Hello,

Wondering if somebody could help me, or advise me.

I have a stereomicroscope and I am trying to repair/renovate its lighting system, as it ceased working the other day, which didn't surprise me, as I have used it alot over the last few years.
The microscope is lit with two bulbs, both 12 volt 10w standard bulbs which are dimmable.

The system is supplied with 240 volt mains power, which (via a fuse) goes to a small and simple circuit board which incorporates a variable resistor for dimming the lights, plus a few inline resistors and a few other unknowns (including a pair of tubes wound with several turns of copper wire (a resistor of sorts??)) . Power then goes to a chassis mounted 240v>12v 20VA transformer and goes via a switch to a pair of lights.

It stopped working, completely, I guessed it might be the transformer, as it had been making a slight buzzing sound at high loads over the last few months. I tested it with a multimeter, and it didn't appear to be working properly. I replaced the transformer, but it still doesnt work. I have bypassed the switches etc to test them, but still it doesn't work. I guess that the problem must be in the circuit board.

My question is, does a 240>12V transformer need to be receive power via a circuit board and resistors etc, or can it take 240 volt power directly, after which I would then put in a variable resistor at the 12v side of things. I ask, as it seems overly complicated for two dimmable 12 volt bulbs.

Cheers

David

 

Generally, a 240 volt transformer can be connected to the 240 volt outlet, but you are working with medical electronics. They have some complications to ensure their safety and that they do not radiate interference to other machines. A photo would be of some help, a schematic would help a LOT! A brand name, a model number, anything.
Please post which country you are in.

 

Generally, a 240 volt transformer can be connected to the 240 volt outlet, but you are working with medical electronics. They have some complications to ensure their safety and that they do not radiate interference to other machines. A photo would be of some help, a schematic would help a LOT! A brand name, a model number, anything.
Please post which country you are in.

Thank you

I am in England, if it helps.

Above is a photo of the main setup, out of the microscope for simplicity.
240v power comes in via the white wires in the left of the photo, the variable resistor on the right is on the 240 volt system. The two lights work of the 12volt power coming from the yellow wires coming from the transformer. I freely admit I dont have the skill or knowledge to start fiddling with the circuit board, but I would like to know if I can safely connect the transformer to 240 volt power and run my two 12 volt 10 watt lights off the transformer, via a 12 volt variable resistor to dim the lights as necessary?

I only use the microscope for a hobby, and I am not worried about interference with other electrical equipment.

This is the transformer I have, which is identical to the old original part; http://uk.rs-online.com/web/p/chassis-mounting-transformers/0504280/


Many thanks for your help/advice

 

I'm guessing that's a fairly conventional phase-controlled dimmer circuit, the metal-tabbed component in the board centre being a triac and the most likely culprit for the failure. Do you fancy your chances at a repair by replacing it (~£1.50)?
To answer your question, the tranny could drive the lamps via a variable resistor (rheostat), but it would need to be a high wattage type (at least 25W, preferably 50W) and would get very hot in use. A tad expensive, too (~£33).

 

I would like to see the other side of the PCB. And did you check the bulbs (both of them in case they are wired in series?)

 

I'm with Alec. Transformers almost never commit suicide. If you don't see any burnt looking places, find the number on the triac, about dead center of the board.

 

start by measuring the mains input to the transformer primary on the brown and blue wires, should be 230V, then the secondary side 12V,

if no voltage on the primary i would connect the mains directly to the brown and blue and see if works.

 

Thanks for all your input, very helpful. I will have a fresh go at testing it over the next few days, hopefully make some progress.

Cheers

 

If you decide to go with the transformer only (less noise filter and mains dimmer), adding a bridge rectifier and a simple 12V PWM dimmer, which can be bought in kit form, will get you the full control range back.