The AM Forum

In looking at some of my old gear I noticed none of it had a MOV (metal oxide varistor) in the AC input line.  It would seem like cheap protection at only 0.31 cents from Mouser or $1.99 at Radio Shack.  I know they may not save all equipment from a surge but for the price wouldn't it seem worth it to install them in our old gear?  I would be interested in hearing from others.

Al n7ioh

The failure mode of an MOV is a short and they have a limited number of transients they can handle. I have a few R3030 power supplies trashed by shorted MOVs. Transorbs are much better. We have not been able to use MOVs in MIL stuff since the '70s

MOV's also can wear out and fail open after years of mild surges.  The problem is that you are relying on them and don't know if/that the MOV is open and you don't get any surge supression.  As Frank says, they have shorted too and then can start fires.  Frank probably sees the shorts a lot with the repetitive testing he does which can be unrealistic since the MOV gets heated and then gets another surge soon.  There is the surge energy itself the MOV has to handle, the (MOV) shunt device conducts and then ac line power additionally flows through the device, called let-through current, and then kaboom. 

Thanks guys.  That's the big problem with technology, once you finally learn something it's outdated.  OK so MOV are out and Transorbs are in.  I was looking up transorbs on the Mouser website after reading Frank's post.  I'm going to need some help here in selecting the right ones for my application.  I would like to install them in my gear and replace the MOV's that are in some of my newer gear.  I have an Astron 70 amp supply, some Drake AC-4 power supplies, R-4B and R-4C receivers, and my two Ranger ll transmitters.  Because they are all 120VAC input would I use the same transorbs in each?  I am hoping to buy an Ameritron AL-1500 amp so I am guessing I would need two and they would go from each hot lead to ground?  OK now for the help, which ones do I need to get?  I copied the Mouser link to make it easier. This stuff id Greek to me.
http://www.mouser.com/Search/Refine.aspx?N=254140&Keyword=tvs&FS=True

Thanks guys, Al n7ioh

I also prefer transzorbs.  They are the ones we used in out switching power supply designs.  The ones we used were also faster than MOV's. They are available in both DC and AC versions.  1.5KE160C sticks in my mind for ones I used.  They can be series without any problem. 

Concerning MOV's, they have save a bunch of Astron supplies that I use up at the repeater site from near-by lightning strikes.  If your going to use MOV's put the fuse prior to the MOV so it pops when they short.  Some of the cheapie outlet strips would use MOV's for surge protection and actually have caused fires.  UL came out with some recommended guidelines for their usage.  The plastic outlet strips were the worse.  Our neighbor had their carpet burned from a MOV'ed outlet strip! 
 
I once saw a MOV used on a small flyback Keep Alive supply in which the manufacture was using it for clamp protection rather designing the correct snubber.  When as was said before they only take so many hits and the threshold voltage starts to go lower and lower until they are conducting all the time and pop they go!  That was a wrong application for them and that circuit was re-designed.
Joe, W3GMS     

This is a big jump-up in price from a single shunt device but check for these two companies who made a complex device that would take thousands of high energy hits no sweat - a professional ac line clamping device:

Zero Surge and Surgex.  Cheapest residential model  $129. 

Although $129 or more, these products can be cheap insurance for many scenarios.  (Yes I did work for Zero Surge at one time.)

The transzorbs are just big zener diodes.
Available in unidirectional and bidirectional (in effect, 2 zeners wired cathode to cathode)

1.5KE160A  is a unidirectional, 160volt.
1.5KE160C,  or CA would be a bidirectional 160volt.


Their failure mode is also a short.

Tom,
I believe we use to buy them from G.I..??  General Semi makes more sense but I think it might of been G.I.  I would assume that the junction capactance is lower on the transorbs than a typical zeners since I believe the transorbs are much faster than the typical zeners.  They were more than MOV's for sure but well under 10 bucks!   They sure got us through all the required surge testing without any problems... 
Joe, W33GMS

I stand corrected, the 1.5ke160C is a bidirectional device.
But 10% tolerance.

Transzorbs cost less than $1  each.

The 1.5ke160CA  is $0.62 each at newark.

We use a lot of Microsemi parts We usually use 3 17 volters in series on a 28 VDC bus. I've done lightning tests on them and they will handle some real peak power for a short time.  Cubic used MOVs after the rectifier in the 3030 supply. As they die they get less resistive until they fry the rectifier and traces around them. I have never put them across a 120 volt line. A power linear supply might want one on the secondary

In my humble opinion, these guys make the best on earth.  I've used them for many years protecting gear fed from commercial power mains and never had a  failure.

The larger units usually cause the high tension fuses on the pole pigs to open for a big surge, and save everything on site from damage.  I've seen other sites using generic stuff just explode,  these just keep on working.

This is with them installed on your usual load center breakers.  They work on the 'continuous conduction' principle, with rise times in the 2-3 nS range.  Very robust, very fast.

Many types available as well.

http://www.joslynsurge.com/product.xhtml?Product_ID=js_1250Series

Another good producer of mains type suppressors is ICE :  http://www.iceradioproducts.com/3.html

ICE (Industrial Communications Engineers) also make antenna, telephone, and control line suppressors and have some useful reading material on their site:  http://www.iceradioproducts.com/10.html

The first year after I bought my house in the country I lost several items including a well pump from two different lightning surge events.  I installed the ICE suppressors at the main distribution box breaker (where separate lines go to the house, barn, and both outdoor garages) and also the panels in each building  have one of the mains suppressors along with the incoming phone line.  I have had no further lightning related problems in the 20 years since I installed them.  My well driller friends started using these three years ago after they ran into a lot of problems with the controllers for the newer variable speed pumps and this seemed to have cured those issues.  My only relationship with ICE is that of a very satisfied customer.

I understand direct orders from ICE can sometimes be a bit slow but you can also get them through Array solutions:  http://www.arraysolutions.com/prod_company.htm if you need fast service.

Rodger WQ9E

Another interesting series over current protection technology is the PTC (Positive Temp Coefficient) thermistor.
http://www.rtie.com/ptc/overcurrent.htm

These are like resettable fuses that go high resistance when they have an over current situation. After the short is cleared they return to the normal low z state. How many time they can do this? Dunno.

Mike WU2D

we usually test transorbs with a longer pulse 40 us rise / 120 us to 50%  voltage on the fall. RTCA DO-160E Waveform 5A. We do level 4 750V/750A a lot.

A lot......Every TV has one on its degausing coil...

Tom - AB3FL

Did anyone else notice the $20 handling fee on Newark?

The 1.5KE is a good device but can take only a 1500W hit.

I prefer larger capacity 5KW device, especially with the larger #10 and up wire supplying my gear. Of course wire size may not matter if you get a big enough hit - then only the Transil's capability will matter. (Transorb=littlefuse, Transil=ST - just names.. )

The BZW50-150B and BZW50-180B are rated 5KW, the only difference between the two BzW50's being the 150V or 180V standoff voltage. Note the standoff voltage is that at which 5uA is conducted.
 76 cents at http://www.onlinecomponents.com/buy/SGS-THOMSON/BZW50-180B/

(BZW50 I mentioned have 166V or 200V min. breakdown volts respectively. Breakdown V is where 1mA flows, and is the true start of the conduction.)
Note the 1.5KE160CA breakdown voltage has a 152{min} 168V {max}range (107 to 118 RMS) so be careful selecting or a device can conduct a bit each half cycle and they only dissipate so much steeady state power.
I use the BZW50-180B because its breakdown voltage is 141V RMS (200V peak).

The company I work for makes the 5KW BZW50 devices. Also makes 1.5KE, but not the 160V ones, the 150 and 180V. 180v makes more sense if sticking with a 1.5KW device because the corresponding RMS line volts is 127.3 still a little low but would be OK if your line is stable.

The 5KW devices cost more but what's your gear, time, and trouble worth that you would not use the more rubust item?

It is worthwhile to thoroughly read the datasheets on solid state clamps unless you are very familiar with them.
I work with these kinds of things every day so I can answer specific tech and appliction questions.

I have one each BZW50-180B across the primaries of the two big transformers in the Tucker Transmitter and not only for utility-related issues.
If there is a fault on the secondary side or an abrupt cut-off of the line voltage such as overcurrent trip leaving the primary open, very high spikes of many KV can be generated in these large transformers due to load dump of the core demagnetization and contribute to the destruction of rectifers, etc. mit der spitzensparken and poppencorken. Every time I open a piece of gear it gets one.

p.s. my company ST makes the BZW50, so help me keep my job hehe..

here is a nice appnote that would apply to the general type of voltage clamping device.

How about this one?

The Opto 22   45 amps.. 120 volt.  $34

http://www.opto22.com/site/pr_details.aspx?item=120D45&qs=100610161053,,,18,26&

The debate of Transorb versus Varistor seems to get one sided at the higher power end towards Varistors. I am currently installing a piece of gear at a customer that requires 480 VAC at 100 amps / phase. This is serious power! The power feed after the 125 amp 3 ph breaker has 6 fist size Varistors; 1 between each phases combination, and 1 each between ph and ground. I don't believe any Transorb  would fit this application unless we had a series/parallel Transorb module.

The success of any transient suppression network is helped by examining the transient source impedance (Rs) versus the clamping devices clamping resistance. Since transients are often very short in duration, increasing Rs with series lead inductance via ferrite beads is practical and beneficial. Better yet, make the bead lossy at the clamp ringing frequency to better isolate the source from the load.

That Opto 22 SSR posted earlier is a good candidate for PTT primary keying of a HV plate transformer so long as we swamp the transformer primary with enough resistance to insure continuous conduction of each half cycle. Maybe a small HV on incandescent lamp will suffice.

Jim
WD5JKO

What handling fee?

I've never been charged any handling fee from newark, even if I only buy $10. of parts.

I order from them fairly often, last time was less than a month ago.



The only mention I see are special items, and if they charge you the $20 fee, you don't pay shipping.

"What is the Supplier Direct Ship Handling Fee?
Supplier Direct Ship items that have a service charge indicated are coming from our Farnell warehouse. The service charge will be $20 per order, in lieu of a freight charge for Farnell items."

I have a Polyphaser-branded protector on my house service right across the main breaker panel - installed about 1994. I removed the guts from the Polyphaser box, and installed the 2 big Harris MOVs on the power legs that feed 240 VAC to my shack. The bottoms of the two shunt MOVs go to a short heavy ground rod outside. There is a 2 pole breaker that feeds the shack, and consequently, the MOVs. A small 240 VAC relay remains energized all the time, when the power is on that circuit. If the MOVs were to short, and the breaker open, the relay opens and a sonalert sounds. If the MOVs were to open, would have no protection. I haven't had any problems with them, but then again, they could be open. Only a power supply would tell.

 

This week I hit 3 series 15KW transorbes with a 500KW  lightning pulse
and nothing blew up 750V / 750A WF5