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78L05 used as 27 volt regulator?

I have an application that requires regulated 27VDC at 50mA or less. I used a walwart tranformer that delivers 35 volts peak and a 78L05 regulator with a voltage divider from ground to the regulator output to obtain 27 volts. The ground terminal of the regulator draws a couple milliamps, so the divider doesn't work out to the expected values. Using a 1.5K (1 watt) resistor from the ground terminal to the supply ground, and a 430 ohm resistor from the regulator ground to the output yields about 28 volts (close enough). The output is stable and stays constant with load and no load, so it seems to work ok.

Is this an accepted way of doing things, or should I worry about other problems? I have several 78L05 regulators I would like to put to use, rather than buying something else.

-Bill

Reply to
Bill Bowden
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Hi, Bill. It's accepted to use a couple of resistors as a voltage divider (taking into account the 5mA or so of quiescent current through the GND terminal), just like you're doing. In fact, it's Fig. 12 in this data sheet:

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A couple of caveats: You're right on the edge of the maximum input voltage for the LM7805 at 35V. If it can go greater than that for high line voltage, you might want to cut it down with a few diodes. At turn-on, if there's any capacitive load at the output at all, the difference between the In and OUT terminals will momentarily be the voltage.

Also, you should be aware that the load regulation, noise and other imperfections will be multiplied by 27/5. That may not be a problem, but you should be aware of it. Also, it would be wise to put an 0.1uF ceramic cap across R1, like it shows in Fig. 12.

Cheers Chris

Reply to
Chris

This *approach* is perfectly acceptable but the 7805 isn't keen on that high an input voltage if you short the output !

It would be better to do this say with a 7815. Or a dedicated adjustable regulator like the LM317.

Graham

Reply to
Eeyore

YES, absolutely.

You don't mention capcitors neas the regulator check the datasheet there should be small caps either side to from in and out to supply ground.

Bye. Jasen

Reply to
jasen

Alternatively, use a Zener-Diode:

_____ o------| |-------o + 27 V | | 7805 ------ | --- ZD22V /\\ | o---------o---------o 0 V

Although a 7815 with a 12 V Zener would be safer, IMHO.

Reply to
Dr Engelbert Buxbaum

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