Diode in the ignition light circuit

[Keeping Up]

The ignition warning lamp on Keeping Up's dashboard always glows very very slightly, as a result (much to my surprise) of the 0.6 volts that appear across it in the reverse direction because of the voltage drop in the charge-splitter diode. Sometimes in the dark this confuses me into thinking that there may be a fault, and I've often thought about putting a diode in series with the lamp. Somehow I find myself with a nagging doubt though, are there any possible snags with doing this? Are there any circumstances under which it could damage anything in the alternator, or could cause the lamp not to indicate a fault when in fact one does exist? I think I've convinced myself there is no possibility of it ever causing the lamp to glow when there isn't a fault.

[Scotty]

I cured a similar problem by using a tiny relay,

 

replace the lamp with the coil and feed a new supply for the lamp through the contacts

 

should work?

 

 

 

 

 

But on second thoughts it might be better to cure the problem, not the treat symptom

 

 

 

 

Dr Scott

Edited July 14, 2007 by Scotty

[dor]

Sounds like one for Gibbo.

[Sir Nibble]

A diode in the warning light cable will do no harm at all. However, as you fear, it would prevent the lamp indicating an alternator +ve diode fault. A higher wattage lamp, or a parallel resistor should also help, about 48 ohms?

 

edited to say, since you only want the diode to equalise volt drops, connect two back to back. ie, one going one way and one the other, in parallel, and that should solve the problem.

 

But on second thoughts it might be better to cure the problem, not the treat symptom

In this case, I think the symptom is the problem.

Edited July 16, 2007 by snibble

[RobinJ]

The ignition warning lamp on Keeping Up's dashboard always glows very very slightly, as a result (much to my surprise) of the 0.6 volts that appear across it in the reverse direction because of the voltage drop in the charge-splitter diode. Are there any circumstances under which it could damage anything in the alternator, or could cause the lamp not to indicate a fault when in fact one does exist?

After discussion with hire boat engineers I moved away from using splitters in favour of charge relays, as it eventually prevents the batteries working to their full potential in high charge/discharge situations.

In theory there is no reason why you could not fit two diodes (each way in parallel) in the warning light connection to the alternator to compensate. The first one enables the current to flow from the warning light to energise the alternator, the second allows the current to extiguigh the warning light when the alternator is running. This should work as it drops the alternator vaoltage by the extra 0.6V that the batteries are not receiving!

[John Orentas]

The ignition warning lamp on Keeping Up's dashboard always glows very very slightly,

 

 

I think you have answered your own question, you simply have a slight leakage through the diode so you don't really have a problem at all. It is worth saying again however that diode type split chargers should only be used if you have a 'machine sensed' alternator which seem to be becoming increasingly rare, otherwise you will experience a massive reduction in charge rates.

[chris w]

It is worth saying again however that diode type split chargers should only be used if you have a 'machine sensed' alternator which seem to be becoming increasingly rare, otherwise you will experience a massive reduction in charge rates.

 

That's incorrect. One needs a "battery sensed" alternator which will compensate for the voltage drop across the diodes. There will then be no reduction in charge rate.

 

A "machine-sensed" alternator will cause the very problem you are trying to circumvent.

 

Chris

[Sir Nibble]

That's incorrect. One needs a "battery sensed" alternator which will compensate for the voltage drop across the diodes. There will then be no reduction in charge rate.

 

A "machine-sensed" alternator will cause the very problem you are trying to circumvent.

 

Yea, that occurred to me. This is the problem with split charge diodes, some have an extra terminal for the warning light, but the glow is showing because of voltage lost across the diode.

[John Orentas]

That's incorrect. One needs a "battery sensed" alternator which will compensate for the voltage drop across the diodes. There will then be no reduction in charge rate.

 

A "machine-sensed" alternator will cause the very problem you are trying to circumvent.

 

Yea, that occurred to me. This is the problem with split charge diodes, some have an extra terminal for the warning light, but the glow is showing because of voltage lost across the diode.

 

 

Yes, quite right wrong way round again. The point is that the alternator needs to look at the voltage at the batteries rather than that at it's own output. The alternator needs to increase it's output voltage to compensate for that lost through the diodes.

[Sir Nibble]

If by some chance you happen to have the common-or-garden A127 alternator*, you can sidestep the whole thing by popping the regulator off and putting a diode in the yellow lead from rectifier to regulator.

 

*other alternators are available.

[chris w]

If by some chance you happen to have the common-or-garden A127 alternator*, you can sidestep the whole thing by popping the regulator off and putting a diode in the yellow lead from rectifier to regulator.

 

*other alternators are available.

 

 

............................and just to answer the next question, if you follow Snibble's suggestion, the cathode of the diode (the banded end) should be towards the regulator side.

 

Chris

[smileypete]

The ignition warning lamp on Keeping Up's dashboard always glows very very slightly, as a result (much to my surprise) of the 0.6 volts that appear across it in the reverse direction because of the voltage drop in the charge-splitter diode. Sometimes in the dark this confuses me into thinking that there may be a fault, and I've often thought about putting a diode in series with the lamp. Somehow I find myself with a nagging doubt though, are there any possible snags with doing this? Are there any circumstances under which it could damage anything in the alternator, or could cause the lamp not to indicate a fault when in fact one does exist? I think I've convinced myself there is no possibility of it ever causing the lamp to glow when there isn't a fault.

 

Can you get the bulb out? If so try some permanent marker on it, to reduce the light output.

 

Some other ideas:

 

Another way might be to put an led/resistor across the bulb and use that as the indicator.

 

I wonder if a small resistor would work as well as the bulb for a load?

 

What would be good is a panel that uses high brightness sunlight viewable LEDs for indicators, that also dim at night.

 

cheers,

Pete.

[Keeping Up]

... since you only want the diode to equalise volt drops, connect two back to back. ie, one going one way and one the other, in parallel, and that should solve the problem.

Brilliant! I wish I'd thought of that, thanks. I knew it was a good idea to ask the question here. I'll just go and get another diode out of the cupboard before I forget.

 

In answer to other people's comments, yes I do have a "battery sensing" system (by means of an Adverc controller) and the charging system works very well thank you. Yes there are some problems with splitter diodes but almost all of them are circumvented by the use of a proper controller - and even "split-charging" relay systems have a few problems of their own which are best cured by inserting a diode in each leg (!)

[Sir Nibble]

We aim to please. But don't thank me, Nobber thought of that, I just type faster. (It's difficult with a trunk).