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I have a WWII era RAF chart-board lamp in perfect unused condition. I want to use an LED bulb with the original dimmer as tungsten runs so hot. I want to avoid rewiring or bypassing the dimmer as the original cabling is in such good condition.

The fitting for the bulb holder is BA15D, so I am limited in the range of 24V LED bulbs available.

The dimmer specs I can find are as follows:

  • Ref 5C/726 type G
  • Primary use: Chart-board lamp, 24 volt, 10 watts
  • Total resistance: 50 ohms +/- 5%
  • Number of turns each side: 95
  • Wire: 0108 in. enameled eureka

How can I choose a bulb that will work?

JRE
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Walter Hutton
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2 Answers2

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Reducing lamp voltage will dim the lamp.

I suspect that any LED lamp will draw so little current that you won't get much of a voltage drop. If, for example, you have a 24 V, 6 W lamp you will draw 6/24 = 0.25 A. The 50 Ω resistor will drop approximately 50 x 0.25 = 12.5 V and that should be quite enough. (You can recalculate for any other lamp.) The effect may not be linear because the relationship between current and voltage on LEDs is non-linear.

Transistor
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  • If I understand correctly, you are suggesting that if i find a 6 watt LED bulb to fit, then the rheostat will be able to drop the supplied 24v volts by half and that should be enough to dim the bulb? – Walter Hutton Nov 27 '20 at 11:00
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    That's correct. The non-linearity will make it a little hard to predict. Find a bulb that will fit first and post a link here and I'll comment. – Transistor Nov 27 '20 at 11:29
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    going to 12V or 6V instead of 24V will increase the effectiveness of the rheostat. – Jasen Слава Україні Nov 28 '20 at 03:20
  • Thank you for both your input! I bought the most powerful bulb I could find to fit. https://www.classiccarleds.co.uk/products/24-volt-ba15d-warm-white-led-bulb-p21w-346-344-level-pins?_pos=1&_sid=3882a3fb1&_ss=r. Using the 24v supply the bulb switches on and off perfectly, is nice and bright, but won’t dim, but using a 12v supply the bulb dims nicely, not with a huge range, but enough. So do you think if I drop to a 6v supply I may get a wider range of dimming? – Walter Hutton Dec 01 '20 at 11:35
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Go to a 3W LED and a 5V supply, put a 47 ohm resistor parallel with the LED, and 22 ohms in series.

enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

R3 can be external to the lamp assembly, R1 and D1 can be built into a lamp base, leaving the antique part unmodified,

You can salvage the base of a lamp with a BA15D base, EG Sylvania 1076

Jasen Слава Україні
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    Check your maths there, Jasen. If it's anything like [this one](https://foton.ua/upload/datasheet/3w/3W%20High%20Power%20LED.pdf) it's 2.8 V at 0.7 A (2.1 W). Your 22 \$ \Omega \$ resistor will drop 16.4 V when you've only got a 5 V supply. – Transistor Nov 26 '20 at 23:24
  • I'm sure something like this could work but i want the bulb to fit the original bulb holder (ba15d) and there is nowhere to hide additional electronics except in the small wooden plug adapter (see picture) that connects to the 24v power supply. – Walter Hutton Nov 27 '20 at 10:55
  • that was my intention take a glass bulb and remove the glass bit, then connect the LED and resistor. – Jasen Слава Україні Nov 28 '20 at 02:36
  • @Transistor yeah 4.7 is probably a better choice there. but now the LED needs a heatsink. – Jasen Слава Україні Nov 28 '20 at 02:45