Gate Entry Power Supply "Kaput"

[Crackers]

Hello all,

I have a dual power supply to our front gate entry system. Unfortunately the power supply got wet due to a child with a hose and it stopped working. Being $100 per unit I decided to have a look to see if it was a simple case of a blown fuse, which I thought it was. So I replaced the fuse but when I turned it on it blew again, but this time I noticed a green disc capacitor had a small hole in it.

Now I cannot be 100% sure that the capacitor was also damaged before I put the new fuse in but I am having a hard time identifying this particular part.

The power supply has 240 Volts in and splits it to a 12 VAC and a 18VDC sides.

Any help would be greatly appreciated.

 

[(*steve*)]

I would check t see if you have a bridge rectifier that creates the 18VDC. It is possible that one or more diodes in this bridge have failed short circuit and this is causing fuses to blow.

Show us pictures of the unit (especially the capacitor). It is unusual for capacitors to have holes in them, but there are some that have a small slot cut into them -- however you don't seem to be describing one of these.

 

[Crackers]

Sorry Steve,

I thought I had attached some photo's but it look like they did not show up. The Hole in the capacitor that I talk about was caused buy it blowing up. It is on the opposite side to that in the picture.

I have attached a picture of the complete unit.

Cheers

Craig

 

[(*steve*)]

Thanks for the images.

The chip on that component should not be an issue. Does it come from the position marked TH1? If so, it is a thermistor, not a capacitor.

It would now be very useful to have you read the markings on all the devices attached to the heatsinks and the yellow box, and get an oblique view of the board so we can see the large device between the heatsinks. Also an image just like the last, but from the other side.

I suspect that something has failed short circuit, and I want to find out what the options are before I ask you to check stuff.

 

[Resqueline]

There's an SCR there, although a small one. They're usually used in crowbar circuits that are intended to blow a fuse if they detect an overvoltage on the output.
So it could be that the PSU generates too much voltage now, or that the SCR could be getting a leakage current into its Gate so that it triggers prematurely.
Anyway, more pictures are essential, and don't forget the underside.

 

[Crackers]

Ok this is a picture of the back of the board. I have added in some arrows to help indicate where some of the components are located on the board.

 

[Resqueline]

Ok, forget about the SCR, it seems to be connected in the startup circuit and is not responsible for blowing the fuse.
Ohm for shorts between any of the (4) terminals on the bridge rectifier for starters.

 

[Crackers]

Here is a picture of the Capacitor and the Thermistor. The capacitor was removed as to test it by some of the guys I work with. But after removing it they found that the meter that they wanted to use was away for calibration.

Below the capacitor is the thermistor and the black mark on it is where it has blown out.

 

[Resqueline]

An ohm-meter of some sort is needed. Even a battery & a bulb will do for testing the capacitor, the NTC, & the rectifier.
NTC's will sometimes "blow out" when presented with a short, but most often they'll still work.

 

[Crackers]

I tested the rectifier with my multi-meter. I had some strange readings. Placing the leads across the terminals would show a resistance then within a second a short. I thought that it was my meter so I tried my neighbors and it showed a short.

So in the morning I am going to remove the rectifier and retest it. If the rectifier is shorting would this be the likely cause of the blown fuse and my problems?

 

[Resqueline]

What range? What terminals? The capacitor? What did the display actually show?
There are (at least) 6 measurements to be taken when testing the bridge rectifier.

 

[Crackers]

Hi Resqueline,

Sorry I should have elaborated. I only tested the rectifier. I used the same method as per this link. HERE

As for values I shall go and test it this morning again to record the values, thats assuming the way I have tested it is correct?

 

[Resqueline]

That method is incomplete (and partly erroneous).
1: He did 6 measurements while he should have done 8.
2: He measured + to one ~ but missed doing + to the other ~. Same for - to ~.
3: He measured between the two ~'s. You don't (need to) do this on a bridge out of the circuit.

Besides, I hate having to look at (through) a video when dealing with electronics, but that's just me..

Here's the way (in-circuit) measure both ways with the diode-test:
~ to ~. Should be open (1--- or OL). A short here is not in the bridge rectifier.
+ to one ~.
+ to other ~.
- to ~.
- to other ~.
+ to -. Should be a rising number (going to OL with red on + & black on - , going to 900 the other way around) as the capacitor charges. A short is not in the bridge.

A reading of (close to) 0 means there's a short (somewhere). Black probe on + or red probe on - should give around 450-550. The other way around should give OL.

 

[Crackers]

Ok here is what I got using the diode Ohms setting on my multimeter with the rectifier in the circuit.

~ to ~ = 1
+ to ~ = 1
+ to other ~ = 030
- to ~ = 1
- to other ~ = 1
+ to - = 1 with red on +
Black probe on + & Red on - = 500

I hope I have done it right,

 

[(*steve*)]

OK, one of the diodes has failed short circuit. Replace the bridge rectifier.

 

[Resqueline]

Yep, what steve says.
What gives it away is the reading of 30 (mV) on your third measurement. The reading of 500 (mV) on the seventh measurement may also be an indication.
We assume that by 1 you mean only the far left digit was lit, which is an "overload (OL)" condition, or rather an over-range condition (over 1999 mV).
You measured only one way btw.. A full (both ways) measurement requires the opposite polarity also, making 12 measurements in total.
But that doesn't matter for the conclusion in this case anyway.

 

[Crackers]

Thanks for the help all involved. I am off to the shops this afternoon to get some components. I will let you know how i go.