what size diode do I need

rockfla

rockfla

Well-known member
Building a jumper wire for a 6V coil, is there a particular amp rating or mili-volt rating for the in-line diode i need? OR is the diode really necessary?
 
Not sure about the size rating, but the diode is certainly necessary unless the jumper is on a normally open toggle switch.
 

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The diode should be sized for the current and voltage it is going to be exposed to. What is the resistance of the primary circuit of the coil you will be using and the battery voltage? You can calculate the current using Ohm's Law, E=IR. That should get you in the ball park, unless I screwed something up.
 
rockfla said:
Building a jumper wire for a 6V coil, is there a particular amp rating or mili-volt rating for the in-line diode i need? OR is the diode really necessary?
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Properly installed, you will have a full 6v to the coil while cranking the starter. Stop cranking and the voltage will drop back to where it needs to be.
 
The diode you require should be able to handle about 8 amps. Voltage specs dont matter. An old alternator main (top hat) type of diode would work ok. Make sure you fit it the correct way around otherwise it wont pass any current to the coil. Dont allow it to touch any metal of the car body. Regards, Kevin.
 
koates said:
The diode you require should be able to handle about 8 amps. Voltage specs dont matter. An old alternator main (top hat) type of diode would work ok. Make sure you fit it the correct way around otherwise it wont pass any current to the coil. Dont allow it to touch any metal of the car body. Regards, Kevin.
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Thank all for your guidance!!
 
Thanks for posting my original "quick start" page. The purpose of the circuit is to compensate for draw down on the battery from the starter motor during cranking.
Any "top hat" diode will work, as it just augments the coil voltage until the engine starts,
using low current. Be sure to install the diode as shown to prevent energizing the coil after starting. Polarity is important. If "backward" the engine will start without turning on the ignition. Just reverse the connections to the diode.
 
Would it be simpler to use a relay to achieve the same goal?
Is this a band aid to try and overcome a problem elsewhere?
Millions of Fords started without a hot wire jump, why would we need one now?
Not knocking it, just curious as to why one would be required.
Mart.
 
Strictly opinion... and we all know what that's worth... Sure, it's a 'band-aid'. But, jillions of cars/pickups are using this 'band-aid'. Factory installed, the full battery voltage is picked up off of a terminal on the starter switch (starter solenoid). Ford and Chevy started doing this, 55 for Chevy, 56 for Ford. That's why you see those two small terminals on the starter switch, labelled 'I' (for ignition), and 's' (for start). 'I' gets battery voltage when you hit 'start'.

(For Mart... Do I recall that you have a 'Bird'? 56 or newer? If so, I believe you will find the 'I' terminal on your starter switch....)
 
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Here is how the factory did it. This diagram is copied from the 64 Ford factory service manual for a pickup.
Notice the resistor wire to the coil, and a 'jumper' wire from the 'I' terminal, that bypasses the resistor wire.
So, if there is no 'I' terminal (as per old 6V Fords), use the jumper wire, basically as shown, but connected to the 'big' starter terminal, and put a diode in it (to prevent running ignition voltage being 'back-fed' to the starter).
 

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The solenoid is for a 9N tractor. The solenoid looks like a conventional Ford but the mounting bracket is unique. No I terminal.
 
bobH said:
Here is how the factory did it. This diagram is copied from the 64 Ford factory service manual for a pickup.
Notice the resistor wire to the coil, and a 'jumper' wire from the 'I' terminal, that bypasses the resistor wire.
So, if there is no 'I' terminal (as per old 6V Fords), use the jumper wire, basically as shown, but connected to the 'big' starter terminal, and put a diode in it (to prevent running ignition voltage being 'back-fed' to the starter).
Click to expand...
484676
 
As I am electrically disconnected I sure would appreciate one of you electrical engineer types to splain to me how the diode set-up works; the physical wiring layout is no problem. Regarding the 64 Ford (and many others) what they are doing is jumping out the resistor for a full 12V to the coil on starting and then running on 6V (or 9 or whatever the design voltage is for the coil). This I get. Thanks Don
 
Mart>>>Millions of Fords started without a hot wire jump, why would we need one now?>>>


Agreed. I never had to use one either. When I switched to 12v, I got the 2-terminal solenoid described by bobH just in case. Same price as the single terminal. So why not. But never had to use the bypass terminal. Always started except when the resistor burned out. It does get hot.
 
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