SPDT with high amps

Question

I have 3 DC power supplies that can each supply 320 W. Input to these power supplies comes from AC wall power and is gated by an on / off switch.

I have 2 different loads:

• DC 35 V, 22 A
• DC 35 V, 17 A

I only want to power one of the loads at a time.

I was planning to use a SPDT On-On switch between the power supplies and the loads, but from my searches it's hard to find ones rated for DC 35 V, 25 A. I suppose I could use 3 SPDT switches, one for each power supply, but I would like to keep it to one switch.

Is there a smarter, simple way to accomplish powering one of the loads at a time? Or should I try to find a SPDT switch rated for my requirements?

Thank you.

Answer 1

The specifications for the switch will list the maximum voltage and current, make or break, and voltage type AC, DC, or AC/DC. You want a switch with a DC rating that meets or exceeds your requirements. One of those requirements is Break Before Make, which means you do not want to bridge two power supplies through a switch. It's an excellent way to destroy them unless you do something like Master/Slave.

They make some power supplies, allowing you to parallel units to increase current capacity. It has several market names, but it is a Master-Slave topology. You designate a Master unit and use its voltage regulator to control the Slaves' voltage regulators. Otherwise, you will need a Break before Make switch.

Another good reason to use Break before Make is that DC does not have a zero cross-over voltage. A DC switch must be able to extinguish any arc likely to be encountered. It has to be fast enough and have a significant throw distance. Conversely, AC has zero cross-over voltage, so any arc is extinguished within half a cycle of 1/120th of a second.

Answer 2

The voltage rating is the maximum safe voltage. A switch (or relay) rated to handle 220 volts, such as this 32 ampere switch [shown as an example], can certainly handle just 25 A at 35 V.

However, in your comment, there appear to be an issue: "I have 3 DC power supplies that can each supply 320W of DC power." Many power supplies, and, certainly almost all voltage regulated supplies, cannot safely run in parallel.

Consider that should one supply output 35.0 v, another 35.1 V, and the third 35.2 V, well within normal tolerance,

• One supply would be attempting to feed current into the other.

• The load would be entirely on the 35.2 V supply -- the 35.0 and 35.1 V supplies would not even turn on.

  Some method would be needed to equalize current draw, such three series resistors, and perhaps hefty diodes

If they are ~12 V supplies in series, and all have the same current rating, and all are isolated from each other and from ground, it might work -- if the maximum isolation voltage is not exceeded. If current ratings are not identical, again, one supply would be overrated.

As described, expect heat and smoke, rather than usable power.

Answer 3

Would 10 volts really matter. you can step down the coil with a resister. Amazon has DG85B-8011-96-1024-M1DR AUTOMOTIVE RELAY, SPDT, 24VDC, 60A