What Happens when Zeners are paralleled?

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Ethan

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For example, if you have two 60V zeners in parallel, is the regulated voltage 60? Or 30? They add in series, but I have a feeling that they don't just product/sum divide in parallel?
 
Thank you Voodoo, but I was curious what happens with you have two same voltage zeners in parallel? Better regulation? Or, as you say, the one that's just slightly lower (due to tolerances) will be "on"?
 
[quote author="Ethan"]Or, as you say, the one that's just slightly lower (due to tolerances) will be "on"?[/quote]
Yes. So if you put two zeners in parallel to get a higher power rating, one of them will dissipate most of the power - so it won't work.

You can connect zeners in series to get a higher voltage.

Best regards,

Mikkel C. Simonsen
 
You can't parallel zeners to get a higher power rating unless you match their voltage drops under load, and then it's a bit dicey. If you want a higher power rating, divide the required voltage by the number of zeners you need to get the right power rating. For example, two 30V, 5W zeners in series will be roughly the same as one 60V, 10W zener. But watch the power dissipated - ignore the derating curves and thermal resistances at your own peril...

Tranzorbs, by the way, are ineffective if paralleled and not perfectly matched. Well, they actually have the same power rating as a single one. The one that turns on first will conduct, the others won't. If the spike is really, really, big, they will all fail in a very convincing manner, as each one fails, shorts, vaporizes, and opens in sequence.
 
[quote author="dale116dot7"]You can't parallel zeners to get a higher power rating
[/quote]
You can paralel everything, but if you paralels stabilization diodes,
you have big problem.
To sufficiently paralelize power semiconductors, you must put
small series resistor in series to each. For normal devices, value
of the resistor is 10 ohms.
Yes, you can paralelize zeners with 10 ohms in series to each, but equivalent series resistance of zener is 7 ohms, and
by that resistor you "make" twice higher.
And higher eq. series resistance = worse stablizing ratio.
you will build bad stabilizer with zeners paralel.

xvlk
 
But the purpose of the tranzorb is to have a very low series impedance (much less than an ohm) so large voltage spikes don't damage equipment, so using resistors doesn't really work too well in that application. For applications where you are just burning off a bunch of voltage, of course you can parallel them with the sharing resistors, and if you match their breakdown voltages it'll work.
 
Rule of thumb is that when you directly parallel two or more PN junctions, one will hog the current. That's why you always see small series resistors used when devices are paralleled.

There would be no point in paralleling zeners. The series resistances you would have to add would ruin their regulating properties. Use a higher-wattage zener instead.
 

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