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I've attained most of the material I need to build my first power supply. The specs are mostly dictated by the parts I have. My intention is a dual isolated variable output power supply that I can connect in series to give negative voltages.

Below is the "final" draft of the circuit that I'll be implementing. I'll add in fancy pass transistors and capacitor discharge transistors and other things once I completely understand how they work.

I've set up a basic form of this with a single output and no decoupling capacitors or protection diodes on a breadboard and it worked well.

shcmati

Clarification: I pulled a single bridge rectifier package out of an old TV which is why I have both a packaged bridge and a diode bridge on the schematic. Other than that and the transformers, the circuits are identical.

EDIT: Fixed schematic.

So here are some things I would like to ask:

  • I'd like to solder the primary of the 500mA transformer to the primary lugs of the larger transformer so I don't have to branch the mains wire to it separately, is this fine?
  • Is it fine if I stack the two transformers on top of each other?
  • If I had both outputs connected in series, would I be limited to 1A due to the second transformer only providing 500mA? I don't need that much current but I'm just curious if I'd run into trouble since the transformers are unbalanced. I have a 9V 15VA transformer as well, would that be better matched?
  • What amperage of fuse should I use? I'm worried about magic smoke....
  • How damaging would a shorted/max current output be on this circuit?
  • Any general advice/improvements?

Thanks for your assistance.

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  • \$\begingroup\$ Putting multiple magnetics in close proximity is usually a bad idea unless done deliberately. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 24 '14 at 8:27
  • \$\begingroup\$ Your fixed schematic is still not right. the 10uF capacitors are shorted out. You need to rearrange the resistors in that area. Also, add component designators (C1, C2, R1, R2, etc). \$\endgroup\$ – John Honniball Dec 24 '14 at 9:23
  • \$\begingroup\$ yeah woops, it should be correct (at least for me) now \$\endgroup\$ – merkman Dec 24 '14 at 9:28
  • \$\begingroup\$ @merkman Yes, looks better to me! \$\endgroup\$ – John Honniball Dec 24 '14 at 9:30
  • \$\begingroup\$ If you connect the pot wiper to pot top, if you get wiper bounce or a burned spot or open cct wiper for any reason the mex Vout is set by the pot max value. If the wiper is connected as shown now and goes O/C the Vout will rise to about Vin DC. If you are using the whole Vin the result is about the same. If Vout max is meant to be eg 50% of Vin then an even very temporary o/c wiper may be disastrous. \$\endgroup\$ – Russell McMahon Dec 24 '14 at 14:08
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Resistor arrangement on LM317's does NOT adjust voltages as shown.
Refer to data sheet (carefully) - you need a variable resistor to local ground.

You can only use a second positive supply to form a negative one in the way that you are proposing if the "energy sources" are isolated relative to each other. eg two unconnected windings on the same transformer. The input of the "-ve" supply will be above ground voltage during operation.

To use a single transformer winding or non isolated sources what you want is to use an eg LM337 which is a less well known negative regulator equivalent to the LM317.
Here is a dual +ve & -ve supply circuit diagram using an LM317 and LM337. They use centre tapped transformer and a single bridge rectifier - but you could use the 2 separate windings as you propose and join them appropriately.

LM337 data sheet here

Add a small "spreading" resistor between rectifier output and 10 mF filter cap to widen the conduction angle. This reduces peak currents substantially and reduces RF noise generated by diodes and gives diodes a far less-hard time on conduction peaks.


I'd like to solder the smaller 500mA transformer onto the lugs of the larger transformer, is this safe/do I need to care about polarity?

Slightly unclear what is intended. Mechanically not wise.
Electrically - use wires.

Is it fine if I stack the two transformers on top of each other?

Probably. Minimal flux interactions with closed cores. Cooling may suffer slightly but easily judged and varies case by case.

If I had both outputs connected in series, would I be limited to 1A due to the second transformer only providing 500mA?

When done "properly" each supply can return its own Imax to ground. A current that starts at V+ and ends at V- will be limited by the smallest current capability.

I don't need that much current but I'm just curious if I'd run into trouble since the transformers are unbalanced. I have a 9V 15VA transformer as well, would that be better matched?

As above. As specified you can have 1A V+ to ground, 500 mA V- to ground or 500 mA V+ to V-.

What amperage of fuse should I use? I'm worried about magic smoke....

LM317 is self protecting within limits.
1A fuse fast blow blows at about 2A in moderate time.

How damaging would a shorted/max current output be on this circuit?

As above.

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  • \$\begingroup\$ Yeah I made a mistake on the schematic regarding the pot to ground, I fixed it. I was under the assumption that since the two outputs should be floating I can connect them in a common ground configuration? I'll add in the spreading resistor as well \$\endgroup\$ – merkman Dec 24 '14 at 8:40
  • \$\begingroup\$ @merkman See re LM337 \$\endgroup\$ – Russell McMahon Dec 24 '14 at 8:43
  • \$\begingroup\$ So why specifically wouldn't this work? I'm considering the two outputs to be isolated and positive, no different than two separate positive power supplies except for being in the same case. As far as I know you can connect the negative of one output to the positive of another, call it ground, and the remaining terminals will be +V and -V respectively since they are not commoned. I don't see a need for the LM337 since it would permanently output negative voltage which I don't necessarily want. \$\endgroup\$ – merkman Dec 24 '14 at 9:03
  • \$\begingroup\$ Thank you for addressing everything else though, and yes I would be connecting them with wires. \$\endgroup\$ – merkman Dec 24 '14 at 9:07
  • \$\begingroup\$ Referencing this thread : electronics.stackexchange.com/questions/26610/… "A classic desktop power supply (like the one in the YouTube video linked in the O.P.) is of a "dual positive type". Each channel has its own independent secondary winding (or even a separate transformer) and its own rectifier. The channels can float with respect to each-other, and that allows to connect them in series." \$\endgroup\$ – merkman Dec 24 '14 at 9:12
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Generally, it looks fine, but I as I said in a comment to your previous similar question, I suggest that you add reverse-biased rectifiers directly across each of the two outputs. That way if you connect them in series and a short or overload occurs the output cannot be driven too far negative by the supply with higher output capability (one will always 'win'). This is a general precaution that is useful whenever connecting supplies in series.

schematic

simulate this circuit – Schematic created using CircuitLab

In the above schematic the - supply is 'weaker'. If the R1 is changed from 2K to 1 ohm you'll see the minus supply change polarity and become positive to almost the same voltage at the + supply if you don't put D4 in there. With D4 the reverse voltage is limited by the diode. You can run the simulations and play with the values.

The 9V battery plus diode represents a floating positive regulator that cannot sink current (as most cannot). D3 and D4 are my recommended diodes (but use a 1N4004 or 1N5404 rather than a 1N4148). R1, R2 and R3 are the load resistances.

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  • \$\begingroup\$ would you like to weigh into the "2 positive supplies do not allow a + and - supply" discussion? \$\endgroup\$ – Russell McMahon Dec 24 '14 at 14:18
  • \$\begingroup\$ That's why I asked - disagreement is fine if I'm wrong or what I said is wrong or misleading. If the two windings are independent as proposed then the net affect "may" work. Voltages are interesting. \$\endgroup\$ – Russell McMahon Dec 24 '14 at 20:37
  • \$\begingroup\$ Okay, I added my comment in the first answer and a schematic in my answer. \$\endgroup\$ – Spehro Pefhany Dec 24 '14 at 21:14
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    \$\begingroup\$ Awesome, thank you for the protection diodes. I added these into my LTspice simulation and everything seems to work fine. \$\endgroup\$ – merkman Dec 24 '14 at 22:43

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