Pass transistor heats up... Variable linear power supply

Thread Starter


Joined Jan 9, 2014

I am in school learning about linear and switching power supplies ... I have already built multiple switching regulators and would like to learn more about linear regulators and op-amps.

I have created a power supply similar in fashion to the one below except I didn't include the IC3B op-amp or the circuitry with the 78L12 - 12V voltage regulator since I don't understand why they are included.

I have one really big issue! other then 1) everything else works!

1) The short circuit is being regulated to 1.5A output(tested with 0.33 ohms on output) however the pass transistor TIP3055 heats up very fast and the regulated 1.5A looses control over time! If you keep it on long enough the short circuit CC regulation fails and 5A! comes out. However, when I remove R48 and R52, to increase the short circuit CC limit, the output goes higher ~ 6A and the pass transistor isn't heating up as fast! It seems like it only heats up when the CC limit is regulated to 1.5A! Therefore the heating on TIP3055 has nothing to do with the load current, only when the CC is at it's limit it causes the TIP3055 transistor to heat up really fast(I don't understand!!).

The other issues:

1) I don't understand what the 78L12 - 12 volt regulator circuit is there and why it is connected to -Vs?

2) Is IC3B needed for better current regulation?


Thank you for your help!


Joined Aug 1, 2013
One of the strangest power supply schematics I've seen in a while. It looks like the intent is to create a regulated negative output voltage using a version of a standard positive output regulator circuit.

3. No. +IS is 0.1 ohms away from GND, and IC3B is driving +IS through 1200 ohms, so its effect is attenuated by 12,000:1. Other than tying up an unused opamp section to keep it from oscillating, I've got no idea what it is doing.

2. When the input AC dips down low enough, the 7812 will pull out of regulation and the relay will change state. This could be an auto-ranger to adapt the supply for 120 / 240 Vac operation. When operating from 240 Vac, the relay pulls in and moves the input to the main regulator to the winding centertap to reduce the input voltage and reduce heat in the regulator pass transistor.

1. IC1B is the voltage control loop amplifier. IC1A is the current control loop amplifier. They are diode-OR'd into the drive for the pass transistor, a very common arrangement. The pass transistor drive comes from R45, and is shunted away through D2 and D3 for regulation. The pass transistor needs a significant heatsink to protect it from overheating.

The more you limit the current, the more heat is dissipated in the 3055. This is why it runs cooler at higher current settings.

What is the value of Vref?


Thread Starter


Joined Jan 9, 2014

Thank you for your reply, appreciate it!

3. I added IC3B to the circuit arrangement and it had no effect, never knew unused op-amps in a 4 op-amp IC could oscillate if unused! So I guess it must be just to keep it from oscillating.

2. Thanks, this is something I couldn't figure out! It kinda-of makes sense, so the purpose of the 12V regulator is only for the sole use of the relay?

3. Yes I played around with R45 resistor, when too high there is no current coming out from the pass transistor. However I want a schematic similar to this fashion but with S.C protection at 3 amps. Why is it that the more you limit the current(S.C protection) it heats up faster? It heats up exponentially faster, which doesn't make sense. I have the IC on a huge heat sink and it still heats up within ~ 45 seconds. If you leave it on long enough the short circuit protection fails and the output current goes to my CC limit(5A!) on my power supply input I am using for the input voltage to the pass transistor.

REF = 3.3V

Thank you for your help,