The Shop => Electronics & IC Programing => Topic started by: Muzzerboy on March 28, 2021, 10:49:12 AM

Title: Encoder bulb replacement?
Post by: Muzzerboy on March 28, 2021, 10:49:12 AM
I have a Heidenhain clone encoder that refuses to work. Can't be too difficult to fix, given how simple the electronics are and I don't imagine the mechanics are the root of the problem - it all looks and feels OK.

Sure enough, the mechanical bits comprise an optically slotted glass wheel with a similar slotted (stationary) grid providing a sort of Vernier effect. It's lit from below through a simple lenswith a 5V incandescent bulb the size of a matchstick head. Or was until the filament burnt out. There's also some sort of flat optical receiver for the A, B and Z outputs.

Beyond that, there's a quad comparator and a couple of RS422 differential line drivers. Looks to me as if replacing the light source would be all that's required.

Has anyone any experience of these things? I'm tempted to liberate an LED from a modern light bulb and use that with a suitable resistor, rather than try to source a miniature 5V bulb. Apart from anything else, the available space is an issue, which rather limits my options.
Title: Re: Encoder bulb replacement?
Post by: Bluechip on March 28, 2021, 11:16:40 AM
The things you are probably after are called 'grain of wheat bulbs'.  Wire ended mostly.
Used to be quite common, but not so much now but still obtainable.
Various voltages usually 3, 5, 6, 12V.  About 5 years ago I dumped about 200 of the things, 24 & 32V. Never found a use for the little buggers.  :scratch:
EDIT  New link ...
Title: Re: Encoder bulb replacement?
Post by: BillTodd on March 28, 2021, 11:22:21 AM
What size is the bulb? I'll look in my collection.of odds and sods.

You might get a led to work but you may need an infra red device and it will need to be trimmed (vary the series resistance) to get the right light level (which is usually quite low , i.e 6v bulb running at 5v.)

Title: Re: Encoder bulb replacement?
Post by: Muzzerboy on March 28, 2021, 11:49:10 AM
I will try to measure it although it's embedded within some sort of epoxy(?) adhesive. But 3mm is about right. If anyone has anything suitable, I'd be more than happy to reimburse and/or make a donation.

Must admit, I'm a little unsure about what wavelengths would work here although it's a normal looking bulb so you might guess visible light would work but can't rule out IR. The sensor has no markings, so I can't look it up.

The CPC bulbs are pretty cheap at 1.62 for 5 but as ever they want 4.99 for "handling". Speaking personally, I'd expect more than just a rough grope for a fiver but that's just me.

I may experiment with some LEDs in a bit and see if that might work.
Title: Re: Encoder bulb replacement?
Post by: BillTodd on March 28, 2021, 01:16:46 PM
Photo trannys are often/usually ir  , you won't get any ir out of a 'white, led (just a mix of red,green and blue)  but bulbs  give out more ir than visible when run  cool  for long life.

Ill go have a look in my bulb box....
Title: Re: Encoder bulb replacement?
Post by: BillTodd on March 28, 2021, 01:30:52 PM
These are the smallest I can find  they're 3 diameter by 6mm long , they  seem to have  a range of brightnesses at 5v  (some may be 12v)

let me know if they'll fit and let me have your address by pm.

Title: Re: Encoder bulb replacement?
Post by: Muzzerboy on March 28, 2021, 02:01:01 PM
Thanks for the offer, Bill. I measured the bulb diameter and it was indeed 3mm, which suggests it was a common size. However, the thought of having to wait around for progress didn't sit well, so I had a go at fixing it. Either way I'd have had to clean out the old adhesive, bulb etc.

I found some of those tape LEDs. The package included a 12V battery with a switch and a single segment (3 LEDS per segment) wired up for sales purposes. This showed me that the forward voltage is about 3V under the recommended(?) current. I also measured 2.5V across the 150 Ohm resistor, suggesting a current of 17mA or so. Given that I have around 5V in the encoder supply, simply reusing this 150 Ohm with a single LED would result in around 13mA or so, a cunning plan came to be.

I snipped out one of the LEDs that was neighbouring the resistor and cut back the flex PCB to fit in the cavity where the bulb had been. I also cut and filed the cavity to get the LED roughly where the filament had been, then glued it in place with Araldite Rapid. With a layer of Krapton tape underneath for electrical isolation, it fits back into the original location. This was a bit of an act of faith, as it was only when I powered it up that I could be certain it actually worked. Which it did.

Obvs I couldn't move on without fiddling with the 3 pots. only one channel worked, so I assume the circuit is sensitive to the brightness to some degree. There's one per channel and there seems to be a sweet zone where the comparators do their thing. With all 3 in the middle of their zones,  I seem to have a result.

Pleased with this, as the same encoders are used on the 3 axes of my main CNC machine. This one came with in the bag of spares and claimed (correctly) to be faulty. It's now going to serve as the spindle encoder on my Bantam CNC conversion.

Many thanks for the offers of help and suggestions. Hope this helps anyone else encountering this issue.