DC Switch ratings

Dear All

Is there a rule of thumb for calculating the maximum DC current of a switch for any given DC voltage?

Thanks and regards

Trevor

No, you just need to consult the manufacturers ratings. Essentially there are two problems, one is the current handling capacity of the switch - too much current through the contacts will make them overheat, leading to failure of the switch. Secondly is the breaking capacity of the switch, as the contacts open current will arc across the gap, burning the contacts - too much current and the arc won't stop - this is why switches generally have a much higher AC rating than DC.

This isn't intended as a wise-crack. As a rule (the rule-of-thumb) the switch manufacturer will specify the voltage and current ratings. You need to stay within all of the manufacturer's parameters in order to expect the performance they claim.

Think about some of the issues related to current. One might first think of the contact surfaces but you also need to consider everything including the connection points into and out of the switch. Current flow results in heating and one limiting factor will be how much heat each component of the switch can tolerate. Manufacturers are likely to use different designs and materials for switches that are similarly. That suggests that it's unlikely you can predict what the average switch will do under overload conditions.

Voltage is something that affects the arc that results when the switch is opened under load. It also affects things like materials selection in terms of insulating properties. Separation between conductors is also a voltage issue. It's an oversimplification but the variations among manufacturers would make it difficult to extrapolate outside of manufacturer's limits.

DC ratings

Nigel, Steve

Thanks for the replies. Sorry, I don't think I made myself clear being a numpty with the electrickery most of the time.

If I know the AC rating, is there any way to decide the DC capacity if the manufacturer doesn't state it? Most switches seem to state only the AC rating.

For example, I have a miniature toggle switch in front of me right now with "3A 125VAC, 1A 250VAC" on it and another full-size one with "3A 125VAC, 1.5A 250VAC". The specs do not give any info about DC.

How does one decide if a switch is ok for any given DC application?

Sorry again for not being clear.

Trevor

Trevor Rymell said:

How does one decide if a switch is ok for any given DC application?

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If and only if the DC rating is marked clearly on the switch body.

There is no way to determine what happens if the DC current and voltage is more than 100mA and over a few volts.

That's as high as one can comfortably use for any switch on DC, if one really cares.

Trevor - if I were in your position and the intended use was a non-critical application - I might use a switch rated for 1 amp AC on a 12 or 24 VDC application of an amp or two but only if the consequences of failure were no worse than an inconvenience. Many of us try to be careful with responses because we don't know the application and quite often people who ask for help are looking for what might be considered good, safe, professional practice.

Trevor Rymell said:

Nigel, Steve

Thanks for the replies. Sorry, I don't think I made myself clear being a numpty with the electrickery most of the time.

If I know the AC rating, is there any way to decide the DC capacity if the manufacturer doesn't state it? Most switches seem to state only the AC rating.

For example, I have a miniature toggle switch in front of me right now with "3A 125VAC, 1A 250VAC" on it and another full-size one with "3A 125VAC, 1.5A 250VAC". The specs do not give any info about DC.

How does one decide if a switch is ok for any given DC application?

Sorry again for not being clear.

Trevor

Click to expand...

Normally a switch's contact current rating will be much lower for DC then AC current. This is because the AC voltage passes through 'zero' voltage 60 or 50 times a second and helps 'break' the spark gap as the switch contacts open. This doesn't happen in DC so the contacts are subjected to much more heat as the contacts open a DC circuit. This heat can really damage switch contacts quickly. I've seen many relays with their switch contacts welded closed due to having higher current switched then they were rated for.


If you look at switch ratings in on-line catologs you will see the difference in AC and DC contact current ratings for switches that do have their AC and DC current specs published, let that be a guide for you to 'derate' the current for any switch you have that doesn't have a DC current spec.

Lefty

I would suggest you derate by a factor of five or so! - if the manufacturer isn't even giving a DC value, it's not a good sign!.

Nigel Goodwin said:

I would suggest you derate by a factor of five or so! - if the manufacturer isn't even giving a DC value, it's not a good sign!.

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Yep, /5 would be a good starting point

Nigel Goodwin said:

I would suggest you derate by a factor of five or so! - if the manufacturer isn't even giving a DC value, it's not a good sign!.

Click to expand...

If you can't reach a derating factor of five, an alternative may be to derate (by a smaller factor) both current and voltage specs.


Also, the load that you are switching affects the "nominal DC current" of the switch: it is easier to turn off a resistive load (ie lamps) than an inductive one.

Also choose a switch that opens quickly: DC (and high voltage) switches, when you move the lever (toggle or whatever you move to open it) accumulate energy in a spring and then suddently open. The contact opening speed is independent of the speed at which you move the lever.

"AC only" switches may open slowly, as the zero crossing tends to open the arc.

Remember when we still had so-called "points" in the distributors of our cars? A good way to "divert" the arc from the contacts, was to use a capacitor (also called condenser) in parallel with the contacts. This, plus the length of connecting wire, was equal to a resistor and capacitor in series and is called a snubber network. With a little experimentation, one can have a fair amount of success by just connecting a series combination of a small capacitor (say 0,1mfd) and a resistor (say 47R or 100R) across the switch contact.

It also depends on whether the load is inductive or not.

I've drawn some pretty big sparks from a high current power supply and a large iron wire wound reostat.

I would imagine a swich rated for 240VAC at 10A would burn out very quickly if you used it to control a 10A 48V motor.

No problem

If and only if the DC rating is marked clearly on the switch body.

There is no way to determine what happens if the DC current and voltage is more than 100mA and over a few volts.

That's as high as one can comfortably use for any switch on DC, if one really cares.

Click to expand...

I've used 220/110 rated panel switches with no DC rating for fog lights, electric radiator fans, driving lights, back up lights for years and never had a problem.