LED lights - What about the rheostat ?

[GaryBZ]

Just fitted a set of LED lights on my TR4a. Very happy with the overall performance. The headlight bulbs were difficult to fit, but I managed to install them without having any external wires that are visible under the bonnet. Everything works great apart from the number plate lights. The new bulbs are too big and don’t fit. One other thing to mention is the ignition light. You don’t get a bulb for this in the kit. Having spoken to the supplier they said it can cause a drain on the battery when the car isn’t in use. No big deal as a new standard bulb is fine in this position.

I have some questions about the rheostat. Has anyone found a rheostat that fits and will work with LEDs? I have seen a lot of posts about joining the wires up at the back of the rheostat to bypass it completely. The supplier says this is the best method to achieve maximum brightness for the instruments. At the moment the rheostat is still connected and I get a decent amount of light from the LED bulbs. Turning the rheostat either gives full available brightness or they go off. Given that the wires are all nicely connected to the back of the rheostat, is there a problem in just leaving it like this, or should I bypass the rheostat for other reasons? It’s the original unit, so it isn’t in the best of shape. 
 

Gary

[RobH]

It's probably best to leave the rheostat like it is Gary, unless you really want to have dimming available. Shorting it out completely will make no discernable difference to the brightness.  There have been previous threads on dimming LEDs  so a forum search would be worthwhile if you did want to do that. 

As for the ignition lamp, if you have an alternator fitted it is best to retain a normal bulb anyway, as the bulb current helps an alternator to 'wake up' and generate properly.  Some people have found they have no charging if they fit an LED bulb there.   That doesn't happen if you still have a dynamo. 

 I don't know what the supplier is talking about with a current drain on the battery though. That is just nonsense.

 

[Lebro]

Hi Gary.

Fitting an LED bulb as the ignition warning light will not cause any battery drain ! However it can be problematic with regard to an alternator.

Many alternators of the sort we use, rely on current passing from the ignition supply, through the warning light bulb to the small contact on the alternator.

This provided the "energising current" required to start the thing generating it's output. Once started it produces it own energising current, in the process raising the volts on the alternator side of the bulb to 12V, so the bulb goes out.   LED bulbs often don't draw enough current to get the alternator started.

Dynamos are not affected this way.

Regarding dimming the dash lights, there is no harm in leaving it as it is, but if you did want to dim them, try this circuit (which I use on mine):

The control (replacing the rehostat) is a 1 W 100 ohm potentiometer, like this one:

https://www.ebay.co.uk/itm/391584895037?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

The fixed resistor again a 1W 75 ohm one.

  I modified the shaft of the "pot" to accept the original dimmer knob.

Bob

 

Ha ! Rob beat me too it this time !

Edited March 11, 2022 by Lebro

[GaryBZ]

Thanks, Guys. I couldn’t understand why the battery would drain either. I still have the Dynamo fitted, so I will leave the old rheostat for now. I really like the idea of the potentiometer. I will order one from eBay and have a play. It would be nice to have it functioning at some point.

Gary