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Old 5th November 2012, 07:47 PM   #1
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Default Reasons LT regulator fails under load

Hi All,

I have built 2 identical LM317 regulators, circuit is straight from Valve Amplifeirs 4. Both regs use the same transformer and rectifier, reservoir and common mode choke. Reason for 2 is that I need to power more than 2A heater current, so have split the load into 2 regs. This was the easiest solution with the parts I had to hand.

Both regs test OK reading at 6.5V unloaded, but under load one reg drops out to 0.6V when the other is fine. And when disconnecting the good reg the problem remains. The load is not the issue as it is under 1A, so I am looking for other solutions.

Can anyone suggest reasons why it is failing?

I did search here but could not find the answer, even though I am sure this issue has come up. So maybe this will also help any future searchers too.

Cheers

Charlie
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Old 5th November 2012, 08:55 PM   #2
DF96 is offline DF96  England
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Is this a test load resistor or a valve heater? Heaters take much more current when cold so could trip a regulator into its protection mode.
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Old 5th November 2012, 09:05 PM   #3
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It was tested with valve heaters. Both circuits are identical so am unsure where to look next, other than checking and double checking wiring.

It did regulate at 6.5V for a short time under load, but quickly dropped to 0.6-0.8V. I havn't tested with a resistive load.
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Old 5th November 2012, 09:39 PM   #4
jitter is offline jitter  Netherlands
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Are you powering a single load or two separate loads?
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Old 5th November 2012, 10:07 PM   #5
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There is a total of 6 valves split into 1 reg feeding 4 and the other feeding 2. I tested the working reg with the other section of valves and it works fine, so I am assuming that there is an issue with the reg itself.
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Last edited by Brouilly; 5th November 2012 at 10:24 PM.
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Old 5th November 2012, 11:32 PM   #6
DUG is offline DUG  Canada
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It could be power dissipation.

What is the supply to the regulator?

Either way, one solution could be a current booster for the regulator.

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Old 6th November 2012, 06:46 AM   #7
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Are the two reg outputs connected together w/o some form of isolation?
Doing so will cause the outputs to fight each other. Memory says that basically a diode and op amp will fix this.
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Old 6th November 2012, 08:02 AM   #8
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They are not isolated pre regulator, but they only share a common ground post regulator. The bad reg fails when it is the only device connected so I am assuming it is not interference from the other reg.

The supply is 2 x 9V 1.6A transformer in parrallel, bridge, 10,000uF, common mode choke. So current shouldn't be a problem, and as I said the other reg is working fine under any load up to 1.5A. I will see if I have a PNP to try that out, thanks for the suggestion DUG.
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Old 6th November 2012, 09:43 AM   #9
DF96 is offline DF96  England
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Is the failing one getting too hot? It could be that both are running near their limits, but manufacturing tolerances mean that one is just OK and the other is just over the edge. Have you checked that the initial start up current is within the regulator spec, and that it won't overheat during these first few seconds? The chip can overheat very quickly, as it is small and has small thermal capacity.
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Old 6th November 2012, 11:38 AM   #10
AndrewT is offline AndrewT  Scotland
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Can you arrange for current regulation instead of voltage regulation?

Series connect identical heaters and then feed them with a CCS.
The 317 makes a good CCS and you can trim the output current to just hit the target voltage drop you require.
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