Dual channel bench supply - all linear - Page 1
Forum section: electronics / projects

author: special_K
author_rank: Regular Contributor
timestamp: January 29, 2025, 06:26:28 am
content: Decided to roll a big boulder up a hill for no reason: Building a Linear benchtop supply with two channels. For now, the goal is 1.25v to 25V output at 2A. Why this goal? Felt like it.In a previous thread the inefficiency of Linear has been discussed. Normal solution is to use a transformer with many secondary taps, so that it's voltage is always just high enough to give the minimum overhead needed by the voltage regulator.Unfortunately, here in 2025 there's very limited choice of transformers to buy as a regular customer. Either small (less than 15VA ) jellybean parts, or toroids who have two matched pairs of secondaries (2x 12V, 2x 15v etc). So that limits what we can do with that method.I decided to go with just two taps per channel. I already have a 2x 24V toroid. I decided to match it with a 2x 15V, as 12V AC needs very large (44,000uF) capacitor to give a regulated 12V DC at 2A.Here is the 1st PCB:It's pretty simple: The front panel will have a voltage knob which is actually a stereo potentiometer. we use this as a voltage divider. Inside the PSU is another pot, which makes a reference voltage. When the front panel signal is more than the reference signal, the LM393 makes relays go CLICK and switch that channel from 15V Transformer to 24V transformer.Of course this means that the efficiency of the PSU is quite strange. At 1.25V, very inefficient. Turn output up to 13/14V, it's efficient. But 14/15V and it switches to 24V and it's inefficient again.Here is a second PCB, which is for 1 channel voltage regulator, and also the Current limiting:Voltage regulation is not very interesting, it's basically from the LM317 datasheet. The only change here is that I put two TIP3055 pass transistors instead of one. This is a cheap way to improve thermal transfer to the heatsink.Current limiting is actually a circuit breaker. All the current limiting schemes I seen before seem to work by turning voltage down, but I prefer that power unambiguously shuts off instead.I put the current sensor before R9, who is part of the voltage divider which feeds LM317 adjust pin. Because the current sense resistor has voltage drop, the more current the worse the drop. So by putting it here, the LM317 should be aware of it and regulate more accurately.The circuit breaker behavior is controlled by CD4011 logic, set up as a latch. It's output defaults to low, and the relay by default sends the power output to a dummy load meant to keep the regulator stable. This means that the output is default OFF unless the user hits momentary switch SW4.I haven't actually debugged this design yet, I expect there to be errors. Once those are worked out, then the next thing I want to do is to add thermal protection, and perhaps to disconnect 230V from the 15V Transformer when both channels are running on the 24V one.Feedback appreciated.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2025, 08:42:28 am
content: The shunt at the output side of the regulator means that the resistor is in series to the output and not compensated by the voltage regulator.The current turn off may be too slow to really protect the transistors. There should be at least a crude faster electronic limit of the current to a fixed upper limit at something like 5 A or less.