# DC Current Ratings for AC Switch

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##### New Member
Hello all,

I am working on a ROV (Robot) for underwater missions. I need a beefy switch for main power distribution, which should be able to handle at least 25 Amps.
My current choice is this Rotary Switch:
https://amzn.to/2ukEgs2
According to the seller its rated for 10 Amps at 440 Volts. The Robot uses 12V and about 25 Amps when its using all thrusters.
Now my question is, How much Current will it be able to handle at 12 Volts? Can someone help me out with this Problem?

Kind Regards, Max

##### Well-Known Member
I don't see it lasting that long at that if DC current, especially if any kind of inductive load, In all likelihood the 10amp rating is not only AC but resistive load.
You will need to place some means arc-quench device, snubber etc, whatever kind of switch you use,
What you could do is use a low rated switch to pick up a Automotive starter solenoid.
Max.

##### New Member
Thanks for that.

I should probably add that when power is turned on , only the Raspberry Pi inside turns on and then after 30 secs the motors are ready spin. Until then the current is below 1 Amp. After use the raspberry pi is shut down and the motors won't spin, then the power switch is turned off.
So at start and end of mission the power drops to about half an amp.

##### Well-Known Member
If not actually switching/interrupting the 25amps you MAY get away with it, but the switch may get quite warm or Hot!.
Max.

Alright Thanks!

#### Nigel Goodwin

##### Super Moderator
The main reason for lower DC rating is simply that AC will auto-quench the arc across the contacts when you switch it OFF (as current drops to zero every half cycle). With DC the arc will be prolonged, damaged the switch contacts, and may even not cease at all.

But for your specific question, the rated DC current at 12V will be considerably LESS than the 10A at 440V AC - perhaps only 2A or so?.

Hae you considered a relay?, easily available with high current DC contacts (for use in cars), and use a small switch to control that.

##### New Member
I considered a relay before but I want to keep the dry circuit as simple as possible so rather just a switch on its own. My only concern right now is just the constant current while operation, not the current flow while turning it on or off.

#### Nigel Goodwin

##### Super Moderator
I considered a relay before but I want to keep the dry circuit as simple as possible so rather just a switch on its own. My only concern right now is just the constant current while operation, not the current flow while turning it on or off.
In which case it should handle 10A, as for the AC rating - still a LOT short of the 25A you require. As already mentioned, the issue in this case is overheating of the contacts, and potential failure (either going O/C or the contacts welding together).

##### Well-Known Member
Hae you considered a relay?, easily available with high current DC contacts (for use in cars), and use a small switch to control that.
Essentially what an auto solenoid is as I suggested., but if not actually switching the DC and acting as a dry contact, it may be overkill.
You can get the higher current rated type of automotive relay, most auto wreckers are a good source.
Max.

#### KeepItSimpleStupid

##### Well-Known Member
DC and AC switches have a different kind of construction. With DC, you generally have to pull the contacts apart. With AC and the zero crossings, it's easier to pull them apart.

At the other end of the spectrum, contact material makes a difference. You may not be bale to switch 10 mA with a contactor rated at 40 A. There is a minimum wetting current.

You have to worry about the load that can be swtched. Some high voltage relays (e.g 30 kV) and RF relays need to switch 0 current.

carryig and switching currents can be different. The AC and DC ratings can be different.

#### Nigel Goodwin

##### Super Moderator
Essentially what an auto solenoid is as I suggested.
Except a starter motor solenoid is MUCH more powerful than required, and you wouldn't want to waste the large amount of power that it uses to pull-in.

As I said, and as you also mentioned, there are plenty of car relays that are high enough power - and have modest coil requirements.

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