power supplyvoltage-regulator
Good day,
I have built a simple power supply that converts 220VAC to 5V and 12V outputs. I used only 1 heatsink for both regulators (LM338). The power supply works, but I am unsure what the common output (Vout left on the schematic) will have on the regulators. One voltage regulator's output is 12V and the other one's is 5V. Below is the schematic of the LM338 voltage regulator and a picture on how the voltage regulators and heatsink look like in my power supply.
Do not figure it out, junk it. Current cannot flow from the plus 5 to the ground because these 3 terminal regulators do not sink current. In rare cases it can work if the -5 draw is always greater or equal to the +5 draw. Look for voltage splitting circuits. A common approach is to use an amplifier to force the ground to ground, at low currents even an op amp can do this.
No - not without due precautions.
SOME regulators will withstand this.
Many will not or may not.
Place a reverse polarity diode from output to input of each regulator so that if Vout > Vin the diode will conduct and charge the input capacitor or whatever.
This problem can occur in real world applications.
Largish output cap.
Input short of bus to ground or fuse blows and some other load rapidly lowers Vin.
Vin now low, Vout now high, Vout cap discharges into regulator and destroys it
From page 11 of NatSemi datasheet - here.
Pay no attention to the man behind the curtain when he says a 1N4002 is a fast diode. It's OK here. 1N400X.
Best Answer
You need to insulate electrically one or both voltage regulators from the heatsink:-
If you don't then the resulting output will be 12V on both because a 5V regulator normally uses a series pass transistor to control voltage to its load and when presented with a load voltage higher than 5V it will shut that transistor down.
So, you have to insulate one regulator and probably it's best to choose to insulate the regulator that handles the smallest power dissipation, possibly the 12V device (based solely on least voltage drop across it).