Baffled by simple linear regulator

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
I am making a + & - 12V power supply using L7812 and L7912. The former is a +12V regulator and the later is a -12V regulator. The Pos regulator is supplied with 25.2 VDC and the output is +12.1 V. However, the Neg regulator is supplied with -24.8 VDC and the output is -17.0 V.

At first I thought I had a bad L7912 so I replaced it and to my amazement, I still have -17 volts instead of -12 V.

I am aware that the pin functions are different on these devices.

Looking at the front, the L7812 pins are Input, Ground, Output
The L7912 pins are arranged as: Ground, Input, Output.

This has me baffled. Any ideas?
 

hp1729

Joined Nov 23, 2015
2,304
I am making a + & - 12V power supply using L7812 and L7912. The former is a +12V regulator and the later is a -12V regulator. The Pos regulator is supplied with 25.2 VDC and the output is +12.1 V. However, the Neg regulator is supplied with -24.8 VDC and the output is -17.0 V.

At first I thought I had a bad L7912 so I replaced it and to my amazement, I still have -17 volts instead of -12 V.

I am aware that the pin functions are different on these devices.

Looking at the front, the L7812 pins are Input, Ground, Output
The L7912 pins are arranged as: Ground, Input, Output.

This has me baffled. Any ideas?
It acts like ground on the 7912 isn't grounded. You get voltage out but it changes with the load?
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
Here is the schematic. The voltages are with no load and the -12V is actually at -17V.
Capture.JPG
 
Last edited:

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
It acts like ground on the 7912 isn't grounded. You get voltage out but it changes with the load?
The ground is good on both devices.

With no load on the Neg regulator, output is -17V
With a 3ma load on the Neg regulator, output is -12.1V.
The pos regulator works fine with no load so I don't know why the Neg regulator needs a small load to regulate correctly, but it does.

Problem solved. Thank you HP1729
 

dl324

Joined Mar 30, 2015
9,332
Same for 7812:
upload_2015-12-8_16-28-43.png
Parts will sometimes/often operate outside of their specs, but it isn't guaranteed.

I was breadboarding a circuit with an LM317 which has a minimum load current of 10mA. I was operating at 20mA and it still wasn't regulating. Worked at 30mA. It's out of spec, but who can I complain to??
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
Same for 7812:
View attachment 96196
Parts will sometimes/often operate outside of their specs, but it isn't guaranteed.

I was breadboarding a circuit with an LM317 which has a minimum load current of 10mA. I was operating at 20mA and it still wasn't regulating. Worked at 30mA. It's out of spec, but who can I complain to??
Thanks, That clears it up.
 

SLK001

Joined Nov 29, 2011
1,541
You are dissipating a lot of power in your regulators due to your high input voltage. You should try a smaller transformer, say +/-16V to operate your circuits. If not possible, then provide big enough heatsinks to extend the life of your regulators. At 1 amp output current, your regulators will dissipate 13 watts of power.
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
You are dissipating a lot of power in your regulators due to your high input voltage. You should try a smaller transformer, say +/-16V to operate your circuits. If not possible, then provide big enough heatsinks to extend the life of your regulators. At 1 amp output current, your regulators will dissipate 13 watts of power.
The transformer is actually rated at 15V-CT-15V. I suspect the voltage is so high because there is no load. I will need to see what it looks like when there is a load. If it's still too high, I might try a 12V-CT-12V.
 

dl324

Joined Mar 30, 2015
9,332
You are dissipating a lot of power in your regulators due to your high input voltage. You should try a smaller transformer, say +/-16V
Or he could turn his regulators into switchers with the addition of an external pass transistor, inductor, diode, and some resistors.

Or he could use this self oscillating, switching, tracking, pregulator that I suggested in another recent thread:
upload_2015-12-8_19-10-29.png
 

dl324

Joined Mar 30, 2015
9,332
The transformer is actually rated at 15V-CT-15V. I suspect the voltage is so high because there is no load. I will need to see what it looks like when there is a load. If it's still too high, I might try a 12V-CT-12V.
15VRMS will give you about 20V. 12VAC would give you about 4V less.
 
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