A/C D/C switches. Am I going to burn my car down? - Team Camaro Tech
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post #1 of 24 (permalink) Old Mar 15th, 16, 08:57 PM Thread Starter
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A/C D/C switches. Am I going to burn my car down?

I bought switches before reading up on A/C vs D/C. I got Carling Technologies toggle switches rated at
10A 250VAC
15A 125VAC
3/4HP 125-250VAC

Data sheet

Some of the things I was going to use them for were ignition/accessory, starter solenoid, and headlights. Obviously it would be a bad day if the starter or ignition got welded closed. I've read a few message boards and it is mostly engineers arguing with each other about materials, contact distance, microscopic oxide layers, arcing through atmospheres, resistive vs capacitive, vs inductive loads. Are there any answers besides "buy a dc rated switch?" Looking for something like "No direct conversion equation but at only 12 volts you'll be fine!" Or maybe a "The starter solenoid/headlights only draw X amount of amps, well within the capabilities of those switches!" Or you can just tell me I'm going to burn my car down. Thanks!
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post #2 of 24 (permalink) Old Mar 15th, 16, 09:07 PM Thread Starter
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Re: A/C D/C switches. Am I going to burn my car down?

Just read on this data sheet for the series 'Suitable for low voltage 12/24V DC." I'm going to buy a fire extinguisher and hook up my battery and see what happens.
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post #3 of 24 (permalink) Old Mar 15th, 16, 09:19 PM Thread Starter
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Re: A/C D/C switches. Am I going to burn my car down?

And this from Carling technologies FAQ page "The DC current 'rule of thumb' holds that the highest amperage rating on the switch should perform satisfactorily up to 30 volts DC. For example, if you have an F Series toggle switch which is rated at 10A 250VAC, 15A 125-250VAC, the DC rating is 15A up to 30VDC." I'll let you guys know if my car burns up.
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post #4 of 24 (permalink) Old Mar 16th, 16, 07:59 AM
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Re: A/C D/C switches. Am I going to burn my car down?

Mr. Kabinet, I am so impressed that you actually read the data sheet.

Don't read anything into the data sheet. If it does not mention the switch is rated for DC then you are taking a chance using it for a DC circuit. It might not have been tested for use with a DC circuit. Or, if it was tested, it failed the DC test. The UL, CSA ratings are only for an AC circuit. Believe me, if the manufacturer could advertise that it works on a DC circuit they would. They would be able to sell more switches.

A DC switch has to work much harder to break the circuit than one that is used on an AC circuit. A switch used on an AC circuit takes advantage of the voltage and current sine wave crossing zero as it cycles. A DC voltage or current never crosses zero so the switch has to break the maximum voltage. The switch in question data sheet mentions that the contacts are "slow make" and "slow break". This is just the opposite of what is required to work with a DC circuit.

Now that I scared you, I will tell you the switch will work for a while. It is going to fail prematurely because the contacts will weld themselves closed. This means that after a while, the switch will not turn off.

Patrick
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post #5 of 24 (permalink) Old Mar 16th, 16, 08:11 AM
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Re: A/C D/C switches. Am I going to burn my car down?

DC automotive switches are cheap. Why take a chance?
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post #6 of 24 (permalink) Old Mar 16th, 16, 10:00 AM
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Re: A/C D/C switches. Am I going to burn my car down?

You can design and build the system how you want but have you considered using relays ?. On an old custom car of mine I used some basic lighted toggle switches wired to Bosch relays and have never had issues. These are the small cube style relays with a 30A/40A rating. While it's been years since I did any work on this system, I believe most of the circuits on the 30A/40A relays are only passing or having to switch 20A or less.

On this car I have a row of the lighted rockers with one switch sending a signal to a relay to turn on the parking lights then a switch next to it to send power to a relay for the headlights. I then have even more for turning on things like the wipers (which was nice using the relays as the wipers are switched on and off with grounds).

The older cars headlight system gets power from the battery but it has to go through all kinds of connections, wire and wiring lengths to eventually get to the headlights and then back to the battery to where there have been people that run relays on the headlights to keep the voltage drop to a minimum and then turn these relays on and off using existing wiring or through a basic low amperage switch.

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post #7 of 24 (permalink) Old Mar 16th, 16, 12:25 PM Thread Starter
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Re: A/C D/C switches. Am I going to burn my car down?

Excellent responses! I am now searching for DC switches. The reason they are hard for me to find is I need Mom-Off, Mom-Off-Mom, and Off-On-On all in dual pole and they need to match in appearance AND since I already installed these AC switches, they need to have a 1/2 inch diameter panel hole. AND preferable quick connect but I guess that isn't too important. I appreciate the responses. I do think I will use relays for the headlights as well!
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post #8 of 24 (permalink) Old Mar 17th, 16, 07:22 PM
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Re: A/C D/C switches. Am I going to burn my car down?

Quote:
Originally Posted by tp_smith View Post
Mr. Kabinet, I am so impressed that you actually read the data sheet.

Don't read anything into the data sheet. If it does not mention the switch is rated for DC then you are taking a chance using it for a DC circuit. It might not have been tested for use with a DC circuit. Or, if it was tested, it failed the DC test. The UL, CSA ratings are only for an AC circuit. Believe me, if the manufacturer could advertise that it works on a DC circuit they would. They would be able to sell more switches.

A DC switch has to work much harder to break the circuit than one that is used on an AC circuit. A switch used on an AC circuit takes advantage of the voltage and current sine wave crossing zero as it cycles. A DC voltage or current never crosses zero so the switch has to break the maximum voltage. The switch in question data sheet mentions that the contacts are "slow make" and "slow break". This is just the opposite of what is required to work with a DC circuit.

Now that I scared you, I will tell you the switch will work for a while. It is going to fail prematurely because the contacts will weld themselves closed. This means that after a while, the switch will not turn off.
Right.

How does the switch know to wait for the AC voltage/current to cross zero, or if the load is inductive, capacitive, or resistive, and break the circuit accordingly?

It doesn't.

At any instant the voltage on a load and the current through the switch is dependent on the load type, and where the source voltage is in the cycle.

Assuming a resistive load, the current is proportional to voltage at that instant, regardless of whether the source is AC or DC.
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post #9 of 24 (permalink) Old Mar 18th, 16, 04:48 AM
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Re: A/C D/C switches. Am I going to burn my car down?

Quote:
Originally Posted by 67sc View Post
Right.

How does the switch know to wait for the AC voltage/current to cross zero, or if the load is inductive, capacitive, or resistive, and break the circuit accordingly?

It doesn't.

At any instant the voltage on a load and the current through the switch is dependent on the load type, and where the source voltage is in the cycle.

Assuming a resistive load, the current is proportional to voltage at that instant, regardless of whether the source is AC or DC.
DC can sustain an arc while AC cannot. It is the arcing that wears out the contacts of the switch.

Don
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post #10 of 24 (permalink) Old Mar 18th, 16, 05:35 AM
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Re: A/C D/C switches. Am I going to burn my car down?


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post #11 of 24 (permalink) Old Mar 18th, 16, 08:14 AM
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Re: A/C D/C switches. Am I going to burn my car down?

Excellent demonstration

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post #12 of 24 (permalink) Old Mar 18th, 16, 12:13 PM Thread Starter
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Re: A/C D/C switches. Am I going to burn my car down?

Well! Buying new switches!
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post #13 of 24 (permalink) Old Mar 18th, 16, 06:39 PM
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Re: A/C D/C switches. Am I going to burn my car down?

He was demonstrating 220VDC not 12VDC. Almost every switch I've seen has both an AC and DC rating. Typically you'll see 30VDC as the rating along with an amperage.

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post #14 of 24 (permalink) Old Mar 18th, 16, 06:52 PM
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Re: A/C D/C switches. Am I going to burn my car down?

Quote:
Originally Posted by 69L48Z27 View Post
He was demonstrating 220VDC not 12VDC. Almost every switch I've seen has both an AC and DC rating. Typically you'll see 30VDC as the rating along with an amperage.
Correct. But it's not voltage its amps that counts. So depending on the circuit load you can still get arching with 12VDC
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post #15 of 24 (permalink) Old Mar 18th, 16, 06:57 PM
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Re: A/C D/C switches. Am I going to burn my car down?

Excellent demonstration! Except I'm pretty sure those are not rated hot gloves he's wearing....

But that said, 220v DC and 220v AC are a different animal anyway, because of the nature of "alternating current" vs "direct current". I guarantee you will feel the difference....lol. DC voltage vs AC like for like the DC has the win for intensity by a considerable amount.

HOWEVER.... the example does make the point well, buy a switch rated for equal to or greater than the load it will control, if it's DC rated buy DC rated switches. I'm a commercial/ industrial electrician and in my world if it's not rated for it's use don't use it. People can die and buildings can burn down...

Will those switches work? Of course. But why not just buy the right stuff. Radio shack has crap tons of DC switches and they are not overly expensive.

Also, you don't mention how the circuit will be designed but as mentioned use relays. That's what they are designed for, switches shouldn't make/break a large load repeatedly.

Good luck with your project!

Sean

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