# What is this small circuit board used for in a 12v light?

Repairing a home-made disco light that was used in a nightclub during the 80's/90's and it consisted of 16 12v 50w lamps arranged on 8 legs.

I have discovered a small circuit board within each lamp which has an LED on the back. What was this small board used for and can it be removed?

The lights were powered in serial, so would this board of allowed for the rest of the lights to be powered if one light blew?

It looks like the LED will light up if the 12V lamp blows, so as all are in series and all lamps turn off if one blows, it is easier to locate the broken lamp by just looking at the LED. As it is in parallel with the lamp, it can be removed.

• Excellent - Thankyou. Would changing the old 12v 50w halogens to LED's affect this circuit in any way? Jul 16, 2019 at 10:21

Others have noted that this is a blown-bulb detector.
When a bulb blows you get AC mains across the resistor-diode-LED combination.
The 15K resistor limits current to about 20 mA peak hald cycle DC on 230 VAC.

LEDS:

With incandescent bulbs, WHEN the light blows you get AC mains across
[(the diode resistor LED combination) + (all other bulbs in series)] .
The other bulbs more or less look like resistors of 10% or maybe less of on resistance so for 50W 12V lamps Ron ~= 0.25 Ohms so Roff is maybe 2-10 Ohm range. SO the series 15k dominates. |

With LEDS

• Voltage MUST BE DC.
A full bridge is in order. If you run this on AC with LEDs you will probably destroy ALL of them nearly instantaneously. LEDs do not like high reverse voltage and draw little reverse current. You may be lucky. You may not. Use a bridge rectifier. You MAY get 50 Hz flicker. Better still, see below, abandon this method.

• LEDS MUST be 12V (or more with DC) if 16 used, or the number must be adjusted to run OK on mains if voltage per LED is other than 12V, as you have mains voltage across the string.

• They do not have to be 50W as long as all are of equal power and voltage.

• With a blown LED you get 15 x LEDs in series with the blow-detect-circuit.
It will PROBABLY still work.
I'd consider testing it with a variac and turning it up slowly to see if there are any unexpected gotchas - but, probably OK.

This is a nasty method of operation. It was done to allow 800 Watts of lamps to be run off mains directly with no transformer. Each socket potentially has mains on it at all times. V at a given bulb depends on its place in the string. When a bulb blows ALL bulbs on the phase / live side of the dead bulb are at full mains voltage.
It's lethal.

The same light (or more) can be obtained with perhaps 5% - 10% of the power with modern LEDs.
12V supplies are not overly expensive. Abandoning this nasty kluge is a good idea - and you can easily have an arrangement where a single blown LED (which will be very rare) does not extinguish all the others.

it looks like it only lights the LED and is a place to join brown wires to blue wires.

if you don't need the LED you could find some other way to join the brown and blue wires.