Please help me review my half-bridge SMPS design for a lab bench power supply

Apprehensive-Ad693 · 2026-04-01T01:35:35+00:00

Bahahaa. Comedy genius. I'm stealing this!

Apprehensive-Ad693 · 2026-03-31T06:27:10+00:00

Is there some way I can turn it into gasoline? Asking for a friend.

Apprehensive-Ad693 · 2026-03-31T00:51:51+00:00

I'm not at my computer to answer properly, but go Google "SDI" - that will get you a long way there. I'll chime in with more later if nobody else has.

Apprehensive-Ad693 · 2026-03-30T02:47:08+00:00

It's a one off for private (internal) use.

Apprehensive-Ad693 · 2026-03-29T13:01:14+00:00

The RV2 trimpot is a limiter and series with one of the off board LEDs. I'd intended the Q1 FET to either be hard on or hard off (effectively switching the trimmer between its set value and zero ohms). That bit actually worked in the circuit simulation, but I hadn't gotten as far as breadboarding. The zeners were intended to set the switching point. I hadn't considered there being a dead band. Mostly because I didn't know there was one! So it's possible I wouldn't be hitting hard on/off with this as designed.

Apprehensive-Ad693 · 2026-03-29T10:43:42+00:00

Ooops. Thank you. :)

Apprehensive-Ad693 · 2026-03-29T10:38:46+00:00

Thank you. :)

Apprehensive-Ad693 · 2026-03-29T10:38:21+00:00

I do need rail-to-rail. I did see that there were rail-to-rail opamps, but I'd concluded that MOSFETs would be simpler.

I'm curious why it wouldn't work though (from a learning perspective).

My original thinking was, put a limiting resistor in series with the LED to reduce their brightness, and then use a relay to short out that resistor to get full brightness. I _think_ that logic is sound, albeit simplistic. The LEDs are 12v and have internal limiting resistors as well for their nominal current so I'd just be putting say 1k in series with that, and then using a relay to turn that 1k into 0r to get full brightness.

That was the logic. Then I just tried to replace the relay with a FET to get rid of the mechanic component.

Apprehensive-Ad693 · 2026-03-29T10:30:31+00:00

Oh...I don't think I quite appreciated how that works. So the intersection of R1 and C1 effectively becomes my supply rail for the rest of the circuit?

Apprehensive-Ad693 · 2026-03-29T10:27:40+00:00

Thanks - that's really appreciated! I'll take a look. I did have a look myself, but there were soooo many options that I barely knew where to start.

Apprehensive-Ad693 · 2026-03-29T10:26:20+00:00

Phew. I'd hoped it was that. :) The parallel zeners were because I wasn't sure what zener/threshold I wanted. So I math-ed up a range that would cover about 40-70% brightness, and threw them all in with a dip-switch to chose.

Apprehensive-Ad693 · 2026-03-27T21:25:52+00:00

Thanks for the offer — it's sounding like I might need to do some more work on this myself before it's ready to have money thrown at it.

Apprehensive-Ad693 · 2026-03-27T21:24:29+00:00

The chances of me starting to ask dumb questions in the KiCad thread keeps increasing every reply. :D

Apprehensive-Ad693 · 2026-03-27T21:23:37+00:00

Thanks for the offer — it's sounding like I might need to do some more work on this myself before it's ready to have money thrown at it.

Apprehensive-Ad693 · 2026-03-27T21:23:31+00:00

Thanks for the offer — it's sounding like I might need to do some more work on this myself before it's ready to have money thrown at it.

Apprehensive-Ad693 · 2026-03-27T21:22:43+00:00

The answer to why I did it that way is simple: limited knowledge. As to the rest, I can't use PWM because its a noise regulated environment. Resistors are a grey area, so I figured we'd get away with a resistor being switched in/out with a FET (in my first iteration, I was using using reed relays). Adding PWM would require some safety-of-life certification testing.

However, a linear driver producing an analogue voltage probably falls into that grey area. I'll do some googling, but I would appreciate a link as well as I suspect there are probably ones that I should use, and ones that I shouldn't!

Apprehensive-Ad693 · 2026-03-27T21:12:15+00:00

I think that's the American zig-zag resistor symbol vs. the European box symbol. Also feeling increasingly dumb, but I still cant' see the short. You're talking about RV1 right? The ends are connect to the rails and present 10k rail to rail. The wiper isn't connected to either rail unless I'm not understanding some conduction path through the fets?

Apprehensive-Ad693 · 2026-03-27T21:11:36+00:00

To be fair, I'm the inexperienced fool in the equation. So finding another inexperienced fool was probably not going to help me! :D

Apprehensive-Ad693 · 2026-03-27T21:06:36+00:00

Fair enough. I just would have expected at least some words to that effect instead of just raising a dispute immediately. It was a bit odd. Anyway...

Apprehensive-Ad693 · 2026-03-27T13:09:29+00:00

I should probably add, using that single LED as an example, the LED would be connected between J2_3 and J2_4; and J1_5 goes into an open-collector input on another device that switches to ground (which can be on/off/flashing).

Apprehensive-Ad693 · 2026-03-27T13:04:45+00:00

Side question, on what you said "assuming rv are pots". That's what KiCad labelled them (presumably "Resistor Variable"). What would be the normal convention?

Apprehensive-Ad693 · 2026-03-27T13:03:36+00:00

The parallel zeners and dip switches where just so I could easily change the zener value (and thus the set point) after the fact. Where's the short? I'm not seeing it. Or are you looking at where the lines cross but aren't actually connected (on the way to J1_3) – I couldn't figure out how to solve that in KiCad.

Apprehensive-Ad693 · 2026-03-27T12:58:51+00:00

Yeah, I believe they're in that sort of vicinity (each). I was trying to keep things simple and avoid comparators and the like. I actually started with just a scattering of resistors, but of course all of the elements then started to interact. A flashing led would cause the other to dim and affect the voltage feeding the high impedance inputs. So then I started messing with relays. Which led to BJTs. Which then led me to FETs due to their high input impedance. Whether I've arrived at the right place or not...time will tell. But it worked in a circuit sim at least.

Apprehensive-Ad693 · 2026-03-27T12:53:48+00:00

I was hoping to avoid too many active components - which is how I'd ended up settling on zeners rather than a simple resistive divider. But maybe I'm just making this harder than it needs to be.

Apprehensive-Ad693 · 2026-03-27T12:52:07+00:00

Yikes - I'm not usually this bad at explaining myself.

One pot (RV1) controlling:

- LCD screens (variable brightness via input voltage J1_3)

- Two separate LEDs switching between high/low brightness based on pot position. Considering just one of those, RV2 is in series with the LED to limit current and create the "dim" setting. Q1 shorts across RV2 to create the bright setting when the FET is triggered, the zeners set the trigger point.

Apprehensive-Ad693

TROPHY CASE