Puzzling Alternator bulb question?

[Sunnydays]

We've had our n/boat for just over a year and I've never had a lot of confidence in the original Barrus Shire 10 year old instrument panel which had a suspect ignition switch (lose, floopy key slot) and a tacho that never moved. So I replaced it with the latest version which I rather naively thought would be a straight swap - Same size, same block connecter etc. There was however some wiring mods needed one of which was due to now having individual battery/alternator lights for both the starter battery and the leisure batteries. I explained this to Barrus as they originally told me it would simple swap and they, all credit to them,  sent a guy out to the marina within 48 hours to modify the block connector etc to get things connected. However he identified that the starter battery wasn't charging due to a faulty alternator which was the reason for the tacho problem and the battery/alternator light staying on.

So it was frustrating that having fitted the newly refurbished alternator, both the above problems still continued. However following a suggestion from another marina colleague,  I disconnected the LED bulb and holder from the back of the panel and reconnected to a traditional 12 volt/2w bulb and holder. On starting up....hey presto the light went out and the tacho needle sprung into life!.

So....whilst some might say - well that's the problem sorted....I am puzzled by such things?  

Comments welcome but please bear in mind those of you who posses a far greater understanding of electrical matters that my knowledge is limited and I am merely keen to know whether by simply changing the bulb from an LED really has solved the problem and why the starter battery alternator would work with the LED? 

Thanks in advance

 

[furnessvale]

No doubt far more electrically savvy people than me will be along shortly, but I understand that the lamp in these circuits is more than just an indicator.  It performs a vital role in providing some sort of resistance to the circuit and its watts rating is critical.

Perhaps the lamp you replaced was not the correct rating?

George

[Jen-in-Wellies]

Exactly that. Most alternators of types fitted to boats use the current through the warning bulb to power up some of the windings inside. Once powered up it is self sustaining. Led lights have too much resistance and won't let enough current through, so the alternator doesn't start charging. If you have an led alternator warning light, then you need either a bulb in parallel, hidden away, or a similar value resistor wired in parallel across the led. 12V led warning lights have a high resistance in series built in that causes the current restriction. Without them the Led would burn out the first time it is used, biut is too high for the alternator to start charging.

Jen

[billh]

The normal alternator set up requires  a current through the lamp at start up to excite the alt. field. If it was an LED that would not provide that current unless it was shunted with a resistance of a particular value. The old system used a 12v 2.2w filament bulb which does provide the exciter current. Once the alternator is generating, the voltage across the warning lamp falls to zero and thus it goes out, i.e. there is 12 (or24) volts on either side of the lamp. Clear as mud?

Just what Jen said!

Edited December 11, 2017 by billh

[Jen-in-Wellies]

Reply clash!

[WotEver]

A 75ohm 10W resistor in parallel with the LED would probably do it but there’s no point if you’ve already replaced it with a filament bulb. 

http://uk.farnell.com/ohmite/810f75re/wirewound-resistor-75-ohm-10w/dp/1762307

[Sunnydays]

Thanks folks - I knew there would be an explanation. If its sorted the problem and not going to do any damage then I'll leave the filament bulb and holder in place . Does lead me to another question though.... how come the monitor light for larger alternator (leisure batteries) doesn't have the same problem?

[WotEver]

57 minutes ago, Sunnydays said:

Thanks folks - I knew there would be an explanation. If its sorted the problem and not going to do any damage then I'll leave the filament bulb and holder in place . Does lead me to another question though.... how come the monitor light for larger alternator (leisure batteries) doesn't have the same problem?

Some alternators self-excite. Some need very little encouragement to excite. 

[nicknorman]

1 hour ago, Sunnydays said:

Thanks folks - I knew there would be an explanation. If its sorted the problem and not going to do any damage then I'll leave the filament bulb and holder in place . Does lead me to another question though.... how come the monitor light for larger alternator (leisure batteries) doesn't have the same problem?

Are you sure it’s an LED bulb?

[bizzard]

A 1.2w grain od wheat bulb probably, they look similar to an LED. We used them to light up model railway buildings and things. 

[Sunnydays]

31 minutes ago, nicknorman said:

Are you sure it’s an LED bulb?

Yep pretty sure . All 5 lights look like these...https://www.autoelectricsupplies.co.uk/mobile/product/711/category/57

[forsberg]

It's not good practice to have things dangling behind a panel. But you may have sorted that. I do agree on the resistor suggested above as in 5 years time when the bulb breaks you may have forgotten it is there and do unnecessary repairs. Better still is to mount it close to the alternator.

 

[Jen-in-Wellies]

17 minutes ago, forsberg said:

It's not good practice to have things dangling behind a panel. But you may have sorted that. I do agree on the resistor suggested above as in 5 years time when the bulb breaks you may have forgotten it is there and do unnecessary repairs. Better still is to mount it close to the alternator.

 

The resistor needs to be in parallel with the LED, so behind the panel is probably the best place. As Watever says it will need to be a high power one and they are quite big and chunky. I fitted one on a friends boat when he found his new LED panel light stopped the alternator working.

Jen

[Flyboy]

32 minutes ago, Sunnydays said:

Yep pretty sure . All 5 lights look like these...https://www.autoelectricsupplies.co.uk/mobile/product/711/category/57

Sorry but I think Nicks correct. If you look at the Barrus manual for the latest panel it describes how to change the bulb by pulling it out of the holder. There is no mention of what type of bulb it is however. The LED warning light you suggest does not seem have a replaceable LED. No resistor across the bulb holder is shown on the Barrus  wiring diagram which says to me that the bulb is a standard 1.2W or 2W capless type.

Edited December 11, 2017 by Flyboy

[Tony Brooks]

1 hour ago, bizzard said:

A 1.2w grain od wheat bulb probably, they look similar to an LED. We used them to light up model railway buildings and things. 

I have yet to find one of those that energises the alternator properly. if at all.

[Flyboy]

6 minutes ago, Tony Brooks said:

I have yet to find one of those that energises the alternator properly. if at all.

An LED certainly won't unless it has a resistor shunt across it.

[Sunnydays]

I am pretty sure they are LED but then why would Barrus fit them to a new panel if they dont work with the alternator? I think I'll  call them tomorrow. 

28 minutes ago, Flyboy said:

An LED certainly won't unless it has a resistor shunt across it.

Abd if they are meant to run with a resistor then surely the guy sent out from the factory would have fitted one?

[bizzard]

41 minutes ago, Tony Brooks said:

I have yet to find one of those that energises the alternator properly. if at all.

Ok for dynamo's. It only had to go out when the charge began.

[Flyboy]

16 minutes ago, Sunnydays said:

I am pretty sure they are LED but then why would Barrus fit them to a new panel if they dont work with the alternator? I think I'll  call them tomorrow. 

Abd if they are meant to run with a resistor then surely the guy sent out from the factory would have fitted one?

If it is an LED then it would have a physically large (47ohm 2W) resistor inside the warning light wired across the LED, which is unlikely. Why don't you give Barrus a ring and ask them to clarify? No resistor is shown on the wiring diagram which is the same as for the old panel alternator warning light.

Edited December 11, 2017 by Flyboy

[Jen-in-Wellies]

The OP is fitting a current Barrus panel to an older Barrus engine. It is possible that the current engines have both alternators not requiring the low resistance bulbs to work properly and have LED warning lights. Modern cars are more likely to have led warning lights than incandescent bulbs, so this style of alternator has to be pretty common now. Looks like they have sold him a new panel without realising the implications for the older alternator. Quiet possible as not many people know about this issue.

Jen

Edited December 11, 2017 by Jen-in-Wellies

[Sunnydays]

50 minutes ago, Jen-in-Wellies said:

The OP is fitting a current Barrus panel to an older Barrus engine. It is possible that the current engines have both alternators not requiring the low resistance bulbs to work properly and have LED warning lights. Modern cars are more likely to have led warning lights than incandescent bulbs, so this style of alternator has to be pretty common now. Looks like they have sold him a new panel without realising the implications for the older alternator. Quiet possible as not many people know about this issue.

Jen

Yep this is definately the latest panel and as far as i know they are the original alternators ....so 10 or more years old. Odd that the light from the liesure alternator goes out though? Will call Barrus tomorrow as suggested. Cheers again for your interest and help.

[Chewbacka]

2 hours ago, Sunnydays said:

Yep this is definately the latest panel and as far as i know they are the original alternators ....so 10 or more years old. Odd that the light from the liesure alternator goes out though? Will call Barrus tomorrow as suggested. Cheers again for your interest and help.

I was told many years ago that some alternators had enough residual magnetism that they can self excite without needing a light, maybe that is the case with your leisure alternator.

[ditchcrawler]

3 minutes ago, Chewbacka said:

I was told many years ago that some alternators had enough residual magnetism that they can self excite without needing a light, maybe that is the case with your leisure alternator.

My old leisure alternator use to self excite but it was a heavy 24 volt lorry one, the new modern one doesn't which is a shame.

[Sir Nibble]

For lots of alternators the lamp is switched by the regulator and plays no part in excitation.

[WotEver]

3 hours ago, Sir Nibble said:

For lots of alternators the lamp is switched by the regulator and plays no part in excitation.

Well said, that man