r/electronics • u/Whyjustwhydothat Inductive resistive capacitor. • May 06 '26
Project 10V-60V Discrete Railsplitter 2-5A unbalanced load with propper cooling. And 10A Balanced.
So this is my latest draft for a railsplitter with low noise that is ment to be an accessorie fir my 60V 5A power supply. You could add 1uF film+100nF ceramic between Q5 and Q6 Collectors, and 100pF on Q1 base to Q1 emitter for lower transients and risk for oscillation. If you parallel 2 tip35c + tip36c you could go around 8-10A unbalanced load with propper cooling. As long as the load is symmetrical then this circuit should be able to handle several amps (Atleast 10A). It might be hard to see but i added a 1000uF 80v electrolytic capacitor 1 uF film 100v and 100 nF ceramic 100V at the input for more stability. Can't say for sure that this works like it's intended as i haven't simulated or built it yet, I just now finnished the schematic, will post the results once i am finnished with it. If it works like intended then it could be a good way to be able to run amplifiers using single rail PSU. And the Voltage/Ampers is limited by what components you use. If you switch the small signal bjt's/drivers to over 100V+ and use mosfets as power stage you could theoretically drive ±50V and 50+ Amps.
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u/OdysseusGE May 06 '26 edited May 06 '26
Bias seems pretty unbalanced. I don't see how it's possible for Q1, Q4, and Q6 to be active.
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u/Beggar876 May 07 '26
Nope. This circuit cannot work at any input voltage from 10V to 60 V. At ANY input voltage Vin the base and emitter of Q2 would be at somewhere close to Vin/2. The current through RE would be Vin/(2RE). Since the collector resistors of the diff pair are close to 2 x RE then the voltage at each collector of the pair would be near Vin - (2 x RE(1 or 2) x Vin/(4RE)) = Vin/2. Thus the collectors are close to the voltage on the bases at any Vin. In fact the collectors would tend to be BELOW the base voltage by a small amount. So the diff pair has no room to work in. But now say you reduce the value of the collector resistors to raise the voltage on the collectors and give that some elbow room to work. That puts them above Vin/2 and the output of the circuit would follow that. The feedback would begin to work but the input voltage would not be evenly split as you want it.
A smaller point is that there is no bias voltage separating the bases of the output Darlington pairs by some small amount to avoid crossover distortion but that is secondary right now.
If you REALLY want to build a discrete circuit to do this (hopefully for the purpose of learning) I suggest you build a better op-amp that can swing its output across most of the available input voltage range. If you are OK with using a commercial op-amp then that is the way I would go and there and, as I'm sure you know, there are lots of example of circuits on the interwebs for this.
Cheers.
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u/Wobble-Engineering May 06 '26
I wish I knew how to make stuff like this. Best I can do is wire up a resistor
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u/ViktorsakYT_alt May 06 '26
The feedback only seems to work in one direction - output gnd voltage too high, but I don't see a way where it will correct for output too low
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u/ViktorsakYT_alt May 06 '26
Actually I think it would work if the top Darlington pair was PNP
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u/Independent-Light740 May 06 '26
The negative part is the problem, and even a Sziklai pair wouldn't fully fix it.
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u/tom-ii May 07 '26
Top rail has 2 (sets of) voltages connected by a dead short.
Bottom rail same.
Okay, you've basically made an uncompensated hi-power op-amp. Uncomp'd in that its bias points will vary quite a lot due to thermal effect, and to parrot several others, boy howdy, there'll be a lot of thermal in there.
Also, for circuits like this (current mirrors and complementary amps), the transistors really need to be closely matched- you won't get this with discrete parts without a cherry-picking, but then back to thermal issues.
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u/sswblue May 13 '26
Bjt pairs exist. I think so do mosfet pairs. But yeah, it is a worse op-amp. But good on OP for trying. That's how we learn.
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u/CircuitCircus May 06 '26
The feedback doesn’t really work, as others mentioned. Aside from that, rail splitter circuits are meant for pretty light loads like a few dozen mA. Say you had an unbalanced 5A load connected between VGND and -30V… Q5 has to pass that full current, so it’s dissipating 150 W as heat! Even if the thermal design allows it to handle that it’s just wasteful.
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u/hapemask May 07 '26
I’m still learning and maybe am missing something obvious, but why does Q5 dissipate 150W? Isn’t the power dissipation for a transistor
V_CE * I_Cwhich would be5 * V_CE_SAThere? Still a lot but not 150W.•
u/ludko_pro May 07 '26
The collector and emitter of Q5 are connected between two fixed voltages, so to say. Collector is connected to +30V, Emitter is close to 0V if the feedback loop is operating correctly (there is a voltage drop across the emitter resistor).
So there’s a total of 30V x 5A = 150W.
Depending on the beta of the BJT, which is usually pretty low for high power ones, let’s say 20, you start getting some extra dissipated power due to the base current. 5A/20 = 250mA. 0,25A x 0,6V = 150mW. It’s a fraction of the Emitter-Collector power but still, fun to know.
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
I understand that unbalanced load is not preferd, but i am not gonna use it for unbalanced load either.
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u/CircuitCircus May 08 '26
The terminology is a little odd, but if that’s the case why have a rail splitter at all? Just connect the load between the + and - of your power supply.
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
That's not the same as having both positive and negative voltage.
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u/Apex_seal_spitter May 07 '26
Good on you for giving it a crack. As others have said, it's going to have a few issues. Simulate it and experiment with the simulation, taking note of the power dissipation of various transistors at different voltages and currents for each rail individually, and both at once.
Don't give up - keep at it. Great to see a hand drawn schematic to get your thoughts down. Awesome project and learning opportunity.
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
Thanks! Gave me some motivation to actually proceed with this for better or worse, if it works, wonderfull. If not then I will learn from that aswell.
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u/sswblue May 13 '26
- Use ltspice to test your designs.
- Stability has to do with the phase of the loop. Adding caps at random won't work.
Keep going. )
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u/Independent-Light740 May 06 '26
I would put the 47R resistors between be. Base resistors aren't required, even with parallel output transistors. However each parallel output transistor should have it's own emitter resistor and it would be beneficial to put them on the same heatsink to have a somewhat equal temperature between them.
Q1 cannot active the transistors for low offsets, as it's collector can barely go 1 Vbe below the measurement point.
The main question remains, why would one ever want to use this? It's very inefficiënt and the space/money could be better spend for a 2nd supply rail?
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
This is supposed to be an addon to my 60v 5A bench psu so that I can get variable dual rail from it. I have made one basicly exactly the same but using an op amp instead of long tailed pair/discrete differential amplifier that works without any issues but it can only handle 30v becouse of the op amp.
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u/a_certain_someon May 06 '26
electricity waster 90000. either that or tapped transformers must be stupid expensive where you live
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
Transformers are expensive yes, and how does this waste electeicity? As far as I understand it as long as the load is symmetrical then the curcuit won't do mutch more than split it in to 2 rails and if its unbalanced load then thats where i burn electricity as heat. Don't take this as a last out i'm just asking trying to learn more.
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u/shuzz_de May 06 '26
This looks interesting, I'll check back periodically for progress/improvements.
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
I'm in hospital for another week or so but when I get home i will build it and see if it works like intended as an dual rail addon for my bench psu.
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u/SKRANEX_ May 07 '26
What does the un/balanced mean? Haven't seen that yes
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
Say i draw +30V and -15V then that's unbalanced, if I draw +30v and -30v then thats balanced.
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u/MINING123STUDIOS May 07 '26
I would use an opamp and boost the output current with transistors.
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
A normal op amp won't go to 60v. The end goal with this project is being able to split whatever voltage and current i want even if its 300v+
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u/MINING123STUDIOS May 08 '26
Then why are you using a linear regulator in the first place?
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u/Whyjustwhydothat Inductive resistive capacitor. May 08 '26
To learn?
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u/reficius1 May 08 '26
If your load is high and unsymmetrical for even a few milliseconds, you will pop either Q5 or Q6. You should add protection similar to what you'd see in audio power amps, which remove bias at Q3-base or Q4-base in case of overload.
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u/Brainsample_ May 08 '26
Shouldn't the inputs of Q3 and Q4 be on the same phase? The output of the differential pair Q1 and Q2 are 180 degrees out of phase. That can't be right.
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u/Whyjustwhydothat Inductive resistive capacitor. May 09 '26
Q1 and q2 is a differential amplifier as far as I know and with that i get negative feedback on Q1ftom virtual ground, like i would using an op amp. I could be wrong as i am still a newbie just trying to learn. This whole curcuit could be wrong and don't work at all. And if so then I have learned something from that to.
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u/Brainsample_ May 09 '26
You're definitely going to need a differential amplifier for this concept. And then:
- If the output voltage is exactly in between V+ and V- transistors Q5 and Q6 don't have to provide much current.
- If the output voltage is too high, Q6 must provide extra current, but not Q5.
- If the output voltage is too low, Q5 must provide extra current, but not Q6.
However in your design, if the output voltage is too low:
- The collector voltage of Q1 goes down, reducing the current of Q6
- The collector voltage of Q2 goes up, reducing the current of Q5
And if the output voltage is too high:
- The collector voltage of Q2 goes down, increasing the current of Q5
- The collector voltage of Q1 goes up, increasing the current of Q6
So Q3 and Q5 work the wrong way around.
A separate problem is the voltage swing of the differential amplifier's output. The voltage can go up to the positive rail, but it can't go below the base voltages of Q1 and Q2 (halfway in between V+ and V-).
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u/Whyjustwhydothat Inductive resistive capacitor. May 09 '26
Well it is a first draft and Q1/Q2 was meant to be an option for a op amp, but i guess i need to redo that part then.
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u/bobtrottier Jun 16 '26
Looks a lot like and audio power amp if you take input of diff amp as input and remove filter caps on output
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u/advandro MN3214 May 07 '26
Am I correct in assuming this could be categorized as "Yet Another (room) Heater"?
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u/Linker3000 May 09 '26
Post is approved but should really be in Askelectronics for circuit design advice.
'Project' is for completed, working projects where the build and design details are being shared for others to replicate.