Voltage Regulator Polarity

Mag748

Is there any way to prevent damage to a voltage regulator if the polarity is connected backwards? I just had a 7805 die by useing a wall transformer that had the positive terminal on the outside and the negative on the inside. That was just uncalled for. By now I know to check every single time.

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Forrest

Here's a +5V regulator that protects against reversed polarity http://www.seetron.com/an_vpwr1.htm

I've also seen similar designs with just a diode feeding the input side of the regulator (D1 in the circuit above) and D2 and D3 are omitted.

PJAllen

Yep.· You can use a diode in series with the input so that it is forward-biased with proper polarity.· If you·work in·a full-wave bridge it will work with either polarity, providing the proper output regardless (if you can "afford" the 1.5V V_F drop/s.)

*** Post Edit ***
If you choose to use a F-W-B -- then·its input must be isolated from the rest of the circuit/ry.· I hasten to add that F-W-Bs work with DC just as they do with AC.· You just have to keep your head about you.· Dwg attached.

In matters such as this, the best (and most under-utilized) test instrument you have is·your lookenpeepers, but you can't beat a DMM.· In carpentry they say, "Measure twice, cut once."· In electronics, I say, "Assume nothing, get the voltmeter."

Post Edited (PJ Allen) : 5/14/2006 11:37:01 AM GMT

LoopyByteloose

I just purchase a bunch of 3.3volt regulators that are 4 pin -- KA78R33-3.3·· They come in other voltages too.

The bad news....
These babies have a fourth wire for sensing low input voltage and it seems that 4th one is NOT protected.
There is virtually no reversed polarity protection and they fail by allowing a full voltage to pass.

The good news....
They output a full 1 amp and can go very, very low [noparse][[/noparse]down to nearly 2volts]. So you get the most out of battery options.

Of course, using a polarized plug or battery holder will get you the most power because all diodes have that wasteful voltage drop, even the Schottky Barrier diodes.

If you must use diodes in battery applications, consider the Schottky's. Only 0.25 voltage drop. They are rated to at least 3 amps.

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 5/14/2006 12:50:27 PM GMT