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IC Voltage regulator (LT1083) problem
IC Voltage regulator (LT1083) problem
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Old 7th March 2006, 02:14 AM   #1
trombone is offline trombone  United States
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Default IC Voltage regulator (LT1083) problem

I'm using an LT 1083 to regulate a tube filament supply on a preamp. Being more of a tube maven than a solid state person, I'm not that well versed in solid state technology.

This regulator has gone intermittent. It's on for a while then cuts out fairly quickly. Sometimes it doesn't come on at all, other times it does. I don't think heat is the problem. It's not on that long before cutout--just a few minutes. This has been a problem occasionally in the past, but other than that, it's worked well for years. Now it's a real problem.

I rebuilt the circuit recently. Replacing a small capacitor seemed to help, but the problem has returned. I suppose the new capacitor could have failed, but that would tell me the regular has other problems.

My question arises because I have to live with this regulator for a while until new LTs come into stock. Do IC regulators fail intermittently like this, without heat being a factor? I always thought they worked or they didn't and there was no intermittent. If they do, I suppose I'm stuck. But if they don't I have reason to dig deeper into the problem.
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Old 7th March 2006, 04:43 AM   #2
lineup is offline lineup  Sweden
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Can you tell a bit more of:
Voltage input.
Voltage output.
If you use backwards diode to discharge big capacitors.

best if yu show us a schematic diagram

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Old 7th March 2006, 05:11 AM   #3
gmphadte is offline gmphadte  India
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Have u resoldered the LT
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Old 7th March 2006, 07:25 AM   #4
infinia is offline infinia  United States
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I suspect you might be running into a low voltage drop out condition.
Most V regulators require about 2-3 Vdc from input to output under load conditions. Can you provide more info on Source and load V &I capacities/requirements?
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Old 10th March 2006, 12:07 PM   #5
dutch diy is offline dutch diy  Netherlands
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Default possible solutions

LT 1083 range have current limiting features in them

Cold tube filament has low resistance and might trick the limiting feature.

One other problem I once encountered was running a IC regulated supply into just a resistor. It basically 'ran away'.
A electrolitic capacitor of 100uF on the output side cured the problem.
J-amp powers Avantgarde-Acoustic Trio
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Old 10th March 2006, 02:28 PM   #6
Bobken is offline Bobken  United Kingdom
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Default Lt1083


I am guessing there is a problem with the reg., itself, especially if you have checked over the associated components. Unless something very unusual has occurred here, it doesn't seem like an instability problem, and I assume you had things working OK for some while before this manifested itself as a problem.

I am familiar with these devices, and they will work down to merely 1volt drop-out, unless maybe they are 'dodgy'.

Also, you (usually) don't need any reverse voltage protection diode from output to input as their internal diodes will handle microsecond surge currents of between 50 & 100A!

Only if the input is shorted (crowbarred) and the output cap is say 1000 uF and above, should there be any likelihood of any probs here.

It would be worth checking out the data sheet to ensure your associated components are in line with LT's recommendations, and from most LT data-sheets I have seen, there is always some helpful advice on stability etc.


Edit: Just a thought, you say there are no similar devices to be had right now, but do you have (or can you get) another 3-legged reg to try in place of the LT1083? Substitution is often the quickest way of resolving an intermittent fault like this, and a cheap LM reg. might show what you need to know.
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