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Old 03-25-2009, 05:45 PM   #1
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Starter Solenoid Diode?

Does the starter solenoid have a diode across it's terminals to prevent "flyback"? I'm designing a circuit to allow me to have a pushbutton starter once activated by rfid or just plain old key if the rfid is not activated. I need to know whether I need to install a diode or if there's already one there. I attached the schematic.
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Old 03-26-2009, 03:56 PM   #2
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Re: Starter Solenoid Diode?

I have never seen a diode inside the starter solenoids so I'd definitely put one on the output on your board.

Hope this helps.
Lou
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Old 03-26-2009, 05:54 PM   #3
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Re: Starter Solenoid Diode?

Alright. Would you happen to know how many amps the solenoid draws?
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Old 03-26-2009, 07:11 PM   #4
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Re: Starter Solenoid Diode?

Sorry, don't know the amperage. My educated guess, however, would be 20+ amps.
Also, another educated guess would be that it doesn't exceed 40A.
You can measure the resistance of the solenoid and calculate the current rougly.
It would be safe to use an automotive relay in the starter solenoid circuit.

Hope this helps.
Lou
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Old 03-26-2009, 11:32 PM   #5
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Re: Starter Solenoid Diode?

Yeah it did. Thanks. I was guessing the exact same thing for the amps. I already got my relays picked out on digikey. I just need help with sizing the right diode. Any electrical engineers in the house? Oh, and Lou. Do you see any major flaws in my circuit? Do you see it screwing up the car in any way? Thanks for the help.
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Old 03-27-2009, 12:02 PM   #6
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Re: Starter Solenoid Diode?

You can use any 1A/100V diode as long as it's fast switching. That will take care of the inductive flyback on the big solenoid.
For the small relays that you're using on your board, you can use smaller-amperage diodes, less than 0.5A, no problem.

Also, don't forget that the ignition switch is feeding a lot of circuits when turned on, you'll probably want a few relays feeding them separately.
This diagram might be of assistance: http://www.austinthirdgen.org/mkport...stribution.jpg

Hope this helps.
Lou
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Old 03-27-2009, 02:04 PM   #7
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Re: Starter Solenoid Diode?

Yea I have the shop manual and therefore all those diagrams, which I have been studying fastidiously. Thanks, though. Wow. I'm glad you mentioned using multiple relays. For some reason I just looked for the largest fuse in the diagram and said to myself, "That is the most amperes I will need." Wrong.

I still don't quite understand the diode sizing. 1A? Isn't that at least 20 times too small from what we guessed the solenoid's draw to be? And there are all sorts of ratings. Average current. Reverse current leakage. Voltage Forward/Reverse. Which one is the 1A rating?

Once again. Thanks. You have been very helpful in my quest of not screwing up my car.
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Old 03-27-2009, 03:05 PM   #8
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Re: Starter Solenoid Diode?

If we're talking flyback diodes, they don't need to be rated for the current the solenoid is pulling. They are not wired in series with the solenoid. These flyback diodes are wired parallel to the inductive load and in an OPPOSITE direction to short the high-voltage spikes induced when current stops flowing through the solenoid.
These currents are not big but the voltage could get pretty high (hundreds or thousands of volts), if not shorted.
As long as the flyback diode is connected to the solenoid, it shorts all negative spikes from the coil and prevents the high voltage buildup.

Hope this explains it well.
Lou

P.S.: You're looking at maximum forward current (repeatable) rating.
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Old 03-28-2009, 03:05 PM   #9
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Re: Starter Solenoid Diode?

Ahh. I have been looking all over the internet trying to teach myself, but only you have made it understandable. Thank you.

While reviewing my design to include more relays for the "Run" position, I saw that the ignition coil didn't have a fuse and therefore I don't know it's maximum current. IIRC, I read that ignition coils don't go past 20amps. Even if I am wrong, I'm quite sure that it will not exceed 70A, which is what the relays are rated at. Any input?

Also there is one line that feeds the fan and heater fuse and the window circuit breaker. The values of the fuses and breaker add up to 80A. I don't think this will be a problem as I feel it's unlikely that all three would reach close to their maximum for any significant amount of time to do damage to the relay. For safe measure, I could always just install a 70A fuse before the relay. That would save my relay and board. What do you think?

I really appreciate all your help. Hopefully, once I get this up and running, other thirdgenners will be able to benefit from the end result that spawned from your help.

Edit: Wow that sounded lame. Oh well.

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Old 03-28-2009, 07:09 PM   #10
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Re: Starter Solenoid Diode?

flyback
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Old 03-28-2009, 09:51 PM   #11
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Re: Starter Solenoid Diode?

Taken from wikipedia: "A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode[1]) is a diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed."
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Old 03-28-2009, 11:10 PM   #12
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Re: Starter Solenoid Diode?

Glad to be of help.

Ignition coil: you're right, 20A sounds in the ballpark. However, a coil upgrade (maybe in the future?) would exceed that so you could could with 30A or at max 40. Which is the standard rating for automotive Bosch relays.

80A draw: if your relay is rated at 70A, I would not worry. Look at the wiring, it doesn't support more than 40A anyway.
If you did want to go out of your way, you could make a small modification to the fuse box and cut the window breaker contact off and feed it from a separate relay. It's really easy to do, once you see what you're doing.

Your project is pretty interesting, I myself had a similar idea some time ago .... but no spare time to implement it.

Good luck
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Old 03-29-2009, 12:13 AM   #13
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Re: Starter Solenoid Diode?

Yea spare time is something I got plenty of. I go to community college and work a few hours in the afternoons.

I don't like how most people do pushbutton starters but render the ignition cylinder useless (I will have to disable the steering lock, though) and bypass the VATS permanently. I wanted to keep everything intact and add ease of use. I'm going to start another thread to follow the project. I'll post the link in here once I make the thread.

Here it is: http://www.thirdgen.org/techboard/el...ter-build.html (RFID/Pushbutton Starter Build)

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Old 03-31-2009, 10:13 PM   #14
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Re: Starter Solenoid Diode?

I would not place a diode on the starter solenoid. Just drive the solenoid with a good relay and be done with it. A relay rated for a comparable motor load will handle the flyback from the coil. This is essentially what the factory did with the factory installed ignition switch.

I seriously doubt that a 1A 1kV diode will be reliable with something that large. If it fails it will short across the solenoid and make it impossible to start the car until the diode is removed. Since it really isn't necessary there is no reason to fuss over the diode.

The coil side of your relays driven by logic circuits would be a different story; these should be protected by an appropriate high speed diode.
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Old 03-31-2009, 10:19 PM   #15
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Re: Starter Solenoid Diode?

The relay is rated at 12VDC 70A. That is sufficient, yes? I understand it will handle the flyback, but for how long? I don't want it to shorten the life of the relay significantly. I imagine the ignition switch contacts are much more hefty than those of the relay and thus can take it with ease. I just don't want anything to fail prematurely.

How about this diode http://www.digikey.com/scripts/DkSea...ame=70EPF04-ND. Pretty cheap if it's gonna keep my circuits intact. I picked the ratings based on some technical literature I read on some Panasonic relays. I don't see why the info wouldn't carry over.

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Old 04-05-2009, 01:53 PM   #16
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Re: Starter Solenoid Diode?

A 70A relay should last a long time with that load. I believe the typical harnesses are protected with a 20A fusible link. I went and took a resistance measurement on a stock big block starter and came up with about a 15A load, probably less by the time the starter engages due to voltage drops. A 70A diode would be in no risk from over current damage.

If you insist on having a diode you may want to look at something like the STTH3010. This should be more than enough to handle what you are doing.
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Old 04-05-2009, 05:57 PM   #17
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Re: Starter Solenoid Diode?

Alright. Cool. I will leave the diode out. Thanks for the help.
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Old 04-05-2009, 05:57 PM
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