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Old 13th July 2007, 06:05 AM   #1
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Default psu protection diodes - specified volatge

re: 3 term. voltage regulators....in this case xx7924 (-24v)

A friend suggested that instead of the usual 1n400x diode as the protection diode (between In and Out terminals), that I could:

Quote:
....use a 39 V/5 W zener as this will protect the regulator not only against reverse voltage but also against too high input voltage.
I do trust him, but just looking for a second opinion, and if this is a practice I should continue to use in future psu's.

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Old 13th July 2007, 06:16 AM   #2
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Default Re: psu protection diodes - specified volatge

Quote:
Originally posted by redrabbit
re: 3 term. voltage regulators....in this case xx7924 (-24v)

A friend suggested that instead of the usual 1n400x diode as the protection diode (between In and Out terminals), that I could:



I do trust him, but just looking for a second opinion, and if this is a practice I should continue to use in future psu's.

=RR=
There is nothing about 'opinion' in this. It's a straightforward isssue.

This works but is only necessary if there is a danger that the input/output differential will ever be larger than 40V. This can be the case with an input voltage that is higher than 40V and a large output cap. At switch-on, the input voltage may be there before the cap has had a chance to charge up.
Or if you have an output short with full > 40V input.

If that cannot happen in your supply, if the input voltage is never > 40V, it obviously doesn't do anything.

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Old 3rd August 2007, 06:54 AM   #3
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ditto on jan's comment. Moreover, you have to realize that a 39V 5W zener can only drain 128mA without exceeding its power rating, so that protection is actually only good for transients - and if those are an issue (in car applications they are, for example) I'd rather put a transzorb right across the input. These are essentially zeners with a very high impulse power rating (like 1.5kW for a couple of microseconds).
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Old 3rd August 2007, 10:58 AM   #4
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I agree. But in case of switch-on transients that are essentially one-off events, the zener could take significant more current for the short time it takes the output cap to charge up so that the Vin-Vout difference goes below 40V.

Unless you hang multi-1000 microfahrads at the reg output, which he wouldn't, now, would he?

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Old 4th August 2007, 12:11 PM   #5
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Well, yes - but do you want to pipe any excess juice from the input straight to the output in order to protect the regulator, but at the cost of killing the load?

Putting a zener in that place is counterproductive. A regulator has to dissipate excess energy or die trying ...
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Old 4th August 2007, 12:27 PM   #6
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Quote:
Originally posted by wine&dine
Well, yes - but do you want to pipe any excess juice from the input straight to the output in order to protect the regulator, but at the cost of killing the load?

Putting a zener in that place is counterproductive. A regulator has to dissipate excess energy or die trying ...

Beg to disagree. If the input/output diff goes above 40V because the output cap is not charged yet, the current through the zener charges the cap, together with the regulator, towards the design output value. Nobody kills any load, where did you get that idea?

As soon as the Input/Output difference goes below 40V as the output rises, the zener stops conducting. That's all there is to it.

Remember, we are talking here about a situation where after the initial conditions, things are safe. For instance: Vin = 60V, Vout = 24V. The zener is there to get the system past initial switch-on, when Vout = 0 (cap not charged yet, that takes a finite time) and Vin is already close to 60V. That would kill the reg if the zener was not there.

After things have settled, the zener just sits there doing nothing. That's why I said that for these typical one-off events, the zener may be rated higher than the DC duty max current/dissipation in the data sheet.

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Old 4th August 2007, 02:47 PM   #7
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I do understand what you mean - I still don't like regulators with Vdiff(max) < Vin(max), you can't implement current limiting and there's the startup problem you're talking about.

My point however was that a surge at the input will be seen on the output, only less Vz, and that might still be too much for the load.
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