I looked all over the Internet and couldn't find anything. Normally it's a fairly straight forward search using the markings on the diode. This particular diode came out of a Dukane nurse call patient station:



Looks like "TZB 6.8A". That "B" might even be an "E". I tried every variation. On the opposite side is an "SI" which I assume stands for Silicon. Any help would be appreciated.

This may be of no help to you, potseal, but if you could possibly find a circuit diagram for the device, that should tell you what sort of diode it is.

Also, I think the character you suspect is a B or an E could in fact be the symbol for DC.

That might be a capacitor, 6.8uF
The band may indicate which end goes to the negative side of the circuit.

wa2ise,
I was thinking, on reflection, that this could be a 6.8V Zener diode, not your stock standard silicon diode.

Thanks for the replies. I won't be able to search for a circuit diagram for a few days since I'm currently on holidays. I took another look at the board and the diode in question is identified on the board as "CR 1", "CR2", and "CR3". I believe "CR" stands for control relay. I took a picture as well:



I circled the 3 diodes and I circled the row of capacitors that are directly connected to the Anode of the diodes.

EDIT: I just looked up "CR" and according to one source it stands for "diode".

I had a sacrificial board identical to the one in the above image. It wasn't working properly either. I traced the problem to one of the 3 diodes. I replaced the original diode with one that I had in my spare diode container. I used an FR207. It's described as a "fast recovery" diode. Definitely a shot in the dark but ultimately it eliminated the problem. I'd rather have more information to go on to be certain it holds up in the long run.

A couple of questions:

How do you KNOW it is a diode? Was there a reference designator printed on the printed circuit board it came out of? Perhaps a CRxx or Dxx.

Is there a schematic symbol printed on the circuit board? Any polarity markings?

Have you tested the component? Diodes typically fail shorted followed by burning open from excess current. The component in the picture looks undamaged. Did you check it for continuity? Diode voltage drop in both directions? Resistance value?

How did you isolate the fault to that component?

Any idea of the function of that component in the circuit?

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EDIT
Do the 9 caps sit across a communication data buss? The diodes and capacitors might be used for electronic interference noise suppression. Or voltage termination

Can we please get a wider view of the PCB to see where this fits into the whole thing?

Appreciate the response Larry.
As you can see in the image above the location of the diode in question sits on the circuit board with 2 other identical diodes. CR1, CR2, and CR3 are marked on the PCB at that location. This PCB is directly related to the touchpad of the Dukane patient station for such functions as "call", "staff emergency", etc. The board I repaired and the board I'm trying to repair (in the image above) had similar issues. Both boards when powered had touchpad LED indicator lights lit before the buttons were pushed. These lights could not be cancelled or reset. I traced out the circuit from the malfunctioning signal to various components and eventually arrived at the diodes. The first board I repaired seemed straight forward when one of the diodes didn't test properly using a DMM. The board I'm trying to repair now seems to have the same problem except it's two of the diodes that don't measure correctly. I did several resistance measurements comparing the working board with the board that needs repair. The resistance across the diodes that I suspect is far greater than the others. As well, when I compare the R across the capacitors the ones in the malfunctioning board show "OL" while the ones in the working board show approximately 900 ohms.

I will definitely post that image tomorrow. Calling it a night right now.

The chances are high that it's a 6.8volt zener.
Can you deduce anything from the associated circuit components? For instance it looks as though the three cathodes are connected together; is that point ground? Do the working ones have ~6.8v across them? Or are they perhaps across signal input lines as transient suppression?
Finally, (though should be firstly), have you tried calling the manufacturer? I've sometimes found specialist manufacturers quite helpful with technical queries.

Here is a more complete view of the board I'm working on:

Here is the identical sacrificial board that I attached to a piece of MDF and made more accessible for testing AFTER I replaced a diode that I believed was causing the malfunction:



This unit appears to be working fine. I have removed the touchpad and replaced it with various wires attached to a terminal strip. I can short the wires together which will cause the appropriate indicator light to come "on". Then I can cancel those same lights. Before I replaced the diode one of the "call" lights was always "on" and could not be cancelled.

When I power the board that appears working properly and measure the voltage with positive lead on the anode and negative lead on the cathode I get -3.8V on CR3 and CR2 but 0 V on CR1.

Contacting the manufacturer is definitely a good idea but without being a 100% positive I believe Dukane is no longer a supplier of this equipment. Most Google searches seem to result at a company called Crest. Their website shows older Dukane nurse call equipment but it doesn't look anything like the equipment I'm working on. This equipment is quite old. Still, it may be worth sending an email.

Oops. I guess I forgot to mention... The voltage drop measured across the diodes on the board that is not functioning properly is 0.17 V.

^^^ correction - that previous measurement should be "0 V".

If its any help, Dukane sold off the nurse call station division to this company in 2002 according to the Dukane web site

http://www.ascom.us/us-en/index-us/systems-division/ascom-telligence.htm#overview

"CR" means "crystal rectifier". An old name for a solid state diode, made from a piece of silicon or geranium. So it is a diode.

CR tends to indicate a diode and you seem to have proved it.

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