WTB Obsolete optocoupler

[PaulAm]

I/m looking for an obsolete Sigma optocoupler 301t1-12b1.  Magnecraft made an equivalent, w301t1-12b1.  This is long obsolete and I haven't been able to track down a replacement.  I thought I had a good lead with a surplus dealer, but they had zero stock.

This one looks like a piece of heat shrink with 2 leads coming out each end.  It's somewhere around 3-4 cm long.  It's a 12V incandescent lamp coupled to a CdS cell, but it's a bit unusual in the range of dark/light resistance.  It's 10M dark and < 400 ohms when on.  I'd fabricate a new one if I could find some cells like that.  I haven't found any good candidates for that either.

This one's out of a welder and I can't just bodge in a modern one.

Rebuilding the current one is an option, but I'd like to get a spare before I destroy it.  Anyone have one of these gathering dust?  Any pointers to suppliers with CdS cells with that resistance change?

thanks

[Gyro]

It's more likely to be a lamp filament failure rather than the LDR isn't it?  If the filament reads o/c then you've nothing to loose by opening it up and replacing the lamp. Just a thought.

[exe]

How is it used? I'm new to optocouplers, so, can't really help. But I don't see anything special about this optocoupler.

Is 400Ohm resistance really required? If so, I'd took a fet optocoupler and added 390-400Ohm in series with it to mimic required resistance. Input would need a resistor divider or something to make modern optocoupler accepting 12V input.

[Cerebus]

If an LED driven one will do (e.g. it's not relying on an A.C. response) then those specs are available from the Vactrols you can find listed on eBay. The VTL5C clones have dark resistance of well over 20M, are 400 ohms at an LED I_f of 1.55mA and fall to 155ohms at an I_f of 10mA. Just measured those exact figures on one that I've got.

Listing here: https://www.ebay.co.uk/itm/2pcs-LCR-VTL5C-linear-optocoupler-DIP-4-New/191832658122?hash=item2caa1e10ca:g:RAMAAOSwAuNW8Q22

[Seekonk]

I got a bunch of old Raytheon CK something that are CK something in what looks like a metal crystal case. No chance to get to them till Nov. Seems like something easily made with a CDS cell and some heat shrink. Priced small lamps lately!

[PaulAm]

I will replace the lamp, although I would like a spare before I take it apart.  The lamp in the coupler is 12V 24mA which is a little hard to find.  Everything I've found so far has been either 40mA or 60mA and I don't want to cook the ldr.

This is turned on by 24v AC through a 330 ohm resistor and turns on the SCR drive for the welder.  Besides the AC input, I'm not sure if the soft start implied by the incandescent  lamp is important or not.  A lot of the solid state alternatives have very fast turn on times.  For that matter, they could have used a normal relay, but they chose not to, so there must be an engineering reason.

The LDR portion is in the input circuitry to an op amp.

Anyone know of an LDR with a 400 ohm to 10M light/dark range?  That 10M value is an order of magnitude or two greater than most LDRs I've found that meet the lower value.

[Brumby]

I will replace the lamp, although I would like a spare before I take it apart.  The lamp in the coupler is 12V 24mA which is a little hard to find.  Everything I've found so far has been either 40mA or 60mA and I don't want to cook the ldr.

I once solved a hard-to-find-the-right-lamp ** problem by looking at higher voltage ones and tried a couple which would operate at an acceptable current on the lower voltage.  For example, I might suggest looking at something in the 24v range that is around the 40 - 50 mA mark.

This doesn't guarantee a perfect match, but it may do the job.  For my exercise, though, it worked out very well.

Edit: ** Incandescent

[2N3055]

There's plenty of those:

https://www.digikey.com/products/en/sensors-transducers/optical-sensors-photo-detectors-cds-cells/540?k=&pkeyword=&pv2283=u10+MOhms+%40+10s&pv2283=u11+MOhms+%40+5s&pv2283=u20+MOhms+%40+5s&pv2283=u20+MOhms+%40+10s&FV=ffe0021c&quantity=0&ColumnSort=0&page=1&pageSize=25

[valgamaa]

You might find better success searching for vactrols rather than opt-couplers. Many of these have been discontinued because of the use of cadmium, but some are still made, and I think Cool Audio might make some replacements too.