Calling electricians : Fridge power supply

[Scooter]

Morning all

I have been having a problem with my old fridge, it had a system installed similar to the NL (by the previous owner).
Now the problem is, when plugged into 12V it works great, but when plugged into 220V it works for a while then the fault light comes on.
This light flashes once every 5 sec or so and according to the guide it means low voltage.
So in my wisdom I thought it would be the rectifier diode and so I replaced it, only to find that is doing the same thing, works fine then doesnt.

Now my question to the electricians, could it be the transformer?
I have checked the readings and when it is working I get +-15.5V on its way to the compressor and when it is not working 17.8V.

As with everything I would prefer not to have to take it to some guy who changes a part costing R5 and charges R500

Help Please

[Scooter]

Can nobody shed any light on this subject?
After fidling with it for a while, checking what I could with limited knowledge, it was working fine again.
Now it has started its crap again. I am really at a loss for answers here.

[cprinsloo]

Hey Scott, check that neutral connections!!!!

Seriously, I'm not at all familiar with the NL power supllies, I assume the compressor is a Danfoss, of which you can change the cut-out voltage, with the risk of damaging the battery.

There can be a dip in the supply if the P/S get hot, etc. which will cause the compressor controller to cut out, hence the one flash of the diode.

The Compressor draw quite a few amps when it starts, and you need a P/S with some oemf to get it going, once it's going the current drop substancially.

Is the dual battery still OK?

Cheers,

Chris

[Scooter]

Hey Chris, old faithfull, maybe I should just PM you all my electrical questions
Thanks for the reply. maybe I should elaborate a bit on what I have found. I took the power supply out, it is in fact not a NL P/S but just a run of the mill P/S that the previous owner had installed to start up the danfoss compressor. The system works on the same principal as the NL with different speeds and the fault light.
The P/s consiste of: 1 x Toroidal Transformer, 2 x capacitors (4700mf), 1 x diode, 1 x relay, 1 x schottky diode (bridge rectifier I assume). So as you can see it is fairly simple, or is it? I have replaced the diode and the schottky diodes (at a cost of +- R3) but it still gives the same problem so my next thought was the relay and transformer. The relay gets very hot while the fridge is on and perhapd that is causing a drop in power?
The transformer doesnt get hot at all but perhaps there is a break in the windings when it heats up a little bit?

Dude, the dual battery system has given me so much crap but I think I finally nailed it., I will post an update on my findings.

[cprinsloo]

Hi Scott,

The set-up seems simple and straight forward. The relay getting hot can maybe be a problem, I should think it must get warm, but not hot.

If you can replace everything except the transformer, maybe it will solve the problem. Don't forget the caps, electrolytic caps only have a certain life span. Can you perhaps send me a photo and a layout of the p/s if possible? I also have an old Tropical freezzer with the Danfoss and a separate p/s, also a torroid transformer, but the chip on mine has burnt out, and I cannot see what it was. Maybe I can convert mine to the same set-up as yours. Otherwise I'lll just buy a big battery charger, one that can handle the starting current.

Good luck,

Chris

[Scooter]

Chris I will take a photo this weekend and draw up the layout.
The transformer is a 50-60hz 150VA 12v output
The caps are 4700mf 25V http://www.mantech.co.za/ProductInfo.aspx?Item=14M5852
The diode is a 1N4007 http://www.mantech.co.za/ProductInfo.as ... 1N4007TB/R
The Schottky diode was a PBYR2045CT (I replaced it with a MBR2045CT from mantech http://www.mantech.co.za/ProductInfo.as ... =MBR2045CT
The relay is a siemens http://www.mantech.co.za/ProductInfo.aspx?Item=102-4021

Hope this help for now. Mantech didnt have the relay when I was there but they did give me the name of another place that they thought would have it.
Problem being that it would be an open frame relay which might be problem with all the dust? AP Electronics http://www.apelectronics.co.za/

[Scooter]

I found a relay, after searching numerous component suppliers, at http://www.electrocomp.co.za at a cost of R7.50.

Chris I took a photo and drew up a layout for you. The layout is drawn as viewed from the top of the PCB (as with the photo). I am sure you will understand it but let me know if not.

fridge.pdf

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[Family_Dog]

Scott,

Your drawing of the 20A blade fuse should have a link going to the Schottky Rectifier?

There is nothing wrong with your toroidal transformer, these things seldom go faulty. Is the new relay rated at sufficient amperage for the contacts? Does anything get hot when it runs of AC? Can you measure some voltages with it operating initially (while the fridge is working) and then after it starts to play up. Are you able to measure current drawn (DC Amps) under both sets of circumstances? Make sure you have a suitably rated DC Ammeter and then remove the 20A fuse and connect the ammeter leads there, preferably either soldering the leads or using decent spade connectors. Make sure you use a heavy gauge wire for this, and keep the leads as short as possible.

-F_D

[Scooter]

F_D, thanks for the advice. I will definately check what you have suggested. The only part that gets hot when running on AC is the relay, hence my feeling that replacing it might solve the problem, BUT I havent replaced it yet and the fridge has been working fine for the last 5 days.

I do appologise if there should be a link between the fuse and Schottky rectifier, I will have to check the PCB and make sure I didn't miss it. I will update the drawing once checked.

The new relay I bought is exactly the same as the old relay and both are rated as : http://catalog.tycoelectronics.com/TE/b ... 6A1001C133

Nominal Voltage = 12 VDC
Rated Current @ 85° C = 25 Amps.
Contact Arrangement = 1 Form C, SPDT, 1 C/O

The voltage test you mention, where would you like me to read a voltage from? I have already measured the output to the compressor when working fine and not. I got a reading of +-15.5v when fine and +-17.8v when flashing if that helps.
The amperage test I will set up and do while it works but will have to play the waiting game for it to play up.
What do you reckon my reading should be?

[Family_Dog]

Scott,

The rectifier is a heavy duty one rated at 25A, the relay has 25A contacts, the transformer will suffer a nominal voltage drop when the compressor is drawing current. That all sounds fine as it is an unregulated power supply. The fact that the voltage rises when the "fault" occurs means that the compressor is not drawing current. This can be caused by the thermostat switching off at the desired temperature, faulty wiring and/or plugs (if there are any) or a faulty relay. The relay should not be that hot to the touch; the coil only consumes 1.6W according to the web reference you provided. That means that the contacts might be arcing.

Can you remove the cover of the old relay and insert the relay back into the circuit? You should see a small spark at the points when the relay closes and a probably larger one when the relay de-energizes. If the points (contacts) are heating up, this might be due to a relay having burnt its contacts and thus producing a high-resistance to the circuit. This could be caused by constant usage and wear and tear, although had they connected a cap across the points (as in an old-type distributor), this would alleviate the problem. Or, it could be that the compressor is working against a load and drawing excessive current, but in that case the 20A blade fuse should have blown.

I'm not sure what the current consumption of the compressor should be - I guess about 10-12A when working. Chris?


-F_D

[Scooter]

Jeepers this fridge is keeping me busy SWAMBO thinks I'm giving her the "cold shoulder"

It makes sense that the voltage would increase when the comp isn't drawing current and that is exactly what's happening when it plays up.
The compressor stops and the light flashes. As I said it is working fine for the moment, why? who knows, it is tempremental.

As soon as it decides it has had enough I will replace the caps and the relay, then at least I will be sure that all components, bar the transformer, are new and shouldn't give me hassles.

Keep the advice coming, I love learning new things and electrical is definately not my strong suit.

Thanks F_D

[cprinsloo]

Hi F_D/Scott,

The compressor should draw about 3 Amp (mine does). Can be a little bit more or less. There should be a fan to cool the compressed gas. Is the fan OK? If the fan gets sticky it can cause a volt drop and cause the compressor controller to cut out. Otherwise I agree, the relay is a mechanical part and can get worn out. I'm not too sure about the function of the relay. When does it switch on and off? Is it controlled by the thermostat?

Cheers,

Chris

[Scooter]

Chris, I know the relay switches on when I plug the fridge in but I have not noticed if it switches off with the thermostat. I havent had the patience to sit and watch it till it gives in
If it works with the thermostat what would that mean as opposed to not?
As far as I can tell the fan is fine and works perfectly when the comprssor is running.

[cprinsloo]

Hi Scott,

I was just guessing, I'm also a bit clueless. I'm wondering why there should be a relay at all? My P/S doesn't have a relay, I don't know if the relay does have some kind of safety function. F_D?

Cheers,

Chris

[Family_Dog]

A relay would normally be to isolate the battery when running off 220v, but the sketch isn't very clear on this. Scott did state that it is not the normal NL PSU, so it might be from an old power supply for something else. Don't think it operates as a battery charger!

Scott, unless the electrolytic caps are leaking, I can't see any advantage in replacing them. They won't have much filtering effect at the high amperage drawn by the compressor in any case.

Any chance you can check & redraw the schematic again? And this time, shade in the copper print... I get cross-eyed trying to follow the block diagram Also, indicate which are the relay contacts & coil terminals, should probably be marked on the relay it self.


-F_D

[cprinsloo]

Hi Scott,

I talked a lot of shait, I missed the drawing that you made!!

Agree totally with F_D, the relay should activate when you connect 220V to isolate the battery. Nothing to do with the thermostat. But what doesn't make sense to me is that if the relay activates, the "live" doesn't seem to go through the relay contacts, so the relay should not influence the voltage, because the "live" goes directly to the transformer and then back to the rectifier, to the 20A fuse and on to the compressor. Nothing much that can cause volt drop, except if the "live" also goes through the relay contacts. But maybe I'm completely wrong!! :evil:

Maybe, big maybe, there is a dry joint somewhere. But you replaced most of the components already, so the chance is you resoldered most of the joints in any case.

Can you perhaps make a drawing of the underside of the board, there should be some copper as well? And the lay-out of the relay connectors will also be of much help. I would also like to understand exactly how this works

Cheers,

Chris

[Scooter]

I have attached a new pdf. It is the same layout, just shows both the upper and underside of the PCB. I have not had time to check whether the 20A fuse is in fact joined to the rectifier, I will try check tonight and change the drawing if it does.

As for the relay contacts, I have no idea which ones are for the coil. I have not opened the relay so have to look at the info I have. Maybe one of you can make heads or tales of the product info on the TYCO site:
http://catalog.tycoelectronics.com/TE/b ... 3#features

fridge2.pdf

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