Hi,

I want to make a circuit that will allow me to switch between output voltages of 3.3v, 5v and 12v dc that can handle up to 10amps. It's for a Lab bench ATX power supply with a voltage and ammeter display. I only have room for one display but I want to be able to monitor the volts and amps of each output when switched.

The type of rotary switch that I need can only handle up to a few hundred milliamps and I don't want to use the bulky industrial rotary switches.

Can I do this with transistors while still getting an accurate voltage and amps reading on the display?

Cheers,
Mike.

 

You could connect the ammeter between the ATX 0V and your negative output connector. This will work for all three of the outputs and will not need switching.

That just leaves switching the appropriate ATX output to your positive output connector. You could do this with P-channel MOSFETs, sources to each ATX output, drains to your positive output. Put a 1k resistor between each MOSFET gate and source then your switch would connect the selected output by grounding the appropriate MOSFET gate.

 

If you put a current shunt in the positive of each rail (The negative is common anyway.) you do not need a switch to carry 10 amps. The switch only needs to carry the current taken by the meter.

Les.

 

You could do it with push-button selector switching with a microprocessor and transistor switching. I was going to say a rotary indexer but push-button would be far less prone to selection errors.

 

You could connect the ammeter between the ATX 0V and your negative output connector. This will work for all three of the outputs and will not need switching.

That just leaves switching the appropriate ATX output to your positive output connector. You could do this with P-channel MOSFETs, sources to each ATX output, drains to your positive output. Put a 1k resistor between each MOSFET gate and source then your switch would connect the selected output by grounding the appropriate MOSFET gate.

Thanks for all your replies.

@AlbertHall this is how I was planning on doing it but wasn’t sure if the MOSFET would give lower readings due to voltage/current drain.

Thanks,
Mike.

 

There will be some voltage drop across the MOSFET but with good choice it should be very low.

 

I guess it all depends on how we choose to define:

and I don't want to use the bulky industrial rotary switches

Grayhill and other manufacturers make all sorts of rotary switches for 10 ~ 15 Amps likely about a 2" diameter. Just a matter of looking at data sheets to find whatever fits your needs. No shortage of switches off the boat from China either and low priced.

Another option is as mentioned the use of some MOSFETs and in that case you want a low RDS (on resistance) to reduce the voltage drop and I would look at logic level MOSFETs. You can use a small low current rotary switch to select the voltage you want or get more elaborate. Just as long as whatever you use is break before make in the design.

Ron

 

You could also use the rotary switch to activate the coils of some 10 amp relays.

 

You could also use the rotary switch to activate the coils of some 10 amp relays.

Since you have 12 VDC I would think about 3 Each automotive relays with sockets or just use 1/4" spade lug connections available at any automotive parts store. Automotive relays typically can handle 30 amps DC @ 12 Volts and have 12 VDC low current coils.

Ron