7812 Voltage Regulators in Parallel?

[georgeburns]

I have a couple of 7812 voltage regulators. According to the specs, the maximum voltage for this regulator is 35 volts.

My Power supply is around 50 volts.

If I connected my PSU to two 7812s in parallel, would I get a regulated 12 volts out without blowing up the regulators (increasing the max voltage of the regulators to 70)?

Would 50 volts going into a single one, really cause any problems if it has a good heatsink?

Thanks for your time.

-gb

[martinw]

Dear georgeburns,
I'm not an expert, but IMVHO opinion, putting a couple of regulators in parallel will not make a blind bit of difference.... each regulator "sees" an input voltage of 50 volts, and this is greater than the max. input voltage of 35 volts. Under these circumstances, the 7812 will not perform as per specification.

However, what will actually destroy your 7812s is the temperature of the junction of the device, and this is a function of
(1) the voltage difference between the input voltage and twelve volts
(2) the current drawn at 12 volts, and
(3) thermal considerations such as heat-sinks and ambient temperature.

My guess is that you may get away with a voltage input greater than 35 volts if you reduce the output current drawn through the regulator. By doing so, you should reduce the power needed to be dissipated, and keep the junction temperature from melt-down.

Just my sketchy two cents..
Best wishes,
Martin

[Al_The_Man]

GeorgeBurns, As Gracie would say, ''Not a good idea"
If you need a 12v supply, is there any possibility you could add a winding to the present transformer? if it is linear type, that is.
Al.

[cbcnc]

How about a different regulator?
Here's one that fairly common and available:
The LM317T is an adjustable regulator. It has a 40v voltage differential. That would just squeak in for 12 v. Or rather than push it, use it to reduce the voltage to a level that the 7812 could handle.
Just my uneducated opinion. Here's the spec. sheet:
LM317T

Chris

[mlaws1172]

Take a look at this circuit posted by Mariss. It works great and I have used it alot. Gecko uses it on their new board.

If this post isnt ok please remove

mike

[georgeburns]

Thanks Martin and all...

My original intent was to create a bleed resistor out of a 12 volt fan for my 50 volt supply. Since the regulators can't handle the voltage, I decided not to do it.

However, I still want to make a 12 V and 5 V regulated supply within my main supply.

My transformer has two 18V secondaries combined to give off 36 V AC rectified to 50v DC.

New question... Can I run a separate rectifier/capacitor/regulator set off of just one of the 18V secondaries. Will that extra draw (maybe 1.5 amps) on only one of the secondary legs, screw up the main PSU due to the mismatched secondaries?

[martinw]

Originally Posted by georgeburns My transformer has two 18V secondaries combined to give off 36 V AC rectified to 50v DC. New question... Can I run a separate rectifier/capacitor/regulator set off of just one of the 18V secondaries. Will that extra draw (maybe 1.5 amps) on only one of the secondary legs, screw up the main PSU due to the mismatched secondaries?

Dear georgeburns,

You can do what you want with each winding, as long as you obey a few rules. What follows is not exhaustive...

You must keep an eye on the load regulation specs of your transformer. Load regulation is a measure of what the output AC voltage is under no current at the output of the transformer, compared to the output AC voltage when the transformer is asked to provide its full rated AC current.

Sorry, that was not exactly well explained, was it???

Let's hope a better man posts soon...

Best wishes,

Martin

[Megahertz]

George,

Yes, you can use one secondary winding for your +5 and +12. I would recommend you make the +50V, +5V, and +12V ground reference the same potential for all your supplies (Tie them together). Make sure it makes senses. Don't short the winding of your transformer. Are you going to casade the +5V regulator off of the +12V regulator. If you do, watch the power on your +12V regulator.

[steve323]

Why not just use a resistor in series with the fan? Suppose that the fan requires 100mA at 12V. The resistor will have 38V across it at 100mA. 38V / 0.1A = 380 ohms. Of course, you need to substitute the actual current draw of your fan. You can also adjust the speed (and noise level) of the fan by slightly increasing the resistance to lower the current.

Don't forget that the power will be 38V * 0.1A = 3.8W.

You can come fairly close to the desired value by combining 4 100 ohm 1W resistors in series.

Steve

[tpworks]

http://www.pcloft.com/et2248vdcfan.html

[timetec]

How about using a TL783 ADJ High Voltage regulator ?

TO220 package - same as a LM317T or 78XX regulator.

Manufactured by Texas Instruments.

Input-Ouput differential is whopping 125V !!

Output current is 0.7A - so slightly down on the 1A of standard 78XX

Still, a very versatile 'state-of-the-art' (TI's words), rugged device.

Google TL783 for the datasheet

Richard