Two independent power supplies to create dual-rail power supply

Thread Starter

forummin

Joined Oct 9, 2021
4
Hi there,

I'm pretty new to electronics tinkering, but found a circuit I'd like to try and breadboard that requires an op-amp and that means I pretty much need a source of positive and negative voltage. I don't have a real bench power supply, but I do have:
1) An HP server power (DPS-460BB) supply that I hot-wired that puts out +12 volts (among other voltages)
2) an old terrible Pyramid power supply that puts out +13.68 volts (intended for automotive stuff)

The HP power supply has a three prong power cable, so it's grounded. Through the chassis and everything.
The Pyramid power supply has only a two prong cable, which I take to mean that the ground is floating?

The question is: Is it safe to hook the positive of the Pyramid power supply to the ground of the HP and create a dual-rail power supply of +12 volts (at the positive of the #1) and -13.68 volts (at the negative of #2)? Of course, the rest of the circuit would treat the Ground of #1 and Positive of #2 (they are connected after all) as overall ground of the circuit.

Hope this question is appropriate and I appreciate the help!
 

dl324

Joined Mar 30, 2015
13,506
Welcome to AAC!
The question is: Is it safe to hook the positive of the Pyramid power supply to the ground of the HP and create a dual-rail power supply of +12 volts (at the positive of the #1) and -13.68 volts (at the negative of #2)? Of course, the rest of the circuit would treat the Ground of #1 and Positive of #2 (they are connected after all) as overall ground of the circuit.
Unplug both supplies and do a continuity check from the ground terminal to some metal on the chassis. If the enclosure isn't metal, measure connectivity between ground and the hot and neutral prongs on the power plugs.

If you don't have continuity, they're isolated and you can stack them.

With a single supply, you can still use opamps that expect a bipolar supply. You just create a virtual ground with a voltage divider. Still, it's better to have a bipolar supply.

You can make one with a transformer and half wave rectifiers.
https://forum.allaboutcircuits.com/threads/dual-volt-psu-help-needed.182266/post-1672293
 
Last edited:

Thread Starter

forummin

Joined Oct 9, 2021
4
Hey Dennis, thanks so much for the reply!

Supply #1 is completely grounded: chassis, ground and ground pin on the wall plug
Supply #2 is not: I just checked and there is no continuity between the ground plug on the front and either of the two prongs on the wall plug or the chassis

So it looks like I can move forward! carefully of course...

And thanks for the suggestions. I knew about the voltage divider trick, but didn't know about the half-wave rectifier. Good to know. It even looks like it would work with a non-center-tap transformer. I only have one power transformer and it's not center tapped.
 

djsfantasi

Joined Apr 11, 2010
7,913
Is it an HP power supply for a PC? Most computer power supplies have several voltages. Often, there will be +-12V and +-5V. So you may be all set.
 

Thread Starter

forummin

Joined Oct 9, 2021
4
Is it an HP power supply for a PC? Most computer power supplies have several voltages. Often, there will be +-12V and +-5V. So you may be all set.
Yeah, it's a server power supply and it does have -12... but I've been told that the -12 in the ATX standard is only there for legacy purposes (something to do with Serial port communication) and so it's not really usable for anything that requires more than a little current.
I'm trying to build out a Class AB amplifier circuit and thought it would be better to have two strong rails. :)

Thanks for the suggestion though, and you're totally right.

I was just nervous about trying to hook two power supplies together with one grounded and the other one not grounded (floating?). If they were both grounded, I knew it would NOT be safe and if they were both floating I knew it would probably be ok.
 

DickCappels

Joined Aug 21, 2008
7,955
I use two power supplies often. I usually put diodes across each output so that if short occurs the output cannot be reverse biased by more than a diode drop -the electrolytic capacitors on the output might be protected this way. I have not tried shorting the output without the protection.
 

MrChips

Joined Oct 2, 2009
24,610
1) Using op-amps does not automatically mean that you need dual power supplies. There are circuits and op-amps that work fine on single supply voltage.

2) When the application dictates that dual supply voltages are necessary, there are ways of meeting that requirement with a single supply.

Show us your circuit and then we can advise accordingly.
 

Thread Starter

forummin

Joined Oct 9, 2021
4
I use two power supplies often. I usually put diodes across each output so that if short occurs the output cannot be reverse biased by more than a diode drop -the electrolytic capacitors on the output might be protected this way. I have not tried shorting the output without the protection.
That sounds like a good idea, thanks!
1) Using op-amps does not automatically mean that you need dual power supplies. There are circuits and op-amps that work fine on single supply voltage.

2) When the application dictates that dual supply voltages are necessary, there are ways of meeting that requirement with a single supply.

Show us your circuit and then we can advise accordingly.
Hi, I appreciate the response very much.
This is all mainly for learning, and I'm doing a VERY simple class AB amplifier to power a speaker woofer at low frequency. I believe that the op-amp is used to amplify the signal just enough so that it can open two mosfets (one NPN, one PNP) in push-pull configuration on the output leading to the speaker.
I don't have a schematic, but I'm trying to build this:
https://www.electroboom.com/?p=243
It's not for hi fidelity or anything :)
Just get two 9V batteries, connect them in series (+ to -) and that connection is the ground.

Bob
Thanks for the response! I'm trying to build something with a little more power than 9-volts can provide, I'm afraid. Otherwise, I'd love to just use them.

Just fyi, I went ahead and broke down and got a big SLA 12V battery today... so I'll hook that up to the power supply accordingly to get the negative voltage I need.
After I get this working, I'd love to hear suggestions on how to make it better. I've got some thoughts of my own as well for version 2 :)
 

BobTPH

Joined Jun 5, 2013
4,032
Thanks for the response! I'm trying to build something with a little more power than 9-volts can provide, I'm afraid. Otherwise, I'd love to just use them.
Well, you indicated you needed the dual supply for opamps . Opamps do not use much power.

Bob
 

MrChips

Joined Oct 2, 2009
24,610
You can design audio power amps with single supply or with dual supplies.
The pros and cons are:

1) With single supply you need to drive the loudspeaker via a very large capacitor (1000μF or higher) which will limit the low frequency response. Also this gives rise to the "thump" on power on as the capacitor charges.

2) With dual supplies you can drive the loudspeaker directly and therefore you are able to go down to 0Hz.
The danger here is that if there is any DC imbalance in your amplifiers you run the risk of destroying the loudspeakers.
 
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