Message Forums

Now I am really confused !!!       You can tell it isn't me who knows about electrics!  

Was it the cool box the last time or not???

Should we get an adaptor to use it on 240v instead?

Or was it something entirely different???    HELP!!

Some more info.......We connected the regular battery charger to the battery and were able to charge it up a bit overnight and use a couple of the 12v lights , water pump etc.  But the lights were not as bright as before.  We didn't use the coolbox again as it does draw quite a bit of power.    Does that help the diagnosis? Cos we really want to know.

Quote: Originally posted by wrekin on 28/02/2005

---

A blown fuse is nothing to do with voltage, it is all according to a rise in the current being drawn by appliances!

If these appliances exceed the rating of the fuse, that is what makes them "blow". Starting up a conventional houshold microwave will cause a power surge and your drawn current will spike, thus blowing the fuse.

---

You are right as far as what makes a fuse blow but please read what I said..."a dead short across live and neutral"

To be precise a  voltage dependent resistor or 'VDR' is connected across the supply and normally it is open circuit. When a voltage higher than design (in this case 275volts) is 'pushed' though it, it permanently changes to closed circuit and connects live to neutral....directly!!, this causes the current surge that blows the fuse...but it was over voltage supply that caused the fault rather than an internal fault drawing to much current.

If however an internal fault did cause the fuse to blow that would be another matter but it would not be overloading the 12 volt output that caused it but a component failure which will only blow a replacement fuse which must always be of the correct rating for fire safety

Quote: Originally posted by wrekin on 28/02/2005

---

A blown fuse is nothing to do with voltage, it is all according to a rise in the current being drawn by appliances!

---

Not always true, I dont know the charger in question, but what Arc Systems says is entirely feasible.

Its fairly common in power supplies etc to protect equipment from overvoltage by some component detecting the overvoltage condition and then placing a short across the input or output which is there for long enough to blow the fuse.  

Its called crowbar protection.

True very true...

...my point being though that it is current rather than voltage blowing the fuse.

Anyway lets not argue please, Jennifer had a problem - I have heard before that if you connect your coolbox to a battery independant of a charger, it will drain the said battery completely flat - with no fault.

You will have to trickle charge a battery if you wish to run something like a coolbox.

Quote: Originally posted by jennifernn on 28/02/2005

---

Now I am really confused !!!       You can tell it isn't me who knows about electrics!  

Was it the cool box the last time or not???

Should we get an adaptor to use it on 240v instead?

Or was it something entirely different???    HELP!!

Some more info.......We connected the regular battery charger to the battery and were able to charge it up a bit overnight and use a couple of the 12v lights , water pump etc.  But the lights were not as bright as before.  We didn't use the cool-box again as it does draw quite a bit of power.    Does that help the diagnosis? Cos we really want to know.

---

I don't know what caused your two chargers to pack up but in general these are some thoughts which hopefully will give you a clue.

The big difference between a 'battery' charger and a caravan charger or to give it it's proper name 'power source' or PSU  is the psu is designed to provide power without battery backup, while the battery charger is just that and provides enough power to charge a battery but not to run the vans 12 volt systems alone..so that explains the dim lights.. There are other differences like higher voltages but these are not so important, the main point is the caravan charger is not a very good battery charger but is a very good 'power source' at least since the KT12SM came out in the mid 90's.Some will say the KT is not so good but I have used one I repaired for six years now and it's not missed a beat with or without a battery and all last year with one cell down in the battery!!

The KT12SM and it's successors are all 10 or 12amp power sources which are capable of working at near maximum load for extended periods of time providing they are well ventilated as heat is about the only cause of overall failure unless a faulty component gives up. The big capacitors are the main thing to fail when they get above 85 degC?, they will be fine again when cool but the damage has been done to other components. Under normal conditions psu's only need to deliver about half their output at any one time and the heat produced is considerably less, your cool box perhaps draws 6 to 9 amps and if the battery is down this will be pulling more amps trying to recharge. In these circumstances if the maximum output is exceeded the output fuse of the psu should blow and protect the psu from damage. Many 'main' fuses from batteries are 25 amp which is there to protect the wiring but not the psu which requires a 10amp timed fuse or a 15amp quick blow and this is located in the psu box. I should also add that although some are 12amp output, what they really are is 10amp ones...turned up a bit!, perhaps that's why my own has worked so well because I turned it back down!?. Zig 18amp X8's by the way are 12amp X7's with a fan...no other difference.

A faulty cell in a battery is a common cause of psu failure either because it is pushed to it's maximum for days on end trying to charge it, or if the battery develops a sudden short circuit and the psu is 'over fused' it will kill it very quickly indeed. Again it is heat that kills but the battery caused it.

None of the above will in general blow the input fuse, the most common cause of this is over voltage either from the mains supply which is quite rare or much more commonly...a cheap to buy generator. The problem here is at least one other component will be damaged and more if the 'spike' got passed for more than a micro second.

Thanks for your detailed explanation Gary.  I think it is starting to make sense now .

A couple of questions just to get it straight in my mind:-

Are there 2 fuses inside the PSU   input and output ?

As we have checked the battery and it is fine and as you say voltage spikes are rare , then it sounds like it was the output fuse that went, possibly due to the coolbox as it was the only thing going apart from the battery being charged.  So it would be safer to invest in an adaptor to run the coolbox off 240v?  It is a bit puzzling as it was in September in Aberdeen so the box was not struggling or the van overhot,  presumably these coolboxes just run flat out all the time as there is no thermostat?

We do not , as I said, know what the problem was with the PSU the first time so we are still a bit in the dark.

As our van is a 98 model it is not that far in age from Wrekin's ABI so might be reasonable to think his PSU is the same,or similar , model , yet he has been told by Plug-in that there are no fuses inside.   Can this be correct?   Or are they just keen to sell him a new unit?

OH will understand all you have said so will get him to read through it all when he gets back.      Thanks again.

First P in S are in my opinion a good company that support there products very well in the field which is a darn sight more than some others do!!!.

There are two fuses, the input is on the psu and although PinS are not telling the truth as such, given what I have said before I think you would agree with the reasons behind it. The output fuse is another matter and depends on your setup as to where it is. If you have a stand alone silver or in Wrekin's case black box it will be in the bottom left corner of the end with the on/off switch. A few systems have this box attached to the back of the main distribution board. Often without the box the pcb is fitted inside the distribution boards case where the output fuse will be in the panel at the front.. 

Heat by the way comes from the psu and if it cannot get away then damage will be done...just a little slower if it's cold outside.

It would be interesting to know what wattage/amperage your cool box is before offering a suggestion as to what to buy

Thanks Gary,     I would agree with what you said about Plug-in,  they came out the very next day after our first unit failed while under warranty, and when OH rang them to ask exactly how to get into the charger to check the fuse (which he could not find anywhere else) they told him about having to drill out the pop rivets and were very helpful.  

  Our psu is the one in the silver box attached to the rear of the big board which has all the electrical stuff, switches,RCD, fuses, pump switch, gas water heater switch, alarm keypad etc on it . On the front it says PMS 2000 I think. This is all housed in a slim cupboard next to the end bathroom and below that is where the tables are stored. The only obvious ventilation is a 2 inch diameter plastic grille thingy in the top front of the side facing into the living area and a 4 inch deep gap between the outer wall and the base of the section the unit sits in (above the tables).  The whole little cupboard has a door on it which would normally be kept closed, so could it be that the unit really needs more ventilation than Sterling have provided?

I am not sure how much power the coolbox uses.  It is up in the loft at the moment but will get OH to check and get back to you on that.   Thanks again.

Hi Jennifer n Gary,

Thx for the input, the unit is black as Gary pointed out and it has a couple of automotive fuses "sitting" on the top of the unit, these are ok.

I will remove the unit and try to take a look inside to see if there are any servicable parts that I can tinker with.

Gary I didn't understand your bit "Wrekin's case black box it will be in the bottom left corner of the end with the on/off switch" is this the location of the hidden fuse?

Quote: Originally posted by wrekin on 01/03/2005

---

Hi Jennifer n Gary,

I didn't understand your bit "Wrekin's case black box it will be in the bottom left corner of the end with the on/off switch" is this the location of the hidden fuse?

Thx for the input, the unit is black as Gary pointed out and it has a couple of automotive fuses "sitting" on the top of the unit, these are ok.Gary

---

I have turned your two paragraphs upside down as the second now answers the first I think??     From your description you seem to be describing a KT12 and not the KT12SM which is smaller and much lighter..SM standing for 'switch mode'. The KT12 has a panel on the top with two fuses, (1amp input and a 10amp output), a switch and a very heavy round transformer inside, it is not fused internally. The KT12SM and all later plug in systems chargers are switch mode and the output fuse is as I said and with an internal fuse

But don't forget I am trying to workout what you have from what you say..a picture would be handy.