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High Current DC Switching

abuhafss

Aug 3, 2010
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So to get things clear, you are switching a solenoid, not the starter motor. And it's now just a short time, not like previously where you said it overheats after sometime?
No. I want to switch on the starter motor briefly, of course, enough to start the engine.
I want to use mosfet or high-side switch in place of the solenoid.
 

abuhafss

Aug 3, 2010
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I would think that a FET solution would be cheaper than a solenoid but the car makers use a solenoid. There must be a reason for this.

A dismantled solenoid will show how severe the duty is.

Here you can see a dismantled solenoid switch.

starter-solenoid-bad.jpg

P1020946-600.jpg
 

Bluejets

Oct 5, 2014
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Reason you have problems is the solenoid is completely shagged. Along with the fact that Chinese GY6 is an elcheapo pos. Note zero contact coating, just bare copper which has discoloured from the heat and likewise effect on the release spring. Cleaning contacts like shown in the video might get you mobile one time but it definetely will never last or be looked at as a "repair that avoids buying a replacement". Youtube has many good aspects but there is a lot of bs displayed there as well. With all this though, fit a new one. You will find a $5 solenoid much more economical than an array of suitable mosfets let alone the cost of your finding out the realism of how many you actually will need.
 

abuhafss

Aug 3, 2010
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Almost all the solenoids available does not have special contact coating. Even the most expensive one has the bare copper plate which gets discolored because of arcing. I provided these pics and videos because Davenn wanted to see the dismantled solenoid otherwise, I am concerned about the winding of the solenoid which draws about 3A and get burned after sometime. Therefore, I was looking for some solid state replacement of the solenoid relay.
 

davenn

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Sep 5, 2009
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Even the most expensive one has the bare copper plate which gets discolored because of arcing. I provided these pics and videos because Davenn wanted to see the dismantled solenoid otherwise,

wasn't me ;)

I am concerned about the winding of the solenoid which draws about 3A and get burned after sometime.

will only happen if the current is applied for too longer bursts

Therefore, I was looking for some solid state replacement of the solenoid relay.

I gave one good suggestion good for around 200 - 250 A .... It may or may not be enough depending on the load on the starter motor
as suggested throughout the thread, I think you are going to struggle to find FET's that can handle 150+ Amps

A starter motor from a normal 2 Litre petrol car draws between 60 and 200 Amps when turning over the engine when the oil is warm and thin.
Under winter conditions, this current can easily double when the oil is thick.

Diesel engines have a very high compression ratio e.g. 22:1 and require more powerful starter motors. On average they draw between 300 and 500 Amps for average sized car engines 2 to 3 litre while on truck engines the current could easily reach 1000 Amps at initial turn over.

The real problem you need to take into account is, initially there will be a very high current spike before it settles down to the average cranking current
I saw a test on a Hyundai 4 cyl car, around 2 litre, where the initial spike was around 200 Amps then it settled down to 100A as it continued to operate for 10 sec

So if your FET cannot handle that initial big spike, it will die every time


Dave
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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There is something badly wrong if cars and trucks can use starter solenoids for years without them failing, yet the starter on your very small motor fails.

Nevertheless, can you measure the resistance of your starter motor. Note that you will need a 4 wire resistance device or possibly pass a relatively high current through it (say 1A) and measure the voltage drop. With this, we can estimate the stall current, and hence the maximum current.
 

duke37

Jan 9, 2011
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A solenoid will need a fair voltage to turn on, then a lower voltage to hold on. Thus it will start an engine even if the voltage drops to less than 10V.

A mosfet does not have this hysteresis so if the voltage drops significantly, then the mosfet is not turned on fully and will dissipate a lot of power. Poof.

You will need a very high current flywheel diode across the motor for the mosfet and it would doubtless increase the life of a solenoid also, reducing arcing.
 

duke37

Jan 9, 2011
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Most of the arcing occurs at startup.
You may be right but how did you determine this?
Normally, two cold contacts will not spark until they are very close. When they open, an arc is always generated and if the inductance is large, a lot of energy has to be dissipated.
 

Bluejets

Oct 5, 2014
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Contact bounce. A common thing with all switches. There's plenty of documentation out there on the subject.
 
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