Current rating / Voltage rating on switches

[cobolt]

Hi all,

Looking for a bit of quick help.

I've got a genuine Carling SPST switch, included with a compressor kit.

I'd like to use it on a 24V circuit, the rating on the side states 12V 20A...the switching current even at 12V is less than 5A...so at 24V maximum 2.5A

The question is, is this switch by default also capable of 24V 10A switching...or do I need to find a switch specifically for that?

 

[grumpy_o_g]

Hi all,

Looking for a bit of quick help.

I've got a genuine Carling SPST switch, included with a compressor kit.

I'd like to use it on a 24V circuit, the rating on the side states 12V 20A...the switching current even at 12V is less than 5A...so at 24V maximum 2.5A

The question is, is this switch by default also capable of 24V 10A switching...or do I need to find a switch specifically for that?

Assuming the switch is working properly and very simplistically excess current will make it get too hot and too much voltage will break down the insulation leading to arcing in the switch or even possibly an electric shock if you go mad with the voltage. Having said that I'd happily use it as you said - 24V through the switch should be fine as long as you keep the current below 10A.

 

[William_H]

Assuming the switch is working properly and very simplistically excess current will make it get too hot and too much voltage will break down the insulation leading to arcing in the switch or even possibly an electric shock if you go mad with the voltage. Having said that I'd happily use it as you said - 24V through the switch should be fine as long as you keep the current below 10A.

The concern re rating of switches aprt from actual current carrying capacity is the arcing at moment of opening the switch. With an inductive load this can produce a higher voltage more of a problem with DC. The motor of the compressor may well produce that back EMF to induce arcing. Back EMF being higher with the higher circuit voltage. Back EMF (electromotive force) is a voltage that appears across an inductor when current is stopped. It is caused by the collapse of the magnetic field and is always in a direction trying to maintain the current. Back EMF is what produced the spark in ignition systems. It can be managed with a diode across the switch or capacitor. Now I am just waffling because I think the switch will be fine for OPs purpose. ol

 

[john_morris_uk]

The concern re rating of switches aprt from actual current carrying capacity is the arcing at moment of opening the switch. With an inductive load this can produce a higher voltage more of a problem with DC. The motor of the compressor may well produce that back EMF to induce arcing. Back EMF being higher with the higher circuit voltage. Back EMF (electromotive force) is a voltage that appears across an inductor when current is stopped. It is caused by the collapse of the magnetic field and is always in a direction trying to maintain the current. Back EMF is what produced the spark in ignition systems. It can be managed with a diode across the switch or capacitor. Now I am just waffling because I think the switch will be fine for OPs purpose. ol

Agreed (except for the waffle)

 

[lw395]

See what the data sheet says for the switch?
https://www.carlingtech.com/amp-hp-volts
""DC Rule of Thumb

For those switches that list an AC voltage rating only, the "DC Rule of Thumb" can be applied for determining the switch's maximum DC current rating. This "rule" states the highest amperage on the switch should perform satisfactorily up to 30 volts DC. For example, a switch which is rated at 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform satisfactorily at 15 amps up to 30 volts DC (VDC).""

There are switches about which are really not much good for breaking DC.
Particularly inductive DC circuits.
What are the consequences of it not being up to the job? Is it safety critical if the switch welds shut? Do you need a long life from it or will it only be used a few dozen times?
Can you get something that has the correct rating for a couple of quid from Farnell/CPC/RS/Mouser/Digikey?
How sure do you need to be?

 

[cobolt]

Whilst I'm fairly sure the switch would be fine, I've ordered the same switch, but with an explicit stamping on the side that it is suitable 12V 20A and 24V 10A....the switches are easy to dismantle, so it will be interesting to see if there is any physical difference inside the switch at all.

I suspect not, and the Carling Contra switch bodies are all made the same, then offered with different "ratings/certifications" on the side to keep the clipboards happy.

Interesting the comment above about diodes....there are indeed a couple of diodes in the pre-supplied loom. I will need to change these also, from whatever they are, to 24V rated, I guess?

 

[lw395]

If they are common or garden silicon rectifier diodes, they are unlikely to be rated at less than 50V.
A lot of switches look superficially similar when you take them apart, but the contacts may be different metals, silver, tungsten, exotic stuff, depending on the application and life rating needed. Gold sometimes, but that's obvious.