, 11:40 am   #1
Diabolical Artificer
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Default 2N3055 series pass questions.

I was using a TIP3055 as a series pass element for a heater supply, to supply 6.3v @1.8A, looking at the SOA that should be ok, but recently I noticed it was getting bl**dy hot. My idea was then to use two 2N30055's to share the load, one gets %£$^ hot, the other less so, therefore I presume I need to use emitter resistors, I have some 0.1r Welwyn WW's jobbies, is this correct?

My main problem is getting rid of this heat without a proper HS, the TIP3055 was mounted on a chassis of 1.2mm ali, 8" x 10" approx, it melted the paintwork locally. I've tried 2 sheets of 3mm thick ali sandwiched on top of this, problem is I only have 2" x 2" approx to wedge all this in and only 1/2" to 3/4" height tops. My next plan is to mount them over a 4" fan, no HS. Am I dreaming here. Do i really need a gurt big HS?

Circuit is a BR, big 4700u cap, supply to collector 28v, LM317 sets Vbase @ 6.8/7v ish, OP at emitter obviously. Protection diodes fitted.

Andy.
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Old 28th Nov 2020, 11:50 am   #2
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Default Re: 2N3055 series pass questions.

Why such a high collector voltage? That means you're dropping (28-6.3)=21.7V, at 1.8A that's almost 40W dissipated to supply 11W...

If you can't reduce the input voltage, maybe a switchmode psu would be worth looking at?
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Old 28th Nov 2020, 11:59 am   #3
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Default Re: 2N3055 series pass questions.

With 28 Volts to the regulator and 6.3 out at 1.8 amps you have 39 Watts of heat to get rid of. First move would be a lower output transformer, if possible.

Yes the emitter resistors should help distribute the load but depends on the current being drawn. For a fan have you any older square CPU heatsinks with fan?

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Old 28th Nov 2020, 12:00 pm   #4
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by Keith956 View Post
Why such a high collector voltage?
that's what I have available, not ideal. I found with a lower voltage and 2200u cap ripple was a lot higher.

I thought of a SMPSU, it'd make sense, but a lot of changes.

"For a fan have you any older square CPU heatsinks with fan?" yes but too big and noisy.

I've tried 1r emitter R's, but there's still a disparity,should there be individual base resistors too? At present there's just one 10r base R.

Andy.
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Old 28th Nov 2020, 12:07 pm   #5
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Default Re: 2N3055 series pass questions.

I'd agree that the voltage drop you have is big and I'd argue that its a waste of energy/electricity.
But, we are where we are Andy.
Why mess about with pass transistors etc? Just use a 5A reg instead of the 317 on a big heatsink. Its what i did with my final version of my EL34 stereo amp for the signal Valve heaters.
Drop me a line if interested mate and I'll pop one in the post for you, I have a couple or 3 spare here and I only need one at the moment for a preamp supply.
The other way around it would be to split the heater supplies to individual sets of valves and use 2x 1.5A regs like the 317.

T' other Andy.
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Old 28th Nov 2020, 12:11 pm   #6
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Default Re: 2N3055 series pass questions.

You could lose some of the power in a suitably rated resistor in the collector circuit to move the dissipation elsewhere. Aim to get about 4 or 5 volts Vce on the 3055. Load's constant which helps.
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Old 28th Nov 2020, 12:11 pm   #7
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Default Re: 2N3055 series pass questions.

I would consider a small DC to DC 'Buck converter'.
20 Amp ones are less than a fiver...
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Old 28th Nov 2020, 12:31 pm   #8
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Default Re: 2N3055 series pass questions.

I know it may be going against the grain, but thinking about Keith's idea of a switch mode may not be a bad idea, I have used them from time to time.

This one is 70 x 50 x 30 mm approx and Ebay has similar items for less than £6.
Click image for larger version

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Trying to remember and old Lambda series power supply I had, I believe the bases of all the transistors were connected in parallel with just series emitter resistors, perhaps a higher value is needed.

If drawing 1.8 Amps between two transistors and trying to share the load, per transistor 0.9 Amps x 0.1 Ohm is only 0.09 Volt drop across the emitter resistor, I think you need a bigger drop to start controlling it. I would suggest around 0.2 Volt try a 0.2 Ohm emitter resistor.

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Last edited by AdrianH; 28th Nov 2020 at 12:36 pm.
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Old 28th Nov 2020, 12:35 pm   #9
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Default Re: 2N3055 series pass questions.

If you're stuck with a high supply you could try a small fan and heatsink - a Raspberry Pi one might fit?

https://www.amazon.co.uk/GeeekPi-Ras...6563175&sr=8-8
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Old 28th Nov 2020, 12:38 pm   #10
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Default Re: 2N3055 series pass questions.

Forgive my maths in my post corrected it a bit.
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Old 28th Nov 2020, 12:42 pm   #11
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by bikerhifinut View Post
Why mess about with pass transistors etc? Just use a 5A reg instead of the 317 on a big heatsink.
That's an idea, will drop you a PM mate.

I agree a SMPSU or buck convertor would be more efficient, but space is tight, A SMPSU would need a screening can and I'm wary of cheap Chinese SMPSU's, I need something that's going to last. Also i would need to then further regulate a SMPSU,they don't come in 6.3v types and the OP would be mucky. Having said all that i will consider them for future builds.

Whether I use a 5A reg or series pass what sort of size HS an I going to need to dump 40w? Roughly?

Thanks for all your IP.

Andy.
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Last edited by Cobaltblue; 28th Nov 2020 at 12:57 pm. Reason: Fixed quotes readability
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Old 28th Nov 2020, 12:44 pm   #12
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Default Re: 2N3055 series pass questions.

How stable is that 28v?

If it's about as stable as the mains voltage, you could just use a big resistor. It's exactly as efficient as a linear regulator.

With all that wattage you need heatsinking however you do it. This is definitely a suitable case for an SMPS.

David
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Old 28th Nov 2020, 1:31 pm   #13
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Default Re: 2N3055 series pass questions.

It depends of course on the valve(s) you're trying to heat, but could you run the heaters in series or series/parallel?

Bryan
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Old 28th Nov 2020, 1:59 pm   #14
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Default Re: 2N3055 series pass questions.

PM replied to Andy, I have a small transformer too that would do the job for you.

Andy oop north
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Old 28th Nov 2020, 2:00 pm   #15
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by Bryan M View Post
It depends of course on the valve(s) you're trying to heat, but could you run the heaters in series or series/parallel?

Bryan
thats a good idea if the valves are all 300mA for instance.
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Old 28th Nov 2020, 2:13 pm   #16
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Default Re: 2N3055 series pass questions.

Quote:
It's exactly as efficient as a linear regulator.
Slightly more efficient, no quiescent current for the regulator (all of tens of microamps!).
 
Old 28th Nov 2020, 2:27 pm   #17
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Default Re: 2N3055 series pass questions.

But just as wasteful, and you still need to dump the heat.
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Old 28th Nov 2020, 2:46 pm   #18
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Default Re: 2N3055 series pass questions.

Apart from series- or series-parallel connection of the valves, does the setup need a _single_ regulator?

In plenty of professional gear you'll find multiple small regulators fitted near to the individual loads - one or two per PCB - rather than having a single central regulator. Could you re-engineer your setup to use a bunch of 7806 or 7812 regulators distributed around, so each one is handling/dissipating less power and spreading the heat over a larger physical area?
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Old 28th Nov 2020, 2:58 pm   #19
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Default Re: 2N3055 series pass questions.

Do also bear in mind that the die in a power transistor can run substantially hotter than the die in an IC regulator - check the datasheets for each to verify. As a result, switching to an IC might actually require a larger heat sink for the same power dissipation. Especially if it has a higher junction-to-case thermal resistance (which is bad if not considered, as you might believe the thing is running rather cooler than you imagine based on the external temperature).

Obviously that applies if trying to stop the die from frying. If the objective is to get the heat sink down to a wanted temperature - perhaps because it it possible to touch from the outside - then that point is perhaps less relevant.

Honestly, 40 watts is a serious amount of heat to lose - just think about the amount of heat that comes from an old incandescent lamp. To me, it's a non-starter if you haven't designed in provision for the heat sinking from the outset.

Can't you add a winding to the mains transformer?

Otherwise, I'd agree that it is a case of going down the DC-DC converter route. You can always change the caps for better quality ones - assuming they've used genuine semiconductors, those are the only things left to worry about if it works well initially. With that in mind, choose a module that uses through-hole caps rather than SMT types.

Don't forget that you might be able to reduce the size of the main 4700uF smoother cap, as the DC-DC converter will absorb ripple. Many of the available modules have an adjustable output - and if there's no pre-set, you can change one or two of the resistors to make that happen - just check the datasheet for the IC they're using.

As to output ripple, it might not be as bad as you think, especially as it's a heater supply. The bulk of the energy will be outside the audio band, and extra filtering can be added without dropping voltage. A linear post-regulator can be a good idea to clean up switching noise when proven to be necessary, but you'd need to use a low drop-out type if you're not wanting to get back to the original problem of excess heat.

You mentioned protection diodes in the original post. I'd suggest a proper crowbar. Don't forget that while you need to monitor the output of the regulation, you can have the SCR located across the input to the regulators (after the rect+C and fuse), so minimising the damage to the regulator should it fire. That's useful when using multi-stage regulations, such as DC-DC converter followed by linear post-reg, as a fault in one which causes the crowbar to fire could end up damaging the other as a result.

Oh, and current sharing emitter resistors only need to drop enough voltage to swamp the minor differences in Vbe between the transistors - which is a few millivolts typically. In an audio power amp, 0.1 to 0.22 ohms is usually sufficient. Here, 1 ohm would have been more than adequate - there would have been about 0.9V across each in theory. As if sounds like that didn't happen, then perhaps one of the transistors is poorly? It's fine to join the bases and collectors when doing this. As always with an emitter follower, watch for ultrasonic oscillation!

If adding a power transistor to a linear regulator IC, it's always best to use a PNP transistor strapped around it, as shown in the datasheets. That way, the transistor is in the feedback loop, so the output voltage is rather better regulated.
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Old 28th Nov 2020, 3:41 pm   #20
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Default Re: 2N3055 series pass questions.

Capacitive dropper before the rectifier? I wouldn't bother trying to do the (rather complicated and long winded) maths but pop it into LTSpice.
 
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