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Old 5th December 2004, 06:15 PM   #1
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Default Damage to linear regulators

How susceptible to damage are linear regulators in terms of their maximum input voltage rating?

I'm just wondering how careful I should be when designing the transformer, capacitor choke sections before a regulator. From what I remember, the mains is allowed to spike by something like 10% positive? I'm not totally sure.

What I'm asking is, how bothered should be about possible voltage spikes destroying the regulators?
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Old 5th December 2004, 10:06 PM   #2
Stocker is offline Stocker  United States
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Design for mains voltage about 10% high and you "should" be ok, usually.

Of course, the more critical the device, the more care you want to take to protect the power supply rails...

But if you mean the 7815/7915 or 317/337 regulators, they are cheap enough you may ah, push the envelope, if your device is not that dear to you either. Use a fuse at least!
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Old 5th December 2004, 11:31 PM   #3
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Thanks Stocker,

I was looking over the datasheet for the regulators and it seems that the absolute maximum peak the regulators can sustain at 50% of their rated output current is about 5% higher than their rated capacity. However, the devices will only sustain about 8.5% of that difference in the negative direction from their drop-out voltage. So it seems that the easiest way to kill regulators is to allow their input to drop below 0.

-0.2 on the input and the regulator dies.
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Old 6th December 2004, 03:48 AM   #4
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How about when using a variable resistor in the adj chain?

If it's a normal three pin control, which arrangement is safest?

If the adj pin is attached to the wiper, when the wipe fails, the adj pin will no longer be connected in any way. This arrangement has the advantage that the chain current doesn't pass through the wiper.

Connecting the wiper to ground means that the additional resistance, not being used, is bypassed and the chain current flows out to the ground through the wiper.

Conecting the wiper to the output side means that as you slide the wiper you'll get more or less resistance to ground, and more or less resistance in series with the adj's pins route to ground.

In the first examples, if the wiper fails, the chain will default to it's maximum resistance. In the last, wiper failure would disconnect the chain entirely from the output.

If the adj pin isn't connected to anything, won't the regulators output default to 1.25V?
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Old 6th December 2004, 04:07 AM   #5
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Quote:
If the adj pin isn't connected to anything, won't the regulators output default to 1.25V?
Yes u are right and wrong.

In case of LM317, the regulator is capable of maintaing 1.25V at the output with reference to the ADJ pin and not ground.

If the adjust pin is open(not connected to anything), ur R2 is infinite. This will lead to maximum output voltage which is Vin-Vdropout.

Gajanan Phadte
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