Need a momentary switch to switch about 40 amps to a DC moto

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Rodney
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Need a momentary switch to switch about 40 amps to a DC moto

Post by Rodney » Mon Jun 09, 2008 11:58 am

I have a DC motor (a starter off an old marine engine) that pulls about 30 to 40 amps on 12 volts. I want to use it for light intermittent duty (starting a large single cylinder model engine of about 72cc). Any suggestion such as "Will an FET be suitable to use as a switch?" or should a contact type switch be used. Any suggestions or circuitry will be appreciated. Power source is a 12 volt lawn mower type battery or a car battery. The mechanical connection between the DC motor and engine being started is simply a friction cone held against the spinner on the model engine.

gerty
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Post by gerty » Mon Jun 09, 2008 12:32 pm

I would use a "Ford" type starter solenoid. It goes in series with the positive cable, and if the case is grounded, only requires one wire to energize.

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dacflyer
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Post by dacflyer » Mon Jun 09, 2008 4:46 pm

you can get theose starters from any hobby shop for about 40-50 dollars
they already have the switch on the motor..
probably can find them cheaper on ebay.. but then again i am not sure what the limits are ( what size engine is the max for that starter )
but then again, they also make gear reduction starters as well...

good luck

Rodney
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Post by Rodney » Tue Jun 10, 2008 11:25 am

dacflyer wrote:you can get theose starters from any hobby shop for about 40-50 dollars
they already have the switch on the motor..
probably can find them cheaper on ebay.. but then again i am not sure what the limits are ( what size engine is the max for that starter )
but then again, they also make gear reduction starters as well...

good luck

I am aware of the availability for these but, to start this large a motor, the cost is more like $150 so I'm trying to get by for a lot less money. My problem so far is getting a switch that will stand up to the high currents.

Engineer1138
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Post by Engineer1138 » Tue Jun 10, 2008 12:56 pm

trying again after my last response disappeared...

a HEXFET such as an IRFZ44 or greater should be OK for basic ON/OFF control.

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Post by FOB » Wed Jun 11, 2008 4:21 am

Back in the 1970's before commercial model starters were common, we used windshield wiper motors available from salvage yards for less than $5.00. We used flipper switches from old pinball machines to switch the wiper motor. Similar switches are available, I believe, at All Electronics and other surplus stores. If the current eats your switch, I agree with the previous poster that a Ford starter relay will probably outlast you, as they switch several hundred amps of 12 volt for years.

Engineer1138
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Post by Engineer1138 » Wed Jun 11, 2008 8:22 am

Almost forgot: if you go the FET route, be sure that your power supply voltage doesn't droop too much when you are cranking the motor, or use a separate FET supply. If the gate voltage drops along with the motor voltage, the FET will begin to consume a lot of power and probably burn up.

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philba
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Post by philba » Wed Jun 11, 2008 9:31 am

I'd use a lower capacity switch to drive a contactor. My guess is that's what the commercial solutions use.

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