Author Topic: 24 volt switch (Read 6505 times)

(unknown) · « **on:** July 12, 2006, 05:27:37 PM »

We've got an off grid vacation cabin (soon to be a retirement house) where I've just installed some 24volt ceiling fans. I thought finding 24 volt DC rated wall switches would be easy but.....

Could anyone refer me to somewhere I could buy a 24 volt DC wall switch that looks like a normal wall switch that would match the 120 volt inverter powered switches. (I don't want to use an automotive toggle switch) I don't need or want a speed control switch.

Thanks

Ungrounded Lightning Rod · « **Reply #1 on:** July 12, 2006, 11:40:09 AM »

As I understand it (and I trust someone will correct me if I'm wrong):

Just as a switch rated for a high current can generally be used with a lower one, a switch rated for a higher voltage can be used with a lower one. Only current, maximum voltage, and sometimes AC vs. DC, matter.

Non-electronic wall switches are generally suitable for both AC and DC. (Non-mercury types will wear out faster with DC, since the arcs tend to move the metal the same way every time they're opened, rather than randomly. But that's not usually an issue since the lifetime is decades to centuries. If you're worried, switch the wires every decade or so.)

If a switch doesn't say AC on its label it's suitable for DC.

This does NOT apply to circuit breakers, since extinguishing an AC arc is easier. (It does it for you 100 or 120 times per second and you only have to keep it from reigniting on the next half-cycle). You need to get one rated for DC. Fuses are generally rated for both AC and DC, and again it's just current and max votage that matter. (But check the labels on the giant ones.)

Ungrounded Lightning Rod · « **Reply #2 on:** July 12, 2006, 11:45:56 AM »

Just checked some articles on switches and noted that the current breaking rating of a switch may drop by a factor of nearly ten when used for DC.

And I can't dig further right now since I have to get to work, so please ignore the previous post for now.

Flux · « **Reply #3 on:** July 12, 2006, 11:57:42 AM »

In the early days switches were made with a quick action like toggle switches and could be used for ac or dc.

The present slow break ones can usually be used at near full rating on 12 or 24vdc with resistive loads such as lamps. For inductive loads there may be a severe derating.

At 48v the chances are that they may be in trouble for resistive load and will not break inductive load.

Up to 24v with normal loads they will usually be ok but you may have to derate a bit for fans and small motors. With true inductive load without freewheeling diodes the rating will be tiny.

If you know how to do it, a diode across the load is worth the effort but remember you must never reverse the polarity.

Flux

(unknown) · « **Reply #4 on:** July 12, 2006, 12:07:09 PM »

The old style 'click' switches were/are what I'm looking for. No longer made anywhere?

So what would be the result if I used a normal switch for the fans and it failed? Short? Fire?

I don't know anything about diodes so that's probably out.

Thanks

RogerAS · « **Reply #5 on:** July 12, 2006, 12:17:13 PM »

Gray Wolf,

I have no experience with 24V switches but I can speak to 12V which should work for your needs. I use standard 110V wall switches with slight modification. I place an overrtated blocking diode inline and upstream (before the switch) of the load on the positive side to stop back emf. I then place a capacitor, a big uf (3000+)rating well above the anticipated voltage, across the terminals of the switch itself. The capictor absorbs the inrush current and softens the shock to the switch terminals. This is very similar to what was used in old breaker point systems in cars and trucks. It is not going to completely stop arcing, but should help. In my case I do not run motors or relays this way, just lights. At $1 each for switches and salvaging the other parts one can afford to burn one out ever couple years.

I haven't made up one of these in a year or two, and I don't have one handy to confirm the arrangement, I'm not at home right now, so if I've got this backward someone correct me.

RogerAS

nanotech · « **Reply #6 on:** July 12, 2006, 12:24:43 PM »

I have a couple (at least) of the old style "click" switches if you want them.

Drop me an e-mail to darrin.moore(at)gmail.com

You can have them for the postage.

dinges · « **Reply #7 on:** July 12, 2006, 12:27:21 PM »

A while ago there was a very good discussion on the subject of using AC switches for (low voltage) DC applications:

http://www.fieldlines.com/story/2006/4/27/22822/7819

There's more to it than just 'it's only 12VDC, so I should be able to use a 120VAC switch'. (why else would we need all those electrical engineers anyway )

Peter.

dastardlydan · « **Reply #8 on:** July 12, 2006, 01:04:23 PM »

Don't know where you are, ( live ) but most marine

boats that is ones with genrators

are 24 or 48 > and so no.

So look for marine supplyer.

_______________

jimjjnn · « **Reply #9 on:** July 12, 2006, 03:02:08 PM »

When I was a kid(long long long time ago) I lived in a dormitory that had DC power only. The switches were made of ceramic and twisted very hard to open and close the circuit. When we went to both ac and DC , the ac switches were easier to turn off and on. When turning off DC power we could always see an arc as the switch opened up. We also had many small burn marks on the wall when a switch arced bad enough. AC switches dont hold up well with DC and are rated at lower amps per switch.

jimjjnn · « **Reply #10 on:** July 12, 2006, 03:04:55 PM »

BTW, It was 120 volt DC too. Smarts a lot if you stick a coat hanger in a light socket

jimjjnn · « **Reply #11 on:** July 12, 2006, 03:08:27 PM »

How about aircraft switches. Would they work. Aircraft junk yards near airports may have them. Probably charge an arm, leg, and 1st born tho.

dinges · « **Reply #12 on:** July 12, 2006, 03:12:15 PM »

Whether they'd work or not depends on what they're rated for. It should be marked on the switch.

It is my understanding that many A/C electrical systems use 28VAC, at 400Hz...

Peter.

Ungrounded Lightning Rod · « **Reply #13 on:** July 12, 2006, 05:34:01 PM »

I place an overrtated blocking diode inline and upstream (before the switch) of the load on the positive side to stop back emf. I then place a capacitor, a big uf (3000+)rating well above the anticipated voltage, across the terminals of the switch itself.

I'd be inclined to just put a freewheeling diode from the load side of the switch to the return. That will keep inductive kick from raising the voltage across the opening switch to maintain the arc. (Downside is that it will short the line if the battery polarity is ever accidentally reversed.)

Ungrounded Lightning Rod · « **Reply #14 on:** July 12, 2006, 05:37:21 PM »

On second thought: I'd be inclined to look at the electrical code and find out what the approved thing to do is. Switches rated for DC house service, UL approved, should still be available, and at reasonable prices.

That way there's no question of your insurance refusing to pay off if there's a fire because you did something outside of code. B-)

ghurd · « **Reply #15 on:** July 13, 2006, 07:18:05 AM »

Look at the big-box stores for the single, boxed, commercial, loud-clicking, $4 switch.

Maybe 3 years ago they had them and they were rated for DC. It said it on the switch itself, but not the box. I figure it was Leviton or Eagle brand.

I would still use a freewheel diode with a motor.

G-

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