Help me decode a rocker switch spec

I need a high amperage DPDT switch to add forward/reverse to a 24vdc motor. 30A rating would be OK assuming that the switch could cope with an occasional short burst higher than that.

There’s a particular switch which seems to be common on ebay and elsewhere which is persistently described as being “250V 15/30A”.

My understanding is that if the switch can cope with 250VAC @ 30A it would handle 24vdc @ 30A (although if that’s wrong I’d be grateful to know). However, clearly there is a major difference between 15 and 30A but for the life of me I can’t find any spec for this (and similar) switches saying when it is 15A and when it is 30A.

Lots of switches of this sort will give the rated amperage for 250/125V AC respectively and maybe that’s what is going on here. If so it’s unstated.

I asked an ebay seller but he didn’t know. Anyone know?

DC specs for switches tend to include a lower current rating. This is due to the fact that the arcing between the contacts during make or break lasts much longer for DC versus AC. With AC, you have zero crossings, and the arc usually extinguishs itself at the first or second zero crossing. DC doesn’t have zero crossings.

So do you think the 15A might be for DC?

No. I am guessing it’s rated for 15 A @ 250 VAC and 30 A @ 125 VAC. It may not even have a DC spec.

If I were you I would use a DPDT switch that has DC specs for the current you need. For better reliability,

  1. Use a switch with DC current specs that exceed the continuous current of the motor. If, for example, the motor draws 30 ADC, use a switch that is spec’d for 40 ADC or 50 ADC.

  2. Install some RC snubber circuits across the switch contacts.

  3. Use a relay (a.k.a. a contactor) with DPDT contacts. The coil of the relay is controlled using any ol’ switch.

Yeah it seems like this is going to be the most economical solution. Weirdly enough getting a switch rated for 30A at 24VDC is expensive (I haven’t found one in Aust. for less than $30 so far) but I can get a relay rated to that for about $5!

Look for switches designed for controlling winches on ATVs or power jacks on RVs. Most of these are momentary with a center off, which may be useful for your application - the momentary center off means that the motor will only run as long as you press the switch and it stops when you let go.

No, the 30 Amp rating is for the absolute peak lasting a few milliseconds. You can test it and it will work, but it won’t last many turn on’s. It burns the contacts.

Anyway, its sure DC rating of the AC switch is far lower, The DC amps rating is far lower than 30 so its completely wrong.

You could use a larger switch, such as a the big arm thing you see in labs and Frankenstein movies, or a smaller version of the big switches attached to electrical distribution poles.

But to keep it small and neat and reliable, perhaps a DPDT relay does the job ? They last longer as they throw the electrodes far quicker … the higher speed protecting them from burning off their tips too fast.

This won’t be as big a problem as you’d think because there will be no current flowing at switchover. The “throttle” (well, speed controller) will be off when the switch is thrown, and large currents will only be passing through the switch while it is set in one position or the other.

The term for the frankenstein style switch is “knife switch” if you need one. They tend to be the most economical high current switches.

As long as we’re guessing, I’d go with the 15A is the 125VAC and the 30A is 250VAC. (In the US) 120 VAC is the hot line and the other line of the DPDT is neutral. The switch can handle 15A on the hot side (and a similar current in the neutral). For 240VAC, each leg can handle 15A, for a total of 30A.

But, yes, for a 24Volt DC motor, you would need a switch rated for 30Amps at 24VDC. As you have already found out, it is much easier to use a relay controlled by a smaller switch.

Blueseas have a Contura Switch that good for up to 15amps or there is a toggle switch commonly stocked Jaycar or RTM.
To handle a true 30amps plus your looking at some other solution.

I doubt that since it’s not really DPDT if it can only handle the amps by using both poles in parallel (if I’m understanding you correctly).

That’s because the pros do this via the contactor/relay controlled by a low current switch. So there are a lot of contactors/relays, and thus supply & demand brings the cost down. Doing it your original way is pretty unusual, so the part needed is rare (and thus expensive).

I think my original guess was correct. This looks to be the same switch in the DPST configuration. Note the current ratings.

Well, even finding a cheap relay that could cope didn’t prove so very easy because so many have a low current capacity on the NC contacts. But found eventually a JQX-38F that would do the job for A$8 delivered, so that’s alright.

That relay should work fine.

You really need to two switches: one for turning it on/off, and a second one for changing the polarity.

The second switch should simply control the coil of the JQX-38F relay.

For the first switch (power on/off), you can use a big, heavy duty power switch. Or you can use another relay or contactor controlled by any ol’ switch (preferred).

Well as mentioned there will be a speed controller and the reversing will only occur when that is off. My current plan is to use a momentary button that will energise the relay for reversing and I will have an interlock that will prevent the momentary button from energising the relay unless the throttle is off.