Gate Opener Circuit Board Diagnosis Challenge


I have a gate opener system that is powered by a small circuit board, shown in the accompanying photos. The board uses a 12-volt battery backup, and it receives an 18-volt DC current to keep the battery charged. Right now, the board runs exactly as it should when only the battery is connected. The problem is that, for some reason, when I connect the 18-volt power-in from the manufacturer-specified transformer, it blows the thermofuse in the transformer (which cannot be replaced). The first time this happened, I figured it was just a bad transformer. However, the board blew a second transformer, so there is something going on.

While I had the 18v power connected, the green LED in the lower right-hand corner came on, the way it is supposed to, to indicate that the board has external power. Again, everything on the board works just fine when only the battery is connected.

I don’t see anything on the front or back of the board that looks like it could be shorting out the board, resulting in a blown transformer. I would welcome any suggestions about things that I could look at as possible faults. Also, if anyone is interested in trying to repair this for a fee, I would be interested in talking to you.

These boards cost $200 new and can be had for $80 used but I hate to throw it away if there is a simple answer to the problem. Thanks for reading my post.


The problem may be the surge protection MOV (Metal Oxide Varistor), labeled VAR1 on the PCB, has absorbed its lifetime capacity of surges and is now clamping at too low a voltage.

I’d try removing VAR1 and see it that fixes the problem. If you have a test bench power supply with current limiting you can use that in place of the wall wart to avoid burning out another one. If you don’t have a bench PS then wire a fast blow fuse rated for the wall wart current output in series so that the fuse blows instead of the wall wart.


Hi Paul,

Thanks a lot for looking at my post and responding. I am going to give your suggestion a try. I don’t have a bench PS, so I am going to try the fast blow fuse approach after removing the MOV.

The dedicated PS is rated 18v, 40VA (2200ma).

I am sure the blade-type automotive fuses are readily available at the local autoparts store; the glass tube kind I would probably have to order online. Do you think a 40A blade fuse would work for this purpose? Also, do I need to put one in series on each of the two wires between the PS and the board, or just on one of them?

Thanks again for your help - I am very excited that it may be possible to fix this. I was getting ready to send it off to one of the online PCB repair businesses and then I read the fine print on their website, which explains that they only work on industrial circuit boards and have a minimum repair charge of $250!

Best regards,



A 40A fuse will provide no protection for a 2200mA (2.2A) device. You need a fuse with a rating of 2.2A or less so I would use a 2A fuse. If the 2A fuse pops right away I’d try a 2A slow blow or 2.5A fast blow next.

You only need a fuse on one line from the wall wart for full protection.

It can be hard to find 2A blade style fuses in stock anywhere local, but Home Depot and many other hardware/home improvement/auto parts stores carry glass fuses and inline holders, so you should be able to pick them up locally.


Thanks again, Paul. I’ve just ordered some fuses online and I removed the MOV in the meantime. I will report back once I get the fuses and try applying power to the PCB again.

I thought I would try to find a replacement MOV to put back when the time comes. I am attaching a photo of the one I took out. The markings are: ZNR (which I understand refers to a MOV) / V14390 / 94. Here’s a photo:

I found V14390 in a Panasonic catalog and I see that Mouser is selling two versions of it, but I am not sure either is a suitable replacement:

If you have a chance to comment on this at some point in the coming days I would be grateful for your guidance. Thanks again.



That Panasonic part is the correct replacement, here’s the Digi-Key page for it.


That’s great - thanks again Paul. I am glad to support Digikey (they have been the supplier of limit switches for my gate opener arms).

Have a good night.



Hi Paul,

I finally received the 2.0 amp slow blow fuses in the mail today and I soldered one to one of the leads between the PS and the PCB power input. I had it plugged in for more than 10 minutes and the fuse did not blow, nor did the PS, and I could see from the green LED on the board that it had power. So, that seems to confirm your theory that the MOV was causing the problem. I am impressed by your ability to troubleshoot accurately like that based on a photo or two and a description of the problem.

Do you think I need to do any other troubleshooting at this point (like removing the 2 amp fuse and wiring the PS directly to the board to see what happens)? Or should I just go ahead and buy a replacement MOV and solder it in?

Thanks again for your expert assistance.



Your very welcome, I love to help others with problem solving.

I knew that this type of failure will happen given a long enough time in service when I first learned about and added MOV’s as surge protectors to products 35 years ago. Although it was more than a decade before I actually saw a failed unit. For the last almost 40 years I’ve worked at one company designing small AC powered instruments that on average stay in service for 25 years. So I’ve helped our technicians with troubleshooting these types of failure symptoms many times.

No you don’t need to do any more troubleshooting, install a new MOV and you should be good to go.

Since you’ve got it, I’d leave the fuse in place to give extra protection to the AC adapter. At some point in the future, maybe not for another decade or more, some other parts are going to fail and cause a different fault, excess current draw. The fuse will prevent that failure mode from burning out the expensive AC adapter as well as protect against damage from the nearly certain future failure of the new MOV.

I don’t include a fuse in my product designs because of the high cost of a fuse and holder and with my designs the warranty is only 5 years and I’ve never had an MOV fail in less than 10. Most of the power side MOVs we ship are still OK 35 years later so the extra cost isn’t economical for us. Most potential customers already balk at the higher cost of our products (2x to 20x the competition). Many of then don’t buy from us until after half a decade or more of paying to service our multiple competitors 1 year warrantied products. Many more potential customers just give up on the whole product category thinking that warranty length is not an indicator of product life costs. :frowning_face:


Thanks again, Paul. I just placed an order for some MOVs from Digi-Key.

I have always had an interest in electronic circuits but never had a chance to develop the knowledge. When I was a kid my dad bought a HeathKit alarm clock for me to assemble, along with a soldering iron, as birthday gifts. I got impatient waiting for him to show me how to go about it, so I followed the HeathKit instructions and soldered all of the components to the PCB. There probably was something in the instruction manual about using as little solder as possible, but I missed that. I basically covered the bottom of the circuit board with solder. My dad sent the board back to HeathKit and remarkably they were able to get all the solder off there.