Transient Voltage Suppression - TVS - Silent Knight 3320 16v security/fire system

[David Peiffer]

I need to replace a failed transient voltage suppression circuit connected to the 16VAC 20VA transformer for my obsolete (& non-supported) Silent Knight 3320 Security System. The original TVS device was a model 7891 no longer available. The problem is that I can't figure out what zener diodes to use. The TVS device is a simple design using two zeners installed in opposite polarity with each one attached to one of the transformer outputs. the opposite ends of the zeners are tied together and a connecting wire is connected to the 120vac ground in the receptacle box. The original zeners are # GZ14414B but I can't find a cross reference. Initially I bought two 16V 5W zeners # 1N5353B. That was the largest 16V zener I could find. Physically a little smaller than the originals. I connected them as original arrangement and in pretty short order that caused my 16v transformer to expire. I measured the new transformer output and (I have 2 of these systems) and one 16V transformer is producing 18.5V (new one) and the other is producing 19.3V. My system is working fine, but the next thunderstorm/voltage spike may take it out if I don't have the TVS protection. I could experiment but I don't want to release the smoke out of another transformer. (they tell me the manufacturer puts smoke in electrical components to signal you when they fail) Should I be using a higher V &/or W rated zener?

 

[(*steve*)]

I think we need some more background.

Transformers rarely give up their smoke. Did yours *before* you replaced the diodes (I know it did afterwards)?

An arrangement with two zener diodes connected to mains earth seems a little unusual to me. Does the mains earth connect to anything else in the power supply? TVS diodes are not the same as zener diodes. Sometimes they are unidirectional, other times they are bidirectional (A zener approximates a unidirectional TVS diode, but with significantly different knee characteristics).

That protection is not there to protect the transformer, but to protect the rest of the circuit. Any protection for the transformer would be in the primary size.

If it is a 16VAC transformer then the absolute minimum zener rating to prevent overloading and destroying the transformer during normal operation would be 23 volts. However I would probably not use anything under about 30 volts.

If the mains earth is connected to either supply rail internally, I would suggest that bidirectional TVS diodes should be used. I can't find any information on a "model 7891", but there are many replacements that might be suitable.

If you need bidirectional TVS diodes, then you could use one of these.

If you need unidirectional TVS diodes, you could use one of these.

 

[David Peiffer]

Steve, thank you for your response. My homemade "TVS" circuit caused the transformer failure. I also caused the original TVS to fail.... I had unplugged the transformer for some system maintenance to replace a bad arming/disarming keypad (I didn't cause that). The leads on the TVS components were not shielded well and the two leads shorted together when I was plugging the transformer into the receptacle. I knew the TVS was there to protect the system and I wanted to replace it even if the system worked fine without it. This is what my system Owner's Manual states and may give a clue to what diodes they used; "......Transient Suppressor is designed to clamp to electrical ground, all the high voltage spikes coming from the A.C. input that may be harmful to the system control panel. The two terminal spade lugs with the clamping diode should be attached to the transformer secondary coil. The common terminal spade lug with one end of each diode connected to it, should be installed in such a manner that it will maintain a good connection to electrical ground using the screw that holds the receptacle cover in place." .......Steve, I noticed the diodes the manufacturer used did not have a stripe on one end but I can tell from my other system good TVS part that the lettering is reversed, one to the other, when they assembled the two diodes into this Y configuration. I don't know if that reversal was intentional or not relevant. I did call Silent Knight, now Honeywell, I guess owned by TI who makes commercial systems only now and was eventually able to talk to a senior citizen (like me) and he remembered the TVS part they made in the '80s and he said he thought it was clamped to 20V and it might have been 45W. I told him I had tried to use zeners and he couldn't remember any other details. Steve, that said, I don't know if I need unidirectional or bidirectional diodes. Does that information make it any easier for you to help me? Thanks, Dave

 

[(*steve*)]

The most important question at the moment is: Does anything else connect to electrical ground? My suspicion is that the negative supply rail will also connect to ground. If so, bidirectional TVS diodes are required.

20V would be too low a rating (unless that's an AC rating, or they're using bidirectional TVS diodes and there is no other connection to ground). Don't worry too much about the wattage, something bigger will be better .

For more protection you can get varistors to place across the input side of the transformer. These will further help reduce problems associates with mains spikes (and is the normal way of doing this).

 

[David Peiffer]

I have the Installation Manual for my 3320 system showing all wiring connections, for smoke detectors, door contact connectors, alarm horn, arm/disarm controls, 12 v backup battery etc. I do not see an reference to a ground connection anywhere. It shows the connection for the wires from the 16VAC adaptor transformer that powers the entire system. The TVS is not shown on this diagram. Maybe I wasn't clear about the transformer. It is a AC adapter that plugs into a wall receptacle... if that changes anything. Anyway, I have pulled the good TVS from the other system and tested the diodes with my little FLUKE 12 meter and it does not show a forward or reverse reading. Does that shed any light on my mystery?
My two prong transformer plugs into the wall like any extension cord. But, unless a varistor would plug in under the transformer it would be hard to connect to the transformer input.
I wonder if somebody had a cross reference from the late '70s if my GZ14414B diode would be listed. I think it is a National Semiconductor part but I can't find it in any listing now.

 

[(*steve*)]

If it does not show any forward or reverse connectivity then the device is bidirectional.

And if there is no other connection to earth, then 20V bi-directional TVS diodes would be appropriate.

 

[chopnhack]

David, I had really good luck some years ago tracking down a vishay product that had burnt up through digikey. It was a smd component and the surface was charred, but they were still able to figure out what it was. Try and contact someone at Digikey, this was the person that helped me:


Becky Schroedl
[email protected]
Technical Support Specialist
Digi-Key Corp.
www.digikey.com
1-800-344-4539 Ext. 1523

 

[David Peiffer]

Okay, I'm pretty sure I understand what I'm doing now; Bidirectional for AC TVS. I looked around a little for a 20V bidirectional clamp and didn't find anything. I'll go to a nearby electronics supply today and if no luck, I'll call Digi-Key Monday. I'll post the results of my search later. The internet and forums are wonderful things when people are willing to assist people they don't know. Meantime, many thanks to you, Steve, and chopnhack.

 

[David Peiffer]

Okay, I think a 5KP20CA looks like it would work.. operating voltage 20V, breakdown 22.2 and clamp at 32.4 and its axial. The clamp voltage seems a little high but I'll do more research.

 

[David Peiffer]

UPDATE: I was at the electrical supply store and talked to a knowledgeable customer about this and he reminded me that my meter is measuring RMS not peak to peak. So, I should be looking for a bi-directional diode that can handle my peak-to-peak voltage. So, with a 19.3VAC output (16VAC rated) transformer I should use a bidirectional diode with operating voltage higher than the 20V I was looking for, to keep it from being in constant stress. Steve, this is what you were telling me about not using a diode under 30V.
The person I spoke with at the store thinks he can cross reference my original diode to see exactly what the Silent Knight manufacturer had used. He will call me. I have to keep reminding myself that I'm making a surge protector not a voltage regulator. More to follow.

 

[(*steve*)]

Remember that in the configuration you have (two in series with the junction connected to earth) the voltage is effectively doubles. 20V units will clamp at 40V. That's higher than the peak value. Note that 20V is less than the peak, so if the ground is connected elsewhere to the circuit these will conduct (as they also would if they were unidirectional -- because placing them back to back just creates a bidirectional TVS of the same voltage as the two unidirectional ones).

 

[(*steve*)]

Actually a pair of 20V devices would clamp at 64V because each actually clamp at 32V. (they would start to conduct at 44V)

This sounds bad, but it probably OK for short transients.

 

[(*steve*)]

A high clamp voltage i typical for a TVS device. <-- this didn't get posted from sometime between your posts #9 and #10

 

[David Peiffer]

First, DigiKey could not find specs on my GZ14414B. I also tried a couple of other places with no luck. I guess it is too old ('80s) and nobody keeps old cross reference books. Also Sams does not have any wiring diagrams for Silent Knight by Waycrosse Inc. More info on Digikey suggestion at the bottom.

Okay, I'm way over my head here. I'm not an electronics engineer, but I was a technician 50 years ago (50c5 days) but never designed anything. So, to try avoid confusion I'll try to clarify to be sure I'm not confusing anybody; I have a wiring diagram not a schematic for my system. I don't understand the internal power supply of my system. My wiring diagram does not show any earth ground connection to the system or to any of the accessories attached to it; i.e. smoke detectors, alarm horn, door sensors, 12 rechargeable B/U battery etc. ....One of my diodes is connected to one output of my 16V AC adapter and the other diode is connected to the other output of the 16VAC adapter. The opposite ends of the diodes are connected together and then to earth ground. I interpret the diodes to be a parallel connection to ground. Each diode would see positive potential alternately at opposite ends of the normal AC sine wave until there is a transient voltage spike. I see that they would be in series relative to the output potential of the 16VAC transformer. In the event my earth ground connection becomes disconnected my diodes will be managing that AC potential in series.

Digikey suggested I look at the specs of a SAxxCA that is bidirectional and 500W. This is my current uneducated direction; It looks like I will try the SA30CA to catch the big spikes and let the system internal power supply manage the little ones. ...Unless someone can give me an educated recommendation.... This is hard work! Thanks again for your help.

 

[(*steve*)]

The point to remember is that both diodes need to conduct unless there's another conduction path to earth.

If, as you say, there is no other path to earth, then the sum of the holding or operating voltages of the TVS diodes (if bidirectional) should be slightly greater than the peak (1.414 * RMS) voltage from the transformer.

I think the connection to mains earth is probably not necessary for these purposes.

I would probably prefer to place a single bidirectional TVS diode across the transformer winding (rated at a little higher than 1.414 * RMS voltage) and another from one end of the winding to ground (similar rating). This will give you the same amount of protection and will not cause problems if the power supply rail gets (deliberately or accidentally) earthed at some point.

Yes, these will take the largest and nastiest spikes, leaving smaller spikes for the rest of the circuit to handle.