Replacing Starter Solenoid with Solid State Components

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worldgnat

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I made an embarrassing mistake today when replacing the rectifier on my '78 XS650. I got everything pulled out and wired up... only to realize I had replaced my starter solenoid with a rectifier... I said it was embarrassing. :doh:

So the question is, has anyone replaced one of these things with a MOSFET or some other variety of high voltage transistor? I'm sure the relay works fine, but it seems silly to use a mechanical component when a $0.25 part from Digikey should do the trick.

Can you see any potential downfalls if I give it a shot?

Edit: To clarify, the reason I can't just throw the old solenoid back in is that I cannibalized the wire harness from the old one. I could figure out how to solder the thing back together, but I'm curious to try a solid state solution.
 
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After a quick search on this site, the starter motor draws 100A under load.

I suppose you could fit a transistor to do the job but the first downfall in your plan is that a power transistor able to switch that amount of current is going to cost a lot more than 25 cents.
 
Are you getting your components mixed up? The starter solenoid has two heavy cables, one from the battery and the other going to the starter along with a red/white positive wire and a blue/white from the starter button.
 
worldgnat said:
I made an embarrassing mistake today when replacing the rectifier on my '78 XS650. I got everything pulled out and wired up... only to realize I had replaced my starter solenoid with a rectifier... I said it was embarrassing. :doh:

So the question is, has anyone replaced one of these things with a MOSFET or some other variety of high voltage transistor? I'm sure the relay works fine, but it seems silly to use a mechanical component when a $0.25 part from Digikey should do the trick.

Can you see any potential downfalls if I give it a shot?

Edit: To clarify, the reason I can't just throw the old solenoid back in is that I cannibalized the wire harness from the old one. I could figure out how to solder the thing back together, but I'm curious to try a solid state solution.
Click to expand...

If you really want to go solid state to switch power to the stater motor, you should price out what a power transistor rated at 100 amps will cost you. The simple starter contactor (aka solenoid) has been used on cars,trucks and bikes for about a 100 years, and for a good reason..............inexpensive,rugged, and reliable. Just reuse your "old" solenoid and be happy it works so well.
 
Hmmm....don't know if I follow...you have two rectifiers now? The original (untouched...but faulty as I assume that's why you wanted to replace it) And the new rectifier replaced something else (The starter solenoid)?

Maybe post a pic of the part you swapped...
 
Hmmm....don't know if I follow...you have two rectifiers now?
Click to expand...

Well, having noticed my mistake, I unsoldered the wire harness and installed it in the correct place. The bike runs now (and charges at idle now, which it didn't do before).

I suppose you could fit a transistor to do the job but the first downfall in your plan is that a power transistor able to switch that amount of current is going to cost a lot more than 25 cents.
Click to expand...

Fair enough. I did run into a post on another site claiming that $47 was a "good deal" for such a transistor.

What I found on Digikey that seemed to meet the requirements was this transistor, which is rated for a source to drain voltage of 30V and continuous drain current of 100A. Now, given the flimsiness of the package, my gut tells me this won't do the job. Can you tell me which rating will confirm that suspicion?
 
That transistor is meant to be used in a pulsed environment, like 'switcher' power supplies.
It'll cook before the starter motor comes up to speed.
 
worldgnat said:
Well, having noticed my mistake, I unsoldered the wire harness and installed it in the correct place. The bike runs now (and charges at idle now, which it didn't do before).



Fair enough. I did run into a post on another site claiming that $47 was a "good deal" for such a transistor.

What I found on Digikey that seemed to meet the requirements was this transistor, which is rated for a source to drain voltage of 30V and continuous drain current of 100A. Now, given the flimsiness of the package, my gut tells me this won't do the job. Can you tell me which rating will confirm that suspicion?
Click to expand...

Its an interesting idea to try using a power transistor in place of the old traditional contactor (solenoid). DC motors draw very heavy current when starting. You would certainly have to mount the transistor to a fair sized heat sink to handle the heat, especially if the engine was not starting quickly and needed extra cranking time. I think you would have to be quite selective as there are many types of power transistors, and
perhaps a BJT might be better than a MOSFET. Protective fly-wheel diodes and a zener diode may be needed as well.

Try it out and let us know how it works, as it would be a learning experience for all of us.
 
Ok, so it sounds like this idea is a no go, at least not until I feel like playing around with it more.

Can you confirm my knowledge of how the solenoid is wired?

Red/White - Kill switch -> Coil Positive
Blue/White - Starter switch -> Coil Ground
Red - Battery -> Common Terminal
Black - Starter Motor -> Normally Closed
Yellow - Alternator -> Normally Open

I haven't seen this explicitly stated anywhere, so I'm trying to piece things together. The only thing I'm a little puzzled about is why the normally open lead would go to the alternator. Does this prevent the field coil from being energized while the starter is running so there's less resistance on the starter?
 
worldgnat said:
Ok, so it sounds like this idea is a no go, at least not until I feel like playing around with it more.

Can you confirm my knowledge of how the solenoid is wired?

Red/White - Kill switch -> Coil Positive
Blue/White - Starter switch -> Coil Ground
Red - Battery -> Common Terminal
Black - Starter Motor -> Normally Closed
Yellow - Alternator -> Normally Open

I haven't seen this explicitly stated anywhere, so I'm trying to piece things together. The only thing I'm a little puzzled about is why the normally open lead would go to the alternator. Does this prevent the field coil from being energized while the starter is running so there's less resistance on the starter?
Click to expand...

A yellow wire is not part of the starter solenoid (aka contactor). The other 4 seem to be correct. On some years the red/white comes from the kill switch, but on other years it comes from the safety relay.

The yellow wire comes from the alternator stator windings and goes to the safety relay coil.
 
Is there a safety relay and a solenoid relay? Because I definitely had five wires going into this unit, one of them yellow. I can't find two relays in any of the schematics I have. The unit I removed was under the right hand side cover. I can take a picture later, if that would help.
 
The solinoid or starter relay is the black plastic and metal unit with two heavy battery cables run to it and two smaller wires coming out of it. The safety relay is the tin box below it. It has five smaller wires. On my '78, three are in a multi-plug connector and two are separate .....

Electrics-Right.jpg
 
Oooohh... I pulled the safety relay. Thanks for posting the picture, 5twins. My manual doesn't seem to list one or the other, so I assumed they were the same thing. It seems like the safety relay was omitted from some models, and I guess either they didn't include it or I'm just not seeing where they put it.

OK, so now I'm a little bit lost. I understand that the relay should disconnect the starter when there's power from the alternator, and I think I know how to make that happen:
Yellow - Coil Positive
Blue/Black - Coil Negative
Black - Normally Closed
Red/White - Common

Which leaves Red - Normally Open, but that doesn't quite make sense... does it?
 
worldgnat said:
Oooohh... I pulled the safety relay. Thanks for posting the picture, 5twins. My manual doesn't seem to list one or the other, so I assumed they were the same thing. It seems like the safety relay was omitted from some models, and I guess either they didn't include it or I'm just not seeing where they put it.

OK, so now I'm a little bit lost. I understand that the relay should disconnect the starter when there's power from the alternator, and I think I know how to make that happen:
Yellow - Coil Positive
Blue/Black - Coil Negative
Black - Normally Closed
Red/White - Common

Which leaves Red - Normally Open, but that doesn't quite make sense... does it?
Click to expand...

The SR has a coil that must be energized from the alternator yellow wire. Power comes in on the yellow wire and the other side of the coil goes to ground on the black wire. That leaves 3 wires. One red/white brings power from the headlight fuse. The other red/white goes to the starter relay and the light checker. The blue/black sends power to the headlight.
 

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