Two power sources SOLVED

We are building a radio box to store 10 Midland FRMS radios in their chargers. (5 charging units)

The chargers accept a 12v power source either from an AC wall adapter or a 12v dc power source. A cigarette lighter plug for vehicles come with each charger as does a wall adapter.

We plan on wiring all the chargers to a single bus in the box. The bus strip will then be wired with a cigarette lighter vehicle plug so all chargers get powered.

We also want to be able to take the box out of the vehicle and bring it inside and plug it into an AC source. To do this we planned on installing an AC to 12v DC converter into the box. The 12v side would also be wired to the bus strip.

My question is, if the box is plugged into the vehicle, do we need to protect the converter from the 12v DC now coming into the converter from the 12v side? Do we need to insert a diode to prevent that?

Thanks for your help!!

by Tillerman
January 01, 2017

This is the converter I'm planning on using.

by Tillerman
January 02, 2017

1 Answer

Answer by mrobbins

Without knowing more about the power supply it's hard to say for sure, but it's a reasonable guess that you should protect the switching power supply from being fed in reverse from the car.

Here are a few ideas:

(1) Add an SPDT toggle switch to the box so that power can only come from one side or the other at any given time, for example:

(2) Add a cigarette lighter plug on your bus bar, and add a cigarette lighter receptacle to your AC-DC converter. Then you can physically plug the bus bar into one source or the other as needed.

(3) Adding a diode is a possibility, but may be a pain -- consider that if you're pulling 10A through it and there's a 1V voltage drop across the diode, now the diode has to dissipate 10W, which is significant for a small component and would require real thinking about heatsinking and power dissipation.

(4) A more modern solution would be to use MOSFETs as the switching elements, instead of a diode, because they'll waste far less energy. However now you have significant complexity in controlling the MOSFET. See these articles: 1 2 3 4

I'd recommend #1 or #2 for simplicity. If your operating current isn't too high, you can go route #3 but there will be a lot of heat dissipation in that diode which you have to get rid of.

Also note that if you're taking what was a 1-cigarette lighter appliance and putting 5 of them together into a single plug, you may exceed the current limit provided by your car's cigarette lighter socket fuse -- take a look at the fuse size and have some backups on hand!

ACCEPTED +1 vote
by mrobbins
January 05, 2017

Thank you soooo much! I think option 1 is also the easiest and most reliable solution. I think I'll go with a three position switch, one side to the 12V DC from the vehicle, the other side to the 12V from the AC converter and the middle position would be the off position.

Thanks again!!

by Tillerman
January 05, 2017

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