ECN Forum Forums General Topics of the Trade General Discussion Area Need help identifying diode

Powered off, what is the in circuit resistance across each of the diodes?

I see that wa2ise mentioned that CR stands for crystal rectifier. I've just scanned through the 3 pages so I hope I haven't missed anything. There's a pretty good chance that you're looking at unidirectional transient voltage suppressors, or TVSs, although those don't look like any that I've used or replaced. 6.8V TVSs, also called transorbs, are very commonly used to clamp inputs from the outside world and have less capacitance and faster response than a zener. It should look like a normal silicon diode forward biased, and you can test it with your power supply and a series resistor, for its clamping voltage. They aren't supposed to be conducting unless there's a surge coming in that needs clamped, so the voltages don't tell you a lot unless they're above 6.8V. The ones I use are black and are marked 1.5KE6.8A(1500W) or P6KE6.8A(600W). That is the wattage that they can withstand for a millisecond or so. A 1.5KE6.8CA has no band and is a 1500W, bi-directional TVS. The 600W transorbs look about the same size as a 1N400x, 1A diode, while the 1500W parts look about the same as the 1N540x, series 3A diodes.

The photos aren't that clear but I can tell that you have mostly 5 volt supply, hex Schmitt-Trigger inverters, caps, and resistor packs on that board. Just what you would expect to see on a board that accepts and debounces inputs from the outside world. RC networks with zeners or transorbs would be used to protect the very high impedance inputs of the CMOS ICs. The "HC" family of CMOS devices were designed to use the same pin outs and 5V power supplies as the older 7400 and 74LS00 families. If my suspicions are correct, your circuit would work with the zeners or transorbs removed IF other components aren't defective. In that case, a P6KE6.8A or 1.5KE6.8A would be your commonly available replacement part. All of the other parts on that board should also be available from Digi-Key, Newark, Mouser, Allied, Etc.
Joe

Once again, I appreciate the feedback.

I tried emailing a company to assist with identifying the diode and currently awaiting their reply...

Joe, I think you're correct about these diode being Zeners. I removed the diodes from 2 boards - one board was functioning correctly and the other was not. I checked the resistance of each in both forward and reverse bias polarity, I used my DMM diode setting to check forward bias voltage, and I set-up a simple circuit using a 1Kohm resistor, digital ammeter and variable DC power supply to see if they would conduct when reversed biased. Here's what I found (I numbered them 1-6):

- with the meter leads positive to anode and negative to cathode all diodes measured approx. 2 Mohm.
- with the meter leads positive to cathode and negative to anode the resistance varied greatly

CR1 = 15 Mohm
CR2 = 6.5 Mohm
CR3 = OL
CR4 = 42 Mohm
CR5 = OL
CR6 = 26 Mohm

- using the diode setting on my DMM all diodes tested good
- using the circuit for testing if they would conduct when reverse bias all diodes began to conduct at approx. 6.8 V

Hopefully this information further proves that they are in fact Zeners. Btw, on the circuit board all 3 diodes anodes connect directly to the row of capacitors. The cathode of CR1 connects directly to the collector of Q1. The cathodes of CR2 and CR3 connect directly to the cathode of an indicator LED - one for each of the two "call" lights - as well as connecting to an IC on the second circuit board through the ribbon cable.

Potseal, I don't think that these are zeners, but rather transorbs, and I'll tell you why. Zeners by application are shunt regulators and they should continuously be operating at their zener voltage. That's why small ones of that size are rated at 1/2, 1, or perhaps 5 watts steady state dissipation. You didn't see 6.8V across the diodes on your good board. Transorbs of the same size are rated at 600 or 1500 watts for a very short time, and are used to clamp dangerous transients.
http://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf
If you look at the Littelfuse datasheet, you'll see that the 1500W is for <2mS. (Fig.2) But the steady state power dissipation is only 6.5W. That should make sense if you take a look at a 5W, 6.8V zener and compare physical size.
I'm not saying that you can't build a shunt regulator with a transorb, or a clamp circuit with a zener. You can. They just wouldn't be optimized for what you're using them for. So for a zener, think,"regulator", and for a TVSS, transorb, MOV, crowbar, think, "clamp", and "protection".
Joe

My mistake and thank-you for clearing that up, Joe. I was aware of MOV's but the terms "transorb" and "crowbar" are new to me.

Transorb is just another name for a TVS diode. A crowbar circuit is usually found after a fuse on the output of a DC power supply. The gate of an SCR across the load is triggered if the output voltage goes too high, blowing the fuse and protecting the load.
Joe