Author Topic: What were these MOVs before they blew up?  (Read 1018 times)

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Online TerraHertz

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What were these MOVs before they blew up?
« on: July 10, 2016, 08:52:41 pm »
Fixing a small power supply for a 3COM network POE switch. It had two MOVs, each one in parallel with one of the two rectified mains electros. Both MOVs have blown up, which of course also took out the fuse.
Problem is, I'm not sure what value MOVs I should replace them with. The numbers on them don't seem likely to be actual voltage ratings. And they are obviously seriously underrated in power, if they were intended to protect the supply more than once.

Does anyone recognize the manufacturer, or better yet have a link to a data sheet?
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Online Ian.M

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Re: What were these MOVs before they blew up?
« Reply #1 on: July 10, 2016, 10:04:10 pm »
Did you try googling V07K140?
7mm dia 140VRMS rating, though I agree there seems to be a lack of current suppliers and data.

If you've got room to cram in 9mm or 10mm dia varistors, do so - don't forget the flame retardant heatshrink.
 
Assuming a high mains voltage of 253V RMS (EEA upper limit: nominal 230V+10%), there will be 179V on each reservoir cap (Vpeak/2).

http://uk.rs-online.com/web/p/metal-oxide-varistors/7858736/ looks likely:
Varistor Voltage   220V
   Maximum AC Voltage Rating   140V
   Maximum DC Voltage Rating   180V
   Clamping Voltage   360V
   Clamping Current   25A
   Energy   30J
   Series   ZNR
   Diameter   9mm
   Maximum Surge Current   2500A

Or if its too tight, for space try  http://uk.rs-online.com/web/p/metal-oxide-varistors/7858708/:
   Varistor Voltage   220V
   Maximum AC Voltage Rating   140V
   Maximum DC Voltage Rating   180V
   Clamping Voltage   360V
   Clamping Current   10A
   Energy   14J
   Series   ZNR
   Diameter   7mm
   Maximum Surge Current   1200A

To find similar, go to your preferred distributor's parametric search and look for a 180V DC continuous rating, and the lowest clamping voltage + highest current you can find with an acceptable form factor.
 

Offline madires

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Re: What were these MOVs before they blew up?
« Reply #2 on: July 10, 2016, 10:12:46 pm »
Looks like a standard 7mm MOV. 07 for 7mm disc, k for 10% tolerance and 140 for the voltage. 140V types are offered by several vendors. Are the two electrolytic caps in series?
 

Online TerraHertz

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Re: What were these MOVs before they blew up?
« Reply #3 on: July 11, 2016, 12:04:33 am »
Did you try googling V07K140?

No! And I don't know why not. Sigh. Something like 20 years of using the Net, and I still sometimes get caught like that.

Also I forgot that MOVs are generally rated in AC V, and so the "140" couldn't be the voltage, since obviously that would have to be greater than half the rectified mains voltage (240VAC --> around 336V DC.  /2 = 168V. ..ish.  As you pointed out.

Anyway, something interesting. See partial schematic of the board.
It has a mains input MOV, of reasonable size. And equalization resistors across the caps, though they are tiny 1/4W things which is just begging for one to go open circuit due to voltage stress. (They are OK still.)
So why the extra MOVs?

Possibly because there is an auto mains voltage select circuit for 110/240V operation, using a triac instead of the simpler manual selector switch. The 'module' circuit is something I'm not going to bother tracing out, but it occurs to me that if something went wrong (transient?) and the triac turned on with 240V mains present, this thing would try to charge the electros pair to twice the expected voltage.
Maybe the small MOVs are included for that case?

In which case since this will always be run on 240V, I could just remove the triac, and leave the small MOVs out.
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Online Seekonk

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Re: What were these MOVs before they blew up?
« Reply #4 on: July 11, 2016, 12:12:44 am »
Are you saying that these are across electrolytics.   In the US, UL used to allow 130V mov's across the line.  There were lots of failures and they raised it to 150. a long time ago, some manufacturers rated the MOV with DC voltage.  Be careful and check spec if you salvage an old mov.
 

Online Ian.M

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Re: What were these MOVs before they blew up?
« Reply #5 on: July 11, 2016, 12:19:16 am »
I'd be reluctant to remove the additional over-voltage protection the 140V RMS MOVs provide, especially if the location is rural or suburban with overhead power lines   However, as their mounting holes are so near each other if you are removing the thyristor, a single 10mm 360V DC rated MOV could be substituted.
 

Online TerraHertz

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Re: What were these MOVs before they blew up?
« Reply #6 on: July 11, 2016, 12:37:28 am »
Are you saying that these are across electrolytics.

Yes. I think this is rather unusual. It's one reason I started the thread.
Also the presence of a much larger MOV in the customary manner across the mains input prior to the bridge rectifier suggests the purpose of the small ones has nothing to do with mains over-voltage. I now think they are only there as protection against something going wrong with the triac mains 120/240 detection circuit. Which it obviously did go wrong. The triac is not shorted btw.

I have a fair collection of MOVs (see pic), but of course none of the right voltage rating.
So, definitely going to just remove the triac and it's drive circuit, and omit the MOVs across the electros.

Another reason for wanting to sort this out, is I expect to be given some more of these 3COM POE Wireless LAN switch (WX1200) units, and a bunch of the distributed wifi antenna modules they host.
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Offline madires

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Re: What were these MOVs before they blew up?
« Reply #7 on: July 11, 2016, 01:33:41 am »
Also I forgot that MOVs are generally rated in AC V, and so the "140" couldn't be the voltage, since obviously that would have to be greater than half the rectified mains voltage (240VAC --> around 336V DC.  /2 = 168V. ..ish.  As you pointed out.

From a Littlefuse datasheet for a 140V MOV (V140LA2P) :
- Maximum Rating (85°C), Continious
  V_RMS: 140V
  V_DC: 180V
- Specifications (25°C), Voltage at 1mA DC test current:
  V_min: 198V
  V_max: 242V

That matches the 200V caps quite well.
 

Online TerraHertz

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Re: What were these MOVs before they blew up?
« Reply #8 on: July 11, 2016, 01:51:30 am »
Fixed. Apart from replacing the fuse, by removing parts.
Thanks for all the help suggesting replacement MOVs, but really I'm certain they were only there to deal with potential mis-operation of the 120/240V 'universal supply' circuit. In case it turned on the triac while on 240V mains.

So, removed that entire subsystem.
The module might be interesting to trace out, if I had time. A thing that activated the gate of a triac while the mains is 110V, not while 230V.
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Offline SeanB

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Re: What were these MOVs before they blew up?
« Reply #9 on: July 11, 2016, 04:51:27 am »
I hate those auto switch modulers. They suffer badly in that the trigger level is actually reached on power on, so they briefly ( on power on) fire the triac for a single cycle. As well they also tend to trigger on any brown out when AC drops below 180VAC for a few cycles, thus the MOV across the capacitors to prevent this from causing a fire, by blowing the fuse from overcurrent.

There were slightly better ones made  with a small IC that had both switch on delay and better voltage thresholds, but that also had a need for a 2W resistive dropper from the incoming mains to power it, like yours, but which were likely to fail as short circuit from mains spikes.

Either way they are not nice, and thankfully improved SMPS designs that made a truly wide range input practical from 65VAC to 265VAC with no switching.
 


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