Alternator exciter circuit lamp parallel resistor, what size?

[DHV90]

Evening all looking for a bit of guidance.
Im finishing an unfinished project boat and am trouble shooting the no-charge on both 12 and 24v alternators. The exciter circuit indicator lamps are both rated at 3mA so wont pull enough power to the coils from my limited understanding

The dash is reasonably nice and adding a bulb holder big enough to take a 3w incandescent bulb would look rough, so I would like to, if it is possible, use the parallel resistor method to draw current to the field coils and keep the existing idiot lights (for now at least)

Can anyone explain the maths to me so I can figure out which size resistors to wire in to each side to try and ensure the alternators cut in at a low rpm, and at roughly the same time.
Is roughly the same wattage as an incandescent idiot light bulb (3 or 4 watts) and fairly low Ohms of resistance be sufficient?

Many thanks

 

[PaulRainbow]

Evening all looking for a bit of guidance.
Im finishing an unfinished project boat and am trouble shooting the no-charge on both 12 and 24v alternators. The exciter circuit indicator lamps are both rated at 3mA so wont pull enough power to the coils from my limited understanding

The dash is reasonably nice and adding a bulb holder big enough to take a 3w incandescent bulb would look rough, so I would like to, if it is possible, use the parallel resistor method to draw current to the field coils and keep the existing idiot lights (for now at least)

Can anyone explain the maths to me so I can figure out which size resistors to wire in to each side to try and ensure the alternators cut in at a low rpm, and at roughly the same time.
Is roughly the same wattage as an incandescent idiot light bulb (3 or 4 watts) and fairly low Ohms of resistance be sufficient?

Many thanks

Round figures a 3w lamp at 12v is 50 ohms and at 24v is 200 ohms

 

[VicS]

Evening all looking for a bit of guidance. Im finishing an unfinished project boat and am trouble shooting the no-charge on both 12 and 24v alternators. The exciter circuit indicator lamps are both rated at 3mA so wont pull enough power to the coils from my limited understanding
The dash is reasonably nice and adding a bulb holder big enough to take a 3w incandescent bulb would look rough, so I would like to, if it is possible, use the parallel resistor method to draw current to the field coils and keep the existing idiot lights (for now at least)
Can anyone explain the maths to me so I can figure out which size resistors to wire in to each side to try and ensure the alternators cut in at a low rpm, and at roughly the same time.
Is roughly the same wattage as an incandescent idiot light bulb (3 or 4 watts) and fairly low Ohms of resistance be sufficient?

Many thanks

If you know the power, in watts, of the bulb that would normally be fitted you can calculate the value of the resistor needed in its place from R = V² / W
In place of a 3 watt bulb on a 12 volt system that works out at 48 ohms .. call it 50.
Dont forget the resistor will produce 3 watts of heat and must be rated accordingly... Dont use one of the little resistors found in electronic circuits

 

[PaulRainbow]

If you know the power, in watts, of the bulb that would normally be fitted you can calculate the value of the resistor needed in its place from R = V² / W
In place of a 3 watt bulb on a 12 volt system that works out at 48 ohms .. call it 50.
Dont forget the resistor will produce 3 watts of heat and must be rated accordingly... Dont use one of the little resistors found in electronic circuits

That's what i said in post #2

 

[vas]

Morning all,

I hope I'm not hijacking the thread too much, but I have a very similar issue on my boat.
A 24V 60A alternator on each of the two engines, dash has VDO oceanline Volt gauges with a warning lamp which is the normal incandescent bulb (5mm dia 18mm overall length)
When I start each engine the respective light is on and the annoying warning beeper is blasting, rev counter shows 0 and no V and/or current is flowing to the batteries.
Unless I blip the throttle to 800-850rpm, nothing will change. Once engine is revved above the said limit, rev counter wakes up and beeper and warning light on gauge turns off, everything is normal till the next start.
Obviously I can drop revs to idle (circa 600) and everything still fine. Running with single morse on each engine this means every startup has to be on lower helm, start, disengage gear, rev, drop revs repeat on the other side. Nothing too complicated, but annoying nevertheless!

Assuming the bulb is a few W (3 iirc) what am I missing? will a 3W resistor (that's the big square ceramic ones 1in long, right?) help? If so, the 200Ω that Paul mentions in post #2 or more?
And do I fit it across the two bulb terminals behind the gauge on dash or at the alternator side (not that I'm sure which terminal that would be...) Alternator is VALEO (iirc) engines are IVECO
I wouldn't dismiss wrong wiring by the PO or installer when the DD were replaced 25yrs ago as all work done to the boat was atrocious...

cheers

V.

 

[DHV90]

Paul and Vic thank you both, that is exactly what I needed to know.
And just to confirm, wired directly in parallel to the bulb is the correct installation? Ie behind the dash, with the wires of the resistor wired to the terminals on the bulb holder?

vas - obviously wait for more knowledgeable responses but the way i understand it is if your alternators do cut in correctly, just not at a low enough rpm, you can try a slightly higher wattage of bulb, like if its a 2 watt bulb try a 3, if it is a 3 then try a 4, which should give a little extra power to the coils to allow the alternator to charge at low rpm. The resistor figures above are basically to replace a bulb as my bulbs draw virtually nothing, so if you put those resistors in parallel it would be too much. a smaller resistor could work but step one is to check the wattage of the bulbs you have in the dash.

 

[PaulRainbow]

Morning all,

I hope I'm not hijacking the thread too much, but I have a very similar issue on my boat.
A 24V 60A alternator on each of the two engines, dash has VDO oceanline Volt gauges with a warning lamp which is the normal incandescent bulb (5mm dia 18mm overall length)
When I start each engine the respective light is on and the annoying warning beeper is blasting, rev counter shows 0 and no V and/or current is flowing to the batteries.
Unless I blip the throttle to 800-850rpm, nothing will change. Once engine is revved above the said limit, rev counter wakes up and beeper and warning light on gauge turns off, everything is normal till the next start.

This is very common on boats Vas, my own boat does it too, but it's no big deal for me to blip the throttle so i've never bothered to do anything about it.

Obviously I can drop revs to idle (circa 600) and everything still fine. Running with single morse on each engine this means every startup has to be on lower helm, start, disengage gear, rev, drop revs repeat on the other side. Nothing too complicated, but annoying nevertheless!

Assuming the bulb is a few W (3 iirc) what am I missing? will a 3W resistor (that's the big square ceramic ones 1in long, right?) help? If so, the 200Ω that Paul mentions in post #2 or more?
And do I fit it across the two bulb terminals behind the gauge on dash or at the alternator side (not that I'm sure which terminal that would be...) Alternator is VALEO (iirc) engines are IVECO
I wouldn't dismiss wrong wiring by the PO or installer when the DD were replaced 25yrs ago as all work done to the boat was atrocious...

If we assume it is a 3w bulb, you shouldn't need to add a 200Ω resistor, perhaps another watt would be enough.

1 watt at 24v = 0.042a, resistance(ohms) = volts/ amps, so R=24V / 0.042A (571Ω) So to add 1 watt, fit a 500Ω resistor.

2 watts at 24v = 0.083a, resistance(ohms) = volts/ amps, so R=24V / 0.083A (289Ω)

If you have some spare, low wattage, bulbs lying around you could experiment with those. If, for instance, you have a 5w bulb you can fit that temporarily in parallel to the 3w lamp holder, but remove the 3w lamp. if this cures your issue you could change the lamps to 5w as suggested in post #6, but they might be brighter than you like (you also have to consider they might get hotter than ideal too), or you could just add a 300Ω resister in parallel.

The resistors can be added anywhere that's convenient, just bare in mind that if they are can get hot.

 

[PaulRainbow]

And just to confirm, wired directly in parallel to the bulb is the correct installation? Ie behind the dash, with the wires of the resistor wired to the terminals on the bulb holder?

That will work, bare in mind they give off a little heat.

 

[DHV90]

awesome thank you mate, really clear advice, much appreciated. Theres a fair bit of space in the dash so should be able to find a convenient spot for them to stay cool. Thanks!

 

[VicS]

awesome thank you mate, really clear advice, much appreciated. Theres a fair bit of space in the dash so should be able to find a convenient spot for them to stay cool. Thanks!

Mount them properly, rather than dangle in the wiring.
I would opt for a 5 watt ( 50 ohm) wire wound resistor resistor for the 12 volt alternator. It'll be nearly an inch long by about 3/8" diameter.

The same calculation as I did earlier but for 24 volts suggests a 200 ohm resistor however I am doubtful if this will provide a high enough excitation current. If you find it does not, reduce it to a 100 ohm resistor, which will provide the same current as 50 ohms does on 12 volts. ( but it will have to be rated higher, say 7 watts )

 

[PaulRainbow]

Mount them properly, rather than dangle in the wiring.
I would opt for a 5 watt ( 50 ohm) wire wound resistor resistor for the 12 volt alternator. It'll be nearly an inch long by about 3/8" diameter.

Still parroting what i said in post #2

The same calculation as I did earlier but for 24 volts suggests a 200 ohm resistor

Again, posting the same as post #2, why do you do this ?

 

[TernVI]

The resistance of a cold bulb is quite low.
The regulator circuit is designed to use a bulb.
Why not use a bulb?

 

[vas]

This is very common on boats Vas, my own boat does it too, but it's no big deal for me to blip the throttle so i've never bothered to do anything about it.



If we assume it is a 3w bulb, you shouldn't need to add a 200Ω resistor, perhaps another watt would be enough.

1 watt at 24v = 0.042a, resistance(ohms) = volts/ amps, so R=24V / 0.042A (571Ω) So to add 1 watt, fit a 500Ω resistor.

2 watts at 24v = 0.083a, resistance(ohms) = volts/ amps, so R=24V / 0.083A (289Ω)

If you have some spare, low wattage, bulbs lying around you could experiment with those. If, for instance, you have a 5w bulb you can fit that temporarily in parallel to the 3w lamp holder, but remove the 3w lamp. if this cures your issue you could change the lamps to 5w as suggested in post #6, but they might be brighter than you like (you also have to consider they might get hotter than ideal too), or you could just add a 300Ω resister in parallel.

The resistors can be added anywhere that's convenient, just bare in mind that if they are can get hot.

thanks Paul,
checked and the bulbs on the lower helm VDO gauges are these:
24V T5 capless 40mA 1 or 2W (unclear)
7323113 Indicator Lamp T5 Capless, 24V 40ma W2x4.6d, 5x20mm

Then remembered that originally boat had two fugly dashes one on each helm and there were T10 capless W3W bulbs on each behind the battery indicator warning light.
I really don't remember if this problem didn't exist back then as I only used the boat for half a dozen times in 2011 and then everything was ripped as part of the four+ year rebuilt, so it may be that the two T10 bulbs were enough to have things working properly.

TBH, no way I'm going to refit these ugly beasts on my dash, but I'll temporarily wire one (and then two ) in parallel to my tiny bulb and see if things improve, then I'll know what resistor to add to the system. Could even do a tiny panel and place them on the e/r out of the way...

Will let you know.

cheers

V.

 

[PaulRainbow]

thanks Paul,
checked and the bulbs on the lower helm VDO gauges are these:
24V T5 capless 40mA 1 or 2W (unclear)
7323113 Indicator Lamp T5 Capless, 24V 40ma W2x4.6d, 5x20mm

Then remembered that originally boat had two fugly dashes one on each helm and there were T10 capless W3W bulbs on each behind the battery indicator warning light.
I really don't remember if this problem didn't exist back then as I only used the boat for half a dozen times in 2011 and then everything was ripped as part of the four+ year rebuilt, so it may be that the two T10 bulbs were enough to have things working properly.

TBH, no way I'm going to refit these ugly beasts on my dash, but I'll temporarily wire one (and then two ) in parallel to my tiny bulb and see if things improve, then I'll know what resistor to add to the system. Could even do a tiny panel and place them on the e/r out of the way...

Will let you know.

cheers

V.

The T5s are 1w, so those T10s at 3w might just do the trick.

Look forward to hearing how it goes

 

[vas]

The T5s are 1w, so those T10s at 3w might just do the trick.

Look forward to hearing how it goes

well that was quick!
1XT5 (1W) is not enough, adding a second though is just fine! so had some spots to add warning lights on my new dash and have two one on each side waiting for me to think of something to wire them to, so job done

thanks a lot Paul, you're a star!

V.

 

[PaulRainbow]

well that was quick!
1XT5 (1W) is not enough, adding a second though is just fine! so had some spots to add warning lights on my new dash and have two one on each side waiting for me to think of something to wire them to, so job done

thanks a lot Paul, you're a star!

V.

That's great Vas, glad to hear it's sorted

 

[vas]

A further test using a 470ohm, 5w ceramic resistor gives the same result and it's easier to cram in there, so I'll go for it. Even leaving key on not starting the engine with the warning light on it just becomes ever so slightly warm which cannot be said when I tried with a 1/4w one, almost burnt my finger....

Cheers
V

 

[DHV90]

Ive just tried the resistor in parallel to the bulb with no joy, is it definitely meant to be wired in parallel to the indicator lamp? My electronics is really basic but I thought that the current would favour the path of least resistance, and basically bypass the resistor?

Vas - how did you install your resistor, and why did you go with 470 ohm out of curiosity?

Thanks again for all the invaluable advice!

 

[VicS]

Ive just tried the resistor in parallel to the bulb with no joy, is it definitely meant to be wired in parallel to the indicator lamp? My electronics is really basic but I thought that the current would favour the path of least resistance, and basically bypass the resistor?

Vas - how did you install your resistor, and why did you go with 470 ohm out of curiosity?

Thanks again for all the invaluable advice!

There has to be a resistor in series with the LED because the LED operates at typically only around 2 volt... Stick 12 volts straight onto an LED and it will swiftly become an ExLED.
If you happen to have an old Maplin catalogue you will find the details of how to calculate the value of the series resistance to enable an LED to run on 12 volts from its normal forward volts and forward current.

An LED does not provide enough current for the initial excitation of the alternator therefore there has to be a parallel circuit via a resistor ( Typically ITYWF of about 50 ohms.} If the engine and alternator package is designed to operate with LED warning lights this resistor will exist somewhere. Some alternators have it internally and simply need a second direct feed from the ignition switch Where they dont it will be somewhere in the panel or wiring.

 

[DHV90]

There has to be a resistor in series with the LED because the LED operates at typically only around 2 volt... Stick 12 volts straight onto an LED and it will swiftly become an ExLED.
If you happen to have an old Maplin catalogue you will find the details of how to calculate the value of the series resistance to enable an LED to run on 12 volts from its normal forward volts and forward current.

An LED does not provide enough current for the initial excitation of the alternator therefore there has to be a parallel circuit via a resistor ( Typically ITYWF of about 50 ohms.} If the engine and alternator package is designed to operate with LED warning lights this resistor will exist somewhere. Some alternators have it internally and simply need a second direct feed from the ignition switch Where they dont it will be somewhere in the panel or wiring

Thanks Vic, i understand those things, the bulb in question is a filament bulb but a tiny one that runs on a fraction of a watt, not an LED but not enough to excite the alternator (from what I understand)

Im just confused about why the resistor needs to be in parallel to the small filament bulb rather than in series

Im going to try a bigger filament bulb over the next few days to simplify matters, but would like to understand whats happening a bit more rather than avoiding it as it might help in the future.