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I need to route DC power from a single rack power supply (Keysight N5766A) to six different loads, with the capability of switching between them independently and unshorted (break-before-make) via a manual switch, preferably a panel-mounted rotary switch. The switching should not allow more than a single load at a time to be powered from the supply.

The project engineer tells me the expected current injection won't exceed 2A at 1V for any of the loads, but I'm a big fan of over-engineering and want to idiot-proof the circuit, preferably to beyond the supply's stated output of 40V/38A. I'm assuming I will need to route the source to relays - nobody makes a 2 pole / 6 position DC rotary switch that will handle much more than the circuit board milliamp range - so I'm wondering how to accomplish this with as simple (and safe) a circuit as possible.

Another option would be to downgrade this particular supply to something limited to the expected draw but we already have one of these on hand, so...


Addendum 09-06-23: The plan is to use six SPST relays controlled by a rotary switch to route the source supply to the loads, so the rotary switch only has to deal with relay control-level power and I still have a failsafe to keep any one load from being energized at once.

The trouble is that the users say they only plan to run around 1VDC at 2A max, and as mentioned above the source supply we have to use is rated at a comparatively-ginormous 40V/38A Max - so it's proving nearly impossible to find a DC relay with that span of power capacity. I'm assuming whatever relay I select will need to be capable of inadvertent overpower conditions up to those maximums, but there's a big gulf between </= 2A and 38A max. Will we be forced to spring for a smaller source supply, or is it overkill to assume someone may be fool enough to crank the supply power to max?


Addendum 09-11-23: Thanks Peter and mbrig. By "span" I mean the difference between the desired test power of 2W (1VDC at 2A) and the supply's rated max of 1520W (40VDC at 38A.) This will only be switched on once a week for a brief test data capture. I think the solution is to just pick relays that are within a decent range at the low end and install a 5A breaker between the supply and the positive bus for the relays (they said 5A would be the max capacity without frying their samples.) I'm thinking the control source will be 24V, likely a simple Omron-type supply. Thanks again for your advice - we can probably call this question closed.

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  • \$\begingroup\$ Ohmite makes "Power Tap Switches" with ratings up to 100 Amps - but you may have to mortgage your house... \$\endgroup\$ Commented Aug 24, 2023 at 1:02
  • \$\begingroup\$ Confused by what exactly you mean when you say "span of power capacity". A 40A 200V relay will happily also let you pass 2A. You just can't use the 1V as also being the control signal; source 12V (or 5, or 24, depending on the relays you choose) from a wall wart, put that through your rotary switch and use it control the relays. \$\endgroup\$
    – mbrig
    Commented Sep 6, 2023 at 20:21

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Use 6 suitably sized high-side N-FETs, with a high-side charge pump controller to generate the switching voltage. Pull all of the gates low with resistors, then use a break-before-make rotary switch to route the gate drive to the desired channel.

I've used the Analog LTC7000A part in the past with good results, but all the major suppliers have parts that might suit.

There are also versions which include over-current control via a low resistance current shunt, either with limiting or power-off (with or without latching). You can also get enable pins which would give you an easy logic level on-off switch.

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