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 > SPDT switch > 20A @ 32VDC?

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brulaz

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Posted: 01/14/18 06:10am Link  |  Quote  |  Print  |  Notify Moderator

I'm seeing SPDT toggles that are 20A at 125VAC Ebay 20A 125VACand some that are simultaneously rated to 21A at 14VDC DelCity 20A 125VAC

But I want to switch a max of ~700W at ~30A, so maybe 23A?

What about doubling up on the contacts of DPDT to act like a SPDT?

And what about a "Motor Rated" DPDT toggle that's 20A at 277VAC (all the others are 10-15A at 277VAC)? DelCity Motor Rated toggle

Actually, doesn't have to be a toggle, only SPDT and cheaper and smaller than the big HD marine engine switches.

Thanks


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enblethen

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Posted: 01/14/18 07:11am Link  |  Quote  |  Print  |  Notify Moderator

I would not switch that heavy of load. I would install a relay rated for 30 amps min., then install a switch to operate the relay.
You show wanting to switch a 700 watt load, at 12 volts that is nearly 60 amps.
That would require a relay of 100 amps to be on safe side. A relay like that would be one that lookslike the old Ford starter relays, but rated for continuous duty.


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DrewE

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Posted: 01/14/18 07:21am Link  |  Quote  |  Print  |  Notify Moderator

I think maybe you'd be well served to look into solid state relays for this sort of thing.

An example (that looks potentially suitable--no idea if this is the best brand or price out there).

Trying to increase the current capability of a switch by paralleling contacts is probably not a good idea. It's reasonable to expect that one of the contacts would generally switch a fraction of a second before the other, and so the slower switching one would end up breaking the full current anyhow. (It is also not recommended to assume that AC ratings are appropriate as DC ratings, particularly at higher voltage and current levels, as you apparently are already aware. Breaking high DC currents is a lot harder on a switch than breaking the same AC currents since with a DC load there are no momentary zero current crossings where any arc that may have started will break itself down.)





theoldwizard1

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Posted: 01/14/18 08:30am Link  |  Quote  |  Print  |  Notify Moderator

Amps is amps. The voltage does not matter much unless you are taking about over 250V. A relay would be safer.

GordonThree

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Posted: 01/14/18 08:46am Link  |  Quote  |  Print  |  Notify Moderator

Use a Cooper Bussman switching DC breaker. Smallest would probably be 40 amp?

Switching breaker has contacts better designed to withstand repeatedly breaking under load.


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enblethen

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Posted: 01/14/18 09:05am Link  |  Quote  |  Print  |  Notify Moderator

OP:
would you most what you are trying to power? Please give make and model.

brulaz

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Posted: 01/14/18 09:08am Link  |  Quote  |  Print  |  Notify Moderator

I have an extra DC breaker but it's not double throw.
A relay sounds like a better idea.
But one with double throw and centre off?
Could use the DC breaker to provide the Off function I suppose.

* This post was edited 01/14/18 09:18am by brulaz *

brulaz

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Posted: 01/14/18 09:17am Link  |  Quote  |  Print  |  Notify Moderator

enblethen wrote:

OP:
would you most what you are trying to power? Please give make and model.


As inputs:
- Meanwell RSP500-27 Power Supply (have seen it go up to 700W out) at 32.5V on one leg of the SPDT switch.
- 340W of Solar panel Voc=47V, Isc=10A on the other leg.


Centre output tap of switch will go to a solar battery charger. This is just in prep for an experiment; dunno if the Meanwell will work as input to the solar charger. If not, I won't need the switch.

2112

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Posted: 01/14/18 09:19am Link  |  Quote  |  Print  |  Notify Moderator

Quote:

What about doubling up on the contacts of DPDT to act like a SPDT?
At higher current one contact will arc before the other degrading the switch.

The voltage rating is for isolation. It's the maximum voltage that can be applied to the switch. A 20A 250V switch works just as well as a 40V switch at 12V. The contact resistance most likely will be different.

A power supply set to the proper voltage can directly replace solar panels.


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GordonThree

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Posted: 01/14/18 09:22am Link  |  Quote  |  Print  |  Notify Moderator

brulaz wrote:

I have an extra DC breaker but it's not double throw.
A relay sounds like a better idea.
But one with double throw and centre off?
Could use the DC breaker to provide the Off function I suppose.


Normal DC breakers aren't designed for switching loads, but if you're only planning once in a blue moon maybe not a problem.

Why double pole? No need to switch ground if that's the case.

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