LED-Lighting an old American Optical binocular inspection microscope

[TerraHertz]

http://everist.org/NobLog/20141228_led-lighting.htm

This would probably be better on a metalworking forum (like so many of my posts), except for the bit about discovering not to run 12V LED downlights intended to replace halogen bulbs, off small inverter packs intended for 12V halogen downlights.

[SeanB]

The lamps use a 4 shottky diode bridge rectifier inside with a 100uF ( or so, I have seen from 10uF to 220uF in voltages around the 16-25V mark inside)  capacitor, then a single SMD boost converter with a ceramic capacitor and a diode driving the LED string with a constant current sensed with a resistor. They do not work well with the cheap "electronic transformer" units at all, but are fine with anything from 9VDC to 16VDC. You probably can find a used wall wart of around 12VDC at around 1A ( old modem wall wart is fine at 9VAC) to provide power for it.

Nice work there, and a very nice use of the old valve as a face too.

[Zero999]

You can buy 12V halogen replacement LEDs which are compatible with electronic transformers. Philips make them.
http://www.lighting.philips.co.uk/pwc_li/main/shared/led/portal/EMEAtool_Update_13082012/files/Technical%20Application%20Guide%20MASTER%20LEDspot%20LV%2010-50W%20MR16.pdf

Be ware of the different spectrum of an LED which is often deficient in the cyan area. If you're using this for soldering or inspection of electronics, this is rarely a problem (except it may make reading some resistor colour codes more difficult) but if you're looking at biological tissues or gem stones, it could be a problem.

[TerraHertz]

The lamps use a 4 shottky diode bridge rectifier inside with a 100uF ( or so, I have seen from 10uF to 220uF in voltages around the 16-25V mark inside)  capacitor, then a single SMD boost converter with a ceramic capacitor and a diode driving the LED string with a constant current sensed with a resistor. They do not work well with the cheap "electronic transformer" units at all, but are fine with anything from 9VDC to 16VDC. You probably can find a used wall wart of around 12VDC at around 1A ( old modem wall wart is fine at 9VAC) to provide power for it.

I figured there had to be a bridge (and obviously shottky) at minimum. After seeing the way they respond to varying DC input, it was obvious there was also a switching converter. But, looking at the way the thing responds to an AC supply, I suspect there's no smoothing capacitor. I should have a look at the waveform right at the LEDs (it's accessible) out of curiosity. There's one of the lamps left over.
There's always a smoothing capacitor in CFL switching supplies, but they have the space to fit it. There's very little space in the stem of these LED replacements for halogen downlights.

One question, is did they use fast switching shottky diodes, or cheap slow ones? With a 40KHz squarewave drive, that's important. Since I bet they used slow (cheap) ones, that's why I quickly gave up on using the inverter. I did try a few experiments with an intermediate bridge and filter cap, but that had problems too:
 - didn't make the inverter run stably (even with various tries at R/C snubbers),
 - the only beefy bridge rectifiers I had were all slow diodes, so they dissipated too much heat,
 - the lamps draw 500 to 600 mA, x3 = 1.8A. Using lossy fixit diode/R/C networks between the inverter and lamps was just silly even it it worked, which it didn't.

Also, I want to retain the option of using plain halogen lamps for the top illumination. For spectral reasons, see below. This is why I went with the 2-prong downlight sockets. It wasn't *totally* a spur of the moment buy, I'd had this project in mind for a while and already decided to go with that format of lighting socket. As opposed to just mounting a high power LED inside the existing lamp housing.
So the supply must be able to handle the greater load of 2 halogen bulbs plus the LED in the base. That excludes typical wall-warts.

The trouble with halogen downlights is they get finger-burning hot. Placed where they are, exposed above the work area, I know what would happen and how often. So, LED lights for default. They get warm, but not dangerously hot.

I suppose I should include these details in the writeup. It was written in a rush.

Nice work there, and a very nice use of the old valve as a face too.

Thanks! I'm glad someone appreciated the metalworking.
Err, 'old valve'? You mean the big brass terminal block out of an industrial-size circuit breaker?

[TerraHertz]

You can buy 12V halogen replacement LEDs which are compatible with electronic transformers. Philips make them.
http://www.lighting.philips.co.uk/pwc_li/main/shared/led/portal/EMEAtool_Update_13082012/files/Technical%20Application%20Guide%20MASTER%20LEDspot%20LV%2010-50W%20MR16.pdf

Hmm, and they're available for around US$8. Not too bad. Thanks, I may try one.

Heh. The final compatibility table in that pdf is pretty amusing. I can imagine the kind of marginal things happening, with triac phase dimmer driving mains-to-12V inverter, driving another inverter inside the LED light body. They don't define exactly what they mean by 'works' for the green cells in the table, do they?

Be ware of the different spectrum of an LED which is often deficient in the cyan area. If you're using this for soldering or inspection of electronics, this is rarely a problem (except it may make reading some resistor colour codes more difficult) but if you're looking at biological tissues or gem stones, it could be a problem.

Yep. Fully aware of this. It's why I chose the 2-prong lamp fitting, so I can substitute halogens if needed.
This scope is only for simple PCB inspection and stuff like that. Though, not ideal due to the restricted clearance between the view area and the microscope stem.
The LED lighting project on this one was both a 'just get the microscope usable' quickie, and also an experiment. I have two other much better microscopes that are also missing lighting parts. One is an Olympus 'BHM' Metalurgical stereo microscope. Really nice microscope. It has the lamp housing but is missing the odd style bulb. For this one I want to make an insert that can fit either a small halogen bulb, or a high power LED. The LEDs for that one are in the post.

The other microscope is a Zeiss Universal. It came with no lamp housing at all. I've been gradually gathering the necessary bits and now have everything but the 100W high pressure mercury arc lamp. eg ebay 260518249321. They're a bit expensive, and I'm waiting to see if I can find them cheaper somewhere.
Also still missing a C-mount camera adapter for that. They don't come up on ebay often, and get snapped up.