7812 and 7815 voltage regulators

Don't use either.. Get an adjustable regulator..

Or perhaps see Fig. 13 to bring the 7812 up to 13.8V

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

You add (to the 7805) an 8.2V zener (eg. BZX79-C8V2) between the GND terminal and ground to give you around 13.2V. Or use 9.1V to get a bit higher.

Best regards, Spehro Pefhany

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------- --+--! 7812 !--------+--- 13.8V ! ------- ! --- ! R1 ! ---C1 +---+-/\\/\\---+ ! ! ! ! GND \\ --- --- / ---C2 ---C3 R2 \\ ! ! ! GND GND GND

C1 from data sheet C2 0.1uF nominal C3 from data sheet

R1 5mA at 12V see Ohms law

R2 is a bit more complex

R2 has 5mA plus the idle current of the 7812 in it see the data sheet for the amount more. It has 1.8V on it see Ohms law.

If you are intending to charge a lead-acid battery with this, you will probably need a diode between the regulator output and the battery positive terminal so as to prevent reverse current, which would discharge the battery if the mains supply were switched off during charging. If your charging current doesn't exceed 1 amp, a 1N4001 will suffice.

This will mean that you need an extra 0.6v (or thereabouts) from the regulator to overcome the voltage drop of the diode. To get this, put an identical diode in the earth lead of the regulator (in series with anything else you may need to put there). If the two diodes are in thermal contact, they will have approximately the same temperature coefficient of voltage.

Ready-made crib sheet for a (not very efficient) double charger with semi-logarithmic metering at:

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~ Adrian Tuddenham ~
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13.8V is too low to charge a lead acid battery; I tried it, took at least 14.3 V. He probably want to power a radio, but a 78XX won't power much of a radio.

Tam

HI, my fault , i should have told you i wanted it for a hobby power supply for testing etc..

thanks, mark k

You need to make sure that it is short-circuit proof so that it cannot damage itself. Even better, some form of pre-settable current limit would reduce the possibility that you might damage something else with it.

The blocking diode might still be a good idea, just in case it ever finds itself across a battery.

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--- from:

"The LM78XX series of three terminal positive regulators are available in the TO-220 package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible."

JF

How much current do you need?

You might look at a simple switcher type power supply. For example,

?name=3DLM2575T-= ADJGOS-ND

This version is adjustable from 1.23 VDC to 37 VDC out, by using a couple small programming resistors. It's not that difficult to select the external parts required to build it. The datasheet is pretty straightforward. Sounds like a good opportunity to learn about switching power supplies....

Also, I think Sharp used to have a 4-terminal (linear) voltage regulator that you could similarly program with external resistors. Don't know if that part is still around however...

-mpm

The current limit isn't particularly well specified (on the data sheets I have seen) and I have had at least a couple of these blow up under short-circuit conditions.

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I think that\'s covered by the "essentially indestructible" part. ;)

JF

I don't know about the 78xx parts but our part number for the 317 is LM317T-not-ST.

They may have fixed the problem but 10+ years ago they were guaranteed to fail in our app.

Yes, but it's typically set around 150°C and the abs. max. operating Tj is 125°C so allowing it to come on won't do much (good) for the reliability...