How do I create a switched power supply for a 12v circuit?

AnalogKid

Joined Aug 1, 2013
11,146
Diodes or switch - neither one regulates the amount of energy passing to the load; they only control which source is providing it.

For a normal electronic device or circuit, the device requires a certain voltage to operate correctly. When connected to a power source of that voltage, it draws current from the source. The anound of current it draws is controlled by the parameters of the device, *not* the power source. This is why a car battery can run both the radio (1 amp) and the starter motor (50 amps) with straight wire connections. Power source > on/off switch > load.

With two power sources, there are several options for connecting them to a single, common load device. these have been discussed above. Without getting into fuses, fuseble resistors, current limiting circuits, etc., it is pretty straightforward. You pick which source is connected to the load, and the load decides how much current it will draw from that source.

In this scenario, you can overload the circuit by increasing the source voltage to an unsafe value. For example, if you connect a 12 V pump motor to 100 V, the motor probably will overspeed and burn up. But as long as the power supply, battery, and load device all are specified for the same voltage, there should be no problems as you switch between the two sources.

ak
 

bassbindevil

Joined Jan 23, 2014
836
Schottky diodes have a lower voltage drop, and can be salvaged from scrapped switching power supplies. Sometimes the part number includes an S, or if you have a multimeter with a diode test function, you can guess the diode type from the voltage drop measurement.
 

AnalogKid

Joined Aug 1, 2013
11,146
The best option, given the OP lack of confidence, would be a switched barrel power jack.
I haven't re-read the entire thread, but I don't recall a requirement that the switching between power sources be automatic and/or prioritized. The original question was about there being too many wires for a SPDT electromechanical switch, and his lack of understanding what to do with them. Once that was cleared up with a diagram in post #17, the rest is just us spinning our wheels about diodes, connectors, etc.

ak
 

K OBrien

Joined Nov 28, 2020
14
Here's a link to a double pole double throw switch you can connect with six wires at lowes. Lowes switch. It has a ten amp limit and if you want you can mount it in a handy box with a metal cover that has a slotted hole in it. The center terminals go to your load.
 
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prepka

Joined Oct 5, 2020
30
I have a unit that is powered by a 12v battery, but I can also power it with a 12v mains unit. To switch between them I have to physically unplug one from the unit and plug in the other.

But I'm wondering about having both the battery and mains adaptor permanently connected and installing a switch in order to switch between them. Is it a SPDT switch I need? Only I can't find one suitable.

I'd have six cables: +/- of the battery and +/- of the adaptor being switched to feed the +/- of the unit.
Best to use a DPDT so that both sources isolated from each other otherwise could be unsafe.
 

LowQCab

Joined Nov 6, 2012
4,216
It could only be "un-safe" if the Diodes are extremely under rated and BOTH OF THEM Short-Out at the same time.
If there are no Diode failures, it is 100% "safe".

Prepka quote "" Not sure of the amperage, but it'll be very small. ""
In that case, standard 5-Amp Diodes would be complete over-kill, and have almost zero chance of failure.
 
I have a unit that is powered by a 12v battery, but I can also power it with a 12v mains unit. To switch between them I have to physically unplug one from the unit and plug in the other.

But I'm wondering about having both the battery and mains adaptor permanently connected and installing a switch in order to switch between them. Is it a SPDT switch I need? Only I can't find one suitable.

I'd have six cables: +/- of the battery and +/- of the adaptor being switched to feed the +/- of the unit.
A simple trip to your local boat dealer and $25 dollars will fix you right up. Just ask for a 2 battery disconnect switch. When switch, you want the sweep or contacts to break, then re-connect. You have two sources of voltage using an unknown current. The battery switch will break contact from #1 before connection to #2 if you desire in parallel circuit. The marine battery will handle 800+ amps thru copper contacts on back.
(Suggest covering with dielectric grease to avoid oxidation and less arcing) Battery #1 to battery #2, both or both off. When switched to both, if wired in parallel, you still get 12.8 vDC but at twice the available amperage. A small $3 switch with only handle 10 amps, so to be safe and provide longevity to your project, you may have to pony up the world most common dual battery switch. They are also make for boats with 3 batteries, used to start port engine, starboard engine and house battery. How to charge them is battery voltage (12.8) X 20% and terminate at the B+ end of your device.
I was a service manager at a dealer for 3 years with boats up to 34’ in stock in my ASE Master Tech certification since 1978. It does not really expire but it’s suggested to take all 8 test every 5 years. I’ve gone back 7 times but taken advanced parts (P2) and advance emissions (L1) which is 100 questions. Missed one!
 
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