I've been searching the web for a few answers but this seems to be the best place to get help and advice on electronics circuits. I've inherited an old microscope which I'd like to get working again. The optics are still very good but the dimmer for the lamp is of a very old rheostat design and is not very reliable. I'd like to replace it with a more efficient alternative. I think a trailing edge dimmer circuit would be the best option to help protect the lamp from sudden voltage changes. The lamp is rated 6V AC 15W and I'd like to run it off 240V AC mains in the UK. I can find a commercial AC-AC PSU (either 24, 12 or 9V) but would like to include a dimmer circuit that didn't include just a potentiometer. Can anyone suggest an appropriate trailing edge dimmer circuit that would drive this lamp? Any help or suggestions gratefully received. Thanks in advance.

 

How is the lamp powered in the original setting. Is it AC or DC?

 

Thanks for the quick response. In the original setup, it's an AC supply to the lamp.

 

I would consider a DC power supplty with PWM ckt. What do you need for control becides a pot?
What is a trailing edge dimmer?

 

For something simple, get a household dimmer switch(the kind for the average incandescent lamps).

run the dimmer on the 24 volt side of your transformer and put two 10 ohm 25 watt resistors in series with your 6 volt lamp. This will allow direct dimming control of the 6 volt bulb. You can find a cheap supply of such resistors disguised as Audio power amp dummy loads.

a dimmer for 110 VAC would be ideal, but I understand you may not be able to get one easily in your home vicinity. The 240 volt version should work alright on the 24 VAC though.

 

Thank you for the clear and practical advice Now I have a plan, I'll give it a go...

 

I would recommend a small constant voltage (CV) transformer for microscope work to keep your specimen image stabilized. We used such transformers for instruments that needed a stable light source. Running the CV output into a small stepdown (or variable) transformer to power your lamp (6VAC @ 2.5 amp.) should provide you with sufficient and stable illumination.

Cheers, DPW [ Everything has limitations...and I hate limitations.]