8 replies to this topic

[nicklouse]

Simple question and I think I know the answer but. Oh it ain't life or death.


Yet.

So switches you know like we like to fit for accessories etc. Some times we will use a relay and other times a relay is not needed.
I remember the ilium instead plastic toggle switches I used for the spots and fogs in the old days ( they are still available) were rated at 30 A and we ran a relay. Ok current is easy to work out what is needed.

But to the actual question let us say we have a toggle switch that is listed as 250V 10A what would the rating be in a 12V situation?

Presuming that 250V is AC and the 12V is DC. Not sure what else it could be.

[David128]

Hi Nick  It will probably halve the current load so I would reckon about 5amp., due to AC switching and DC being constant

[GraemeC]

I don't see why the max current the switch could handle would change significantly whatever the voltage.

 

Most switches and breakers that are dual voltage rated only ever seem to have one current rating:

Edited by GraemeC, 17 November 2015 - 09:34 AM.

[David128]

That may be acceptable if it is DC Rated.. but please read the following

 

 To err on the side of safety, read the specifications thus:

Rated to allow up to 10A at up to 125VAC or up to 6A at up to 250VAC. Not actually rated for DC, so you're on your own.

In other words, if at all there is a choice, opt for a DC-rated switch, so you know you are within rated parameters. If, however, that is not an option, read on...

Some of the factors that affect a switch contact rating:

1. Voltage at which breaking the contact will not cause unacceptable arcing / pitting
2. Voltage at which the isolation provided by the switch housing starts getting unsafe
3. Current at which the contacts will not overheat enough to melt or damage the housing
4. AC or DC voltage: AC signals are easier to break, i.e. less contact pitting or arcing, since the potential difference falls to zero twice per cycle.

Thus, when used for DC, I prefer to assume 10% of the highest rated AC voltage, while keeping the current rating the same as the lowest rated current for the AC specifications.

For this particular switch, 6 Amperes at 12.5 Volts DC would not trigger a paranoia attack.

[Kaj]

Hi Nick, hope this helps a little. Power (Watts)= volts x amps. In general terms if a higher voltage is used the current (and hence the gauge of the wire) can be smaller. As an example most aircraft systems run using 115v AC so that the current draw can be small and hence smaller and lighter wiring can be used. That is why 12v DC car wiring is reasonably thicker copper.
Switches are rated on the amount of current they can switch without welding the contacts together. This is why a relay is used as the switch only controls the relay coil and the heavier current passes through the relay contacts (which can be switched at very high speed to avoid arcing). This is why relays are used in car systems such as wiper motors, lights,etc which have a relatively high power rating.

[jaydee]

I get whats the point of your question

Yes 10A AC are 10A DC and you'll open a can of worm if you ask this at the uni

The actual problem will be knowing how the switch is rated for 10A and how long do you want the switch to last

[KernowCooper]

Most 240 v switches are designed for use on AC circuit and as has been said above the arc created when the switch opens is extinguished when the alternating voltage goes through the zero crossing point. This means the switch can be opened slowly and still not have a prolonged arc.

Zero crossing point is when the voltage is zero as it changes direction at 120 times a second.

Switches in a DC circuit have to extinguish the arc as there is no zero crossing point.. Most have a snap action to separate the contacts rapidly and far enough apart to break the arc before it can damage the contact surfaces.

As a rule of thumb treat 240v AC switches amperage at a maximum of 20% (divide by 5) of the AC amperage marked on the switch on a DC Circuit

So a 240v AC 10amps switch would only be safe on a 12v DC circuit at 2amps, what usually happens is the switch arcs and welds the contacts together and your unable to switch the circuit off. This is worse when a AC switch is used for example a DC fan motor where the voltage spikes during the on/off switching due to the motor windings

Edited by KernowCooper, 17 November 2015 - 08:37 PM.

[nicklouse]

Cool thanks for the refresher.

So to sum it all up as I see it.

The design of the two types of switch should be fundamentally different do to the lingering spark in the DC system.

So there is no way to safely scale the numbers.

BUT using an AC switch should be OK. To trigger a relay or for a relatively low draw application.

So from my collection of switches some have been used before without relays and were fine but we know how that can end.

I will be running the feed to the ignition system via a fused circuit and a 15A 250v switch. The fuel pump, facet red top 5A supply needed via the same set up. Stater relay will be triggered by an unfused circuit and a suitable MOD 12v mom switch.

Rad fan and water pump and wipers will all be powered via 30A relays triggered by 10A 250v switches. Again fused. All covered by a big cut off switch.

Doing it right is different to just doing it.

[KernowCooper]

The relays is the way forward without doubt and will take the load off the contacts