2n3055 pass transistors keep blowing in power supply - diyAudio
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Old 20th July 2015, 02:06 PM   #1
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Default 2n3055 pass transistors keep blowing in power supply

Hey guys I'm new here this is my first post.

This isn't specifically about audio but I am very much an audio enthusiast and will no doubt be posting asking about some questions for a guitar amp I am in the process of designing.

This post though is about a power supply I have made.

I have a 57v transformer that looks like it's good for about 20 amps, I have the center tap half wave rectified to give me 30v.

I have about 500uf of capacitance (no where near enough I know I need about 30,000)

I have a lm317 variable voltage regulator I.C driving the bases of 2n3055 pass transistors. The 2n3055's are in a common collector (emitter follower) configuration with no emitter resistors or base resistors.

The output voltage remains steady and variable from 0 to about 15 volts.

Problem is as soon as I apply any load what so ever both 2n3055 transistors instantly blow and there's 30v at the output.

Could the problem be -

1. No where near enough capacitance.

2. Input voltage too high.

3. Lack of base/emitter resistors for load sharing purposes.

?
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Old 20th July 2015, 02:15 PM   #2
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Schematic plz.
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Old 20th July 2015, 02:16 PM   #3
Mooly is online now Mooly  United Kingdom
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Welcome to diyAudio

It always helps to see a circuit... a couple of thoughts though.

Are the transistors failing short circuit ? Just because you get 30 volts on the output doesn't automatically mean they have failed. Measure them. They will either be short collector to emitter or OK. I'm thinking that the circuit be latching up when a load is applied. That's why you need to measure them.

A 30 volt ac winding will give around 43 volts dc across the reservoir cap.

What sort of load is causing this problem to show ?
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Old 20th July 2015, 02:52 PM   #4
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Here are two threads that discuss this issue about that you are describing.

Do read through this thread as this is the beginning and the good stuff comes in at about post #60 and later,

Lm3886 problem


Here is another guy that is having the same issues and he has a few threads on the same project but at the bottom of this page is where I explain what is happening

The Back Shed: help with circuit

http://www.thebackshed.com/forum/for...676&PN=1&TPN=3


Here is one more thread here at DIYA about this type of design and I go into a bit more detail of things to consider in the design.
Again do read through the whole thing.

12 Volt 30 Amp Power Supply

Also for the voltages you have stated you should be using a LM317HV to start with.

57v/2*1.414=40.305Vdc, The maximum input voltage for a LM317 is 40V this leaves no room for supply line voltage fluctuations.
The input voltage maximum rating for a LM317HV is 60v and would be much better suited for your application.
And as always, Too see the exact schematic you are working with, would better us to help you.

jer

Last edited by geraldfryjr; 20th July 2015 at 03:13 PM.
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Old 21st July 2015, 01:43 AM   #5
kb52 is offline kb52  United States
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There are way to many variables on this. 1 is that he is not stating what load is being applied, and thus I suspect where the CAP mains is only 500UF you could not possibly expect it to hold up, at much current. It could be nreaking into 60 cycle oscillation and causing the IC to deliver much ripple and such to the pass transistor. So in effect blowing it due to that it might even break into high freq oscillation. You do need to add in cap to the output side also and even add like .05 or some other stuff to prevent things from going into high freq oscillation.

Granted all this is purely a guess, but to create stability it is much better to follow a known design that to just throw together a handful or parts and hoping for the best.

Also unknown and assumed is that you put the pass transistor(s) to a heat sink.
Also unknown is the size and current that the power transformer can deliver.
Also unknown but certainly needed is either a cuircuit breaker or fuse(s) to prevent such a disaster.

Years ago I built a home stereo amp from a design, but then in the testing of it discovered quickly that the power transformer I picked to use lacked enough current ability to make it stable. So the result was that when pushing a high level of low frequency bass, it caused a lot of DC shifting. lesson Learned on that one.
The amp used Darlington outputs 2 per channel, and was fairly DC coupled at all stages. It was stable at idle, and properly biased but when pushed to even 1/3 of what the amp wanted to deliver the transformer was just not powerful enough and in the end it would just not work out. You could tell because I had a pilot light which would go dim on the peaks. However it was a good learning experience if nothing else.
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Old 23rd July 2015, 07:35 AM   #6
lexx21 is offline lexx21  United States
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The 2n3055 handles about 60 volts collector to base, 70v emitter to base. You have a 57 volt transformer. Rectify that and you end up with about 80 volts dc. Poof.

Use a transistor rated at a higher voltage.
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Old 24th July 2015, 02:38 PM   #7
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Some how I missed this detail "I have the center tap half wave rectified to give me 30v".
I was assuming that it was a center tapped full wave configuration.

Even for a single diode half wave configuration your average DCrms will be .9x "IF" you are drawing some current.
But when it is just sitting at idle your circuit will be seeing the higher voltage pulses of the rectified AC waveform as ACpeak.

FWIW

jer
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Old 25th July 2015, 01:12 PM   #8
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Quote:
Originally Posted by geraldfryjr View Post
Some how I missed this detail "I have the center tap half wave rectified to give me 30v".
I was assuming that it was a center tapped full wave configuration.

Even for a single diode half wave configuration your average DCrms will be .9x "IF" you are drawing some current.
But when it is just sitting at idle your circuit will be seeing the higher voltage pulses of the rectified AC waveform as ACpeak.

FWIW

jer
One "almost for sure" way to keep power transistors from popping is to implement a simple current limiter. Basically, you put a current monitoring resistor (R Sense below) into the emitter line of the transistor in question that drops about a 0.6-1 volt when the desired maximum current is reached. Hook a small like-sexed transistor (Q2) with the base and emitter connected so that the transistor is switched on by excess current. Tie the collector of the Q2 transistor to the base of the transistor being protected (Q1.


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Old 26th July 2015, 11:52 PM   #9
kb52 is offline kb52  United States
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True to use the current limiter but will the LM317T as proposed counteract with the current limiting to prevent this?
I feel that the whole thing is that it needs a better regulator IC type that would sense all of this and work together to create a "smart power supply" one that is controlling the whole show, offers a preset shutdown when current reaches within 1/2 of the max rating of a 2n3055. Additionally just to have either a fuse or self resetting circuit breaker etc, of sorts would just be a wise idea no matter what.

I am not following that to rectify 47V AC is going to produce 80 volts DC, especially with any kind of load, it might go up to 60VAC without a load. Then again, simply adding a bleeder resistor would be the smart thing to do anyhow, and that also to add in capacitors to keep things somewhat stable and prevent any possible self oscillation which would lead to failure period.

Most smart supplies monitor the output of the regulator to calculate current flow between input and output of the pass transistor, and limit the voltage to the base of the pass transistor in order to prevent overloading, so that you could completely collapse the output to 0 volts and nothing will happen until the short is removed.
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Old 26th July 2015, 11:59 PM   #10
kb52 is offline kb52  United States
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How about to modify this to have Rbias be a Zener diode of desired output voltage? or to add a Zener diode from B of the Q1 to ground? Then add in a few .1 UF caps and some value Electroylytic caps of fairly good value on both input and output, as well as some kind of high value bleeder resistor, to help keep things stable. It just is too noisy otherwise, to make it an effective power supply, depending on the application of course.
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