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### Topic: Toggle switch, how can I know how much current it does support? (Read 263 times)previous topic - next topic

#### Jay98

##### Sep 16, 2017, 01:29 am
Its current rating is only expressed in AC, is there a way I can calculate DC current rating with that info?

With my poor electronic knowledge I tried calculating R by law's ohm, then using that value for a DC voltage but result doesn't make sense so I guess is not that simple.

#### larryd

#1
##### Sep 16, 2017, 01:40 am
Not easy.
What switch?
What will you be using it for?

.
No technical PMs.
The last thing you did is where you should start looking.

#### Jay98

#2
##### Sep 16, 2017, 02:08 amLast Edit: Sep 16, 2017, 02:08 am by Jay98
It's an on/off toggle switch, I don't have the model with me right now.
I don't have an specific use for it either, I'm just interested in the theory.

#### larryd

#3
##### Sep 16, 2017, 02:41 am
Resistive and inductive loads are two different things as the latter can weld contacts.

AC voltages go through zero crossing; this allows switches to power higher AC current loads than DC.

.
No technical PMs.
The last thing you did is where you should start looking.

#### Jay98

#4
##### Sep 16, 2017, 03:52 am
thank you, so there is no a practical way to calculate it?

#### larryd

#5
##### Sep 16, 2017, 04:03 am
Not that I know of.

No technical PMs.
The last thing you did is where you should start looking.

#### Southpark

#6
##### Sep 16, 2017, 04:11 am
Its current rating is only expressed in AC, is there a way I can calculate DC current rating with that info?
Well, AC current is often measured or conveyed in 'RMS' format, which turns out to be an equivalent DC value for the case where the current really were DC. So..... at a first guess, can take that RMS rated AC current to be the same as the rated DC current. That's for starters maybe.

#### avr_fred

#7
##### Sep 16, 2017, 06:20 amLast Edit: Sep 16, 2017, 06:27 am by avr_fred
Quote
So..... at a first guess, can take that RMS rated AC current to be the same as the rated DC current. That's for starters maybe
Well, it's a start but it's a bad start. As already pointed out, AC current crosses zero volts every 8 to 10 milliseconds for most power systems. DC has no such feature. Once the contacts begin to open with DC, there is nothing to extinguish any arc that forms. The result is contact destruction when there is enough energy in the source of said arc. If the DC source is regulating current, its game over as the voltage rises to maintain current.

Every switch is different. If there is no DC rating published, it's not DC rated. Simple.

Could you use a non-DC rated switch for DC? Sure, if the voltage and power is low enough. But what's low enough? Even 5 volts at an amp might be too much for some. There are no simplistic answers.

#### JohnRob

#8
##### Sep 16, 2017, 06:43 am
Another factor that affects the ratings is the contact plating material and thickness.

In addition the "rating" you are asking for must make assumptions on the voltage & current you are using and of course how often would you switch it.

For a hobby application I would scour the catalogs to find a similar switch that has both AC and DC ratings, add some derating and use that current.

good luck

John

#### slipstick

#9
##### Sep 16, 2017, 01:03 pm
This was bothering me too so I just checked a few switches with multiple ratings:

- 2A @250VAC, 5A @120VAC, 5A @28VDC
- 15A @250VAC, 10A @24VDC, 15A @12VDC
- 6A @250VAC, 15A @ 28VDC

and there are some only single rated and with warnings in the manufacturer's specs not to use for the wrong sort of electricity.

As you can see the numbers are all over the place. The good thing is that the specifications are usually available if you know the manufacturer but when buying from hobby suppliers it can be anyone's guess.

Steve

#### kenwood120s

#10
##### Sep 16, 2017, 01:28 pm
This was bothering me too
How's THIS for a misleading data sheet (or section of web page, whatever you want to call it)?:

At the top of the page it says 5a 125vac / 3a 250vac, so far so good. (625W and 750W respectively.)

Lower down it says Max current 5A, Max voltage 250VAC.

While those max values are actually correct, if you hadn't seen the headline you would very likely assume the max current and max voltage went together (1250W), but they don't.

Interestingly I don't see any mention of VDC on that page, although most of the applications listed in point 6 there are likely to be DC.

#### MarkDerbyshire

#11
##### Sep 16, 2017, 02:11 pm
DC voltage ratings for switches and relays are usually 10% of the AC Voltage.  ie a switch rated at 10A 250VAC will be 10A at 25VDC

#### kenwood120s

#12
##### Sep 16, 2017, 02:18 pm
DC voltage ratings for switches and relays are usually 10% of the AC Voltage.  ie a switch rated at 10A 250VAC will be 10A at 25VDC
That may be true but the whole point of a rating is that the maker says it can handle "this" or handle "that". When there's a fire and the opposing expert witness says "that switch maker didn't give a DC rating so it shouldn't have been used for DC at all", it's too late to remember this:

If there is no DC rating published, it's not DC rated. Simple.

#### Southpark

#13
##### Sep 16, 2017, 02:52 pmLast Edit: Sep 16, 2017, 03:05 pm by Southpark
I reckon it depends on situation. What specs have you got on that AC switch? (if you already have the switch, or if you're looking at a particular AC switch). And what level of DC current and what level of voltage is the switch going to encounter?

If in doubt, ask the manufacturers how they come up with a DC rating.

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