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Old 01-20-2006, 06:52 AM
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ac to dc amp rating HELP????

I am trying to find a switch to use in a 12 volt circuit but all of the switches that I find are rated in ac. Most are listed something like this: 2amps @ 125v ac, 28vdc or..... 10 amps @ 440v Ac 12,24,48 dc.

If a switch is rated at say 5 amps @ 220v ac How do I determine how many amps it can withstand at 12v dc.

The switch I need is to operate a single power window motor and I figure that since it is fused 25 amps in my car for two motors I could use a switch for one motor that would be good to 12.5 amps @12v dc. I just don't know how to figure out if a switch will take that power or not.

Thanks
Ric

Last edited by Ratchet; 01-20-2006 at 07:59 AM.
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Old 01-20-2006, 09:38 AM
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Use a relay, then your switch rating is not that big a deal, and the switch will last longer to boot.

Vince
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Old 01-20-2006, 10:04 AM
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http://www.allaboutcircuits.com/vol_4/chpt_4/2.html

http://www.eaa.org/benefits/sportavi...2_switches.pdf
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Old 01-20-2006, 11:57 AM
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Quote:
Originally Posted by 302/Z28
Use a relay, then your switch rating is not that big a deal, and the switch will last longer to boot.

Vince
Check the diagram attached for the application. The switches I am looking for are push button switches that will carry the current required by the motor until the motor drives the rack into contact with the switch at the end of its travel. At that point the switch will open the circuit and stop the motor. When the relay is triggered to start the motor in the oposite rotation the rack will then travel to the other switch which will stop the motor when the rack comes in contact with that switch.

So as you can see switches are required in this application.

Ric
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Old 01-20-2006, 12:02 PM
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onovakind67.

The second link was very informative and points out the problems of using an AC switch in a DC application. I still need to find out how to convert amp ratings in AC volts to amp ratings in DC volts as most switches even though they say they are good for DC don't indicate how many amps they are rated for. The generally give an AC amperage rating an AC voltage a DC voltage but now DC amperage.

Ric
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Old 01-20-2006, 07:55 PM
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Doc here,

Both links are very informative, and make good reading.

To Simplify Your Task at hand, Look at the lowest Current reading of the switch (AC or DC) and make that +/- 25% of the max rated draw of your circuit..(I.E. If your switch is lowest, 10 amp rated, your devices should draw no more than 7.5 amps)

The easy way to view the Voltage rating , Is that it is the voltage at which IT MAY be possible for an Arc to travel across the contacts, OR will weld/burn or pit the contacts closed. Stay below that and you'll be fine.

Is this for Limit switches on a Headlight circuit? Or a revolving license tag? (looking at the drawing) If so, you need to re~position the limit switches to activate the relay Coils, Not the motor..let the relay Contacts do the work..For that, you may have to add a second relay (reverser) .I can't really tell by the lay out the CW/NO/NC contacts on that drawing as they aren't Included...so I can't really say how that will operate or not..

If , On your motor circuit, you are using the relays (Which ALWAYS should be done, and on those items drawing more than 25 amps) Your Task is further Simplified..

You need only be concerned with the Relay coil draw, of each coil, which will be about 1/2 amp to an amp each, Far less than the motor draws..Just about ANY standard aftermarket Switch is rated at about 5 amps..so almost any off the shelf switch will do the job. AND is soooooo much cheaper than a high amp switch!

Doc
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Last edited by docvette; 01-20-2006 at 08:06 PM. Reason: Added info
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Old 01-21-2006, 08:28 AM
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Quote:
Originally Posted by docvette
Doc here,

Both links are very informative, and make good reading.

To Simplify Your Task at hand, Look at the lowest Current reading of the switch (AC or DC) and make that +/- 25% of the max rated draw of your circuit..(I.E. If your switch is lowest, 10 amp rated, your devices should draw no more than 7.5 amps)

The easy way to view the Voltage rating , Is that it is the voltage at which IT MAY be possible for an Arc to travel across the contacts, OR will weld/burn or pit the contacts closed. Stay below that and you'll be fine.

Is this for Limit switches on a Headlight circuit? Or a revolving license tag? (looking at the drawing) If so, you need to re~position the limit switches to activate the relay Coils, Not the motor..let the relay Contacts do the work..For that, you may have to add a second relay (reverser) .I can't really tell by the lay out the CW/NO/NC contacts on that drawing as they aren't Included...so I can't really say how that will operate or not..

If , On your motor circuit, you are using the relays (Which ALWAYS should be done, and on those items drawing more than 25 amps) Your Task is further Simplified..

You need only be concerned with the Relay coil draw, of each coil, which will be about 1/2 amp to an amp each, Far less than the motor draws..Just about ANY standard aftermarket Switch is rated at about 5 amps..so almost any off the shelf switch will do the job. AND is soooooo much cheaper than a high amp switch!

Doc

You know I just new you were going to say I would have to add relays.

But just for curiosity if a switch says it is 10 amps at 125v ac how many amps will it handle at 12volts DC? Whats the formula?

Thanks
Ric
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Old 01-23-2006, 04:53 PM
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I don't remember ever seeing a formula for the conversion of switch ratings. There are to many different types.

Switches are designed and manufactured to meet certain criteria. They are tested by Underwriters Laboratories (UL) or CSI in Canada to certify that they (safely) perform as rated.
The example of 10 amps @ 125 volts is for commercial controls. Sometimes refered to as "pilot duty" which is not for power switching.
We also see "motor rated" switches. You will actually see a maximum HP rating label on the switch. These are designed for power switching and to handle the inrush of current when a motor is started.

Low voltage DC is hard on switching devices. 12 volts DC @ 10 amps produces more contact arcing than 125 volts AC @ 10 amps.

As a very general guide, try to limit the total load to 50% of the rating when used on 12 volts DC.

With motor rated AC switches, you should OK with 75% of the AC rating.

vicrod
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