# What happens if you connect 2 isolated power supplies backwards?

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
What would have happened if you connect 2 isolated power supplies and connected them to work agains each other? + to - and - to +

Let say one was 3v and the other 5v both same current, would connecting them to work against each other only yield a 2v power supply? Seem weird. And would they damage each other? Or that depends on the circuit

Wanted to try this with two little battery powered buck converters just to see what happens but figured its easier to ask than damage stuff just to find out

#### AnalogKid

Jun 10, 2015
2,688
It will damage stuff.

#### ChosunOne

Jun 20, 2010
465
Depending on the kind of batteries and size you use, it will likely damage stuff outside the circuit, including the immediate environment and the experimenter.

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
Depending on the kind of batteries and size you use, it will likely damage stuff outside the circuit, including the immediate environment and the experimenter.

Lol I forgot to mention I was going to try this at very low current 20-30ma, still the question remains would the multimeter say 2v between the 2v circuits. What would the voltage be across the red circle.

PS if the red circle was a incandescent bulb what voltage would it see all other issues aside

Last edited:

#### duke37

Jan 9, 2011
5,364
The power supplies are in series so there is 8V to play with.
You can get the voltage across the bulb if you know its resistance and the value of the resistors.
The 5V psu will try to pull excessive current out of the 3V psu.
Connecting one psu the other way round, will push current back into the 3V psu. It will almost certainly object to this. Some circuits are shown with a diode added across the regulator to take this into account.

#### Colin Mitchell

Aug 31, 2014
1,416
No problems connecting two supplies backwards - but in series. I recommend it all the time. If one is 1amp and the other 500mA, you can only draw up to 500mA. And the voltage will be the difference between the two.
The same applies when connected in series. The max current is 500mA and the voltage will be the addition of the two.
You can also connect them in parallel as the only supply that will deliver anything is the one with the higher voltage.
If you connect them in parallel around the wrong way, (as shown in the diagram above) only the supply with the higher voltage will deliver a voltage and it will charge the electrolytic in the other supply in the reverse direction and very soon it will get hot and explode.

#### BobK

Jan 5, 2010
7,682
Connect the + of the 3V supply to the minus of the 5V supply. You now have two supplies in series making an 8V supply.

Now connect the other two terminals and you have shorted the 8V supply.

If the two supplies are high current lithium ion batteries, you have created an incindiary device.

Bob

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