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MG MGB Technical - Diode, effect of

I am trying to supply power to the coil when the starter is engaged because it makes the car start easily. The starter relay provides power to the coil and the ignition relay is providing power at the same point. The ignition relay is "off" when the starter relay is on. However, it seems that current is flowing back up the circuit to the starter because the starter remains engaged when the ignition relay is "on". Will a diode prevent "backflow"?

A diode has an anode and a cathode end. It seems that power flows from anode to cathode. But, do the dc molecules actually flow from negative to positive? If so, should the cathode end be pointed toward the positive side?

Thanks.
Charles Edwards

Sounds as though you are busy generating a headache for yourself. You don't say if the car is equipt with a ballasted coil. If it is, just wire it correctly. A diode on the wire to the starter relay with the cathode toward the coil would prevent current from flowing from the ignition relay to the starter (on a negative ground car), but would also reduce the voltage from the starter relay by about .7 volts. Why would the ignition relay be off when starting?
George B.

George. The ignition relay is powered as part of the white wire circuit which is energized when the key is in the run position rather than when in the start position.

Charles. It sounds as if you are describing a condition within which the starter is still turning over when the engine has already started and the ignition switch key has been moved to the run position. To me, this would indicate one of two problems. First would be the starter relay is sticking in the on position and continues to energise the starter solenoid. Second, the starter solenoid is bad and does not allow the starter relay to disengage when the key switch is moved from the start postion to the run position. As George notes, the problem of current flowing to the starter, after the key has been turned from the start position, should not be corrected by a "patch", but should be corrected by finding out what the root cause of the problem is and correcting that. Paul Hunt is the expert on these things and should be able to provide some guidance tomorrow. If he does not post, you might contact him through his website. Les
Les Bengtson

Hi there Paul, Les.



I too have a late coil ( 6v ) fitted to my early car. So It would be nice to have 12v at start time, can I just jumper from the start spade on the starter motor, red and white I think it is looking at this http://www.mgb-stuff.org.uk/ site, To the coil. This I think would not effect starting motor volts.
Darcy

Darcy. If you have an "early car", and have not wired in a ballast resistor, you have a full 12V feeding your 6V coil all the time. The RB cars had a second spade terminal on the starter which was energised when the starer was energised. This terminal was connected to the coil and provided a full 12V to the system. When the ignition switch was moved to the run position, after the car had started, the white wire circuit provided a reduced voltage to the coil. If you are running a constant 12V input to your 6V coil, it will cause problems at some point. Why are you doing this? Les
Les Bengtson

Hello Les, Sorry I should have explained my car better.

My coil was fitted by the RAC man when the car broke down, he also fitted a resistor next to it. He did not have a 12v coil in his van, but was lucky to think this 6v one would work. He did explain that the starting would not be as good with this new 6v coil.
Darcy

A diode is one way of supplying the boost voltage during cranking to the 6v coil of a ballasted system if you are using a solenoid that doesn't have the extra contact for the white/light green wire. You can't just connect the coil +ve to the solenoid operate terminal (or its equivalent on the starter relay), 'cos the running voltage at the coil may well be enough to keep the solenoid operated, and even if it doesn't will have the effect of noticeably reducing the running voltage of the coil. Diodes usually have the positive end marked in some way, current flows from its negative end to its positive, which is in the direction of the 'arrow' if you look at its circuit symbol. Current flows from the +ve terminal of the battery to its -ve, so the -ve end of the diode is connected to the +ve supply i.e. the wire from starter relay to solenoid, and its +ve terminal to the coil +ve. You will need a diode capable of carrying a minimum of 8 amps (10v across 1.5 ohms), say 10 amps for safety.

An alternative method is to either use a starter relay with two make contacts, or use a second single-make contact relay operated from the starter relay, using the extra contact in both cases to connect 12v to the coil +ve.

Darcy - that may have got you home but you would be better off reversing what the AA man did and fitting a 12v coil.
Paul Hunt

Paul,

All the diodes with which I am familiar have the cathode marked. The cathode must be at a more negative voltage potential than the anode for current to flow, and obviously, at a more positive voltage potential to prevent current flow. The direction of current flow is immaterial for most DC purposes, no?
George B.

Gentlemen, thank all of you very much for your help. Paul, relay to relay is an excellent idea.
Charles Edwards

George - surely if a circuit requires a diode then the direction of current flow *is* material. Whilst there are few components on the MGB where current flow direction matters e.g. electronic tach, alternator, radio, electronic or diode-suppressed fuel pump compared to the number of components where the direction does not matter, in modern cars this is certainly not the case, they are riddled with them, and doing a conversion to +ve ground would be a challenge.
Paul Hunt

Thank you Paul and Les. I shall get a new coil.
Darcy

Paul,

You misunderstood my meaning. Yes, the polatity matters, but the direction of flow depends on what theory (ei.holes vs. electrons) is being used. You can see that I understand the polatity issue if you noticed the direction I placed the diode and my reference to the eathing of the car.
George B.

George,

This BBS is surely not the place to be talking about semiconductor physics :o)


All; if I wanted to be absolutely sure of getting the maximum coil voltage when cranking, I would use a relay instead of a diode as there is a 0.6 volt drop across the diode, whereas there would be none across the relay's contacts (well, nearly none). If being spectacularly attentive, I might also run a separate cable from the battery to power the coil during this time, so that the voltage drop across the starter's feed wire only affects the speed of cranking.

In all, I like having a 12V coil and a healthy 12V battery 'cos my car fires first compression every time even when it's cold :o)

ttfn,
--
Olly
Oliver Stephenson

With sound connections you will only be getting about 0.2v dropped between the battery post and the solenoid post, by far the greatest volt-drop during cranking is inside the battery and will be 2v or more. A relay *is* probably safer, though, as those owners of certain North American spec cars have found out when the brake-test diode goes short-circuit - pulling the handbrake up energises the starter!
Paul Hunt

This thread was discussed between 14/01/2003 and 16/01/2003

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