Help fixing Bosch charger please (capacitor polarity)

[Paul Baldwin]

Hi,
I was given one of these ages ago and have just got round to looking at it, hopefully it will come in usefull? http://www.screwfix.com/prods/47144...hargers/Bosch-AL15FC2498-Fast-Charger-7-2-24V Didn't realise they sell for 100 quid either!
Basically it's been out on site and been dropped and two of the caps inside have been snapped off at the base. I have soldered one back in already as it was obvious which way round it went, the one next to it is the problem! Does it matter which way round I connect a kx472m capacitor? I have found a datasheet here www.ttiinc.com/object/murata_medium-high_voltage_presentation
I've also found a thread on them here http://forum.servomagazine.com/viewtopic.php?f=1&t=15011

I can't see anything relevant to polarity but don't really want to explode the cap or bugger up any other components.

Thanks

Paul.

 

[Selectron]

That's a non-polarised disc ceramic capacitor, probably being used for mains-filtering purposes. Because it's non-polarised, it can fitted into circuit either way round. You were right to ask the question though because fitting a polarised electrolytic or polarised tantalum with the wrong polarity can have serious consequences.

Sounds like the unit hasn't been powered-up for months, in which case the various capacitors will likely have discharged themselves, but after you've finished the repair and powered it up, be extremely cautious about handling the circuit board again, because it's probably switched-mode, and some of the capacitors even on the secondary (non-mains) side of the circuit can retain a hefty and potentially dangerous charge for some time.

If it was dropped then also examine the PCB's copper tracks very closely, looking for hairline cracks. A magnifying glass is useful when doing that because they can be almost invisible and difficult to spot.

 

[Mr Happy]

It appears to be a ceramic capacitor and therefore does not have a polarity. When capacitors do have a polarity there is usually a distinctive marking next to one of the leads. If the case or capsule has no indication that one lead is different from the other then you can freely insert the component either way round.

 

[Paul Baldwin]

Brilliant thanks for the info both! I did some basic electronics at school many years ago and remember the dangers of charged caps thanks, IIRC I was told thay can hold charge for up to 3 weeks!
I've already found 3 cracked solder joints at the bottom of one of the coils, I'll check the other tracks carefully too.
Having never owned one of these power supplies and seeing it is variable voltage, depending I assume on which battery it senses is plugged into it, what do you think it's default output will be?
I'd like to try and run a buckpuck on it and was hoping it would be kicking out more than the current 12v transformer I'm using.

 

[Foxx510]

I wouldn't use it as a powersupply, it will only work as a battery charger as an IC will be controlling the output and expecting a battery there and anything else will confuse it. If you need a cheap 12v supply with heaps of grunt, grab an old computer power supply and use that. Google will tell you which pins to connect to make it fire up.

 

[Paul Baldwin]

That's what I didn't want to hear lol. I already use pc power supplies for various projects but I need more than 12v, ideally 18+ hence thinking of this charger. The buckpuck is powering a string of 6x xre's and I need more overhead voltage to get them to light fully. I don't need the extra amps this thing is capable of but it was free

 

[Paul Baldwin]

It's still dead at the mo. There seems to be an inline fuse that's blown on the ac side inside the case. It looks similiar to a 13 amp plug fuse but it has 3 narrow colour stripes blue, orange and red and a thicker blue stripe. Its soldered in to two end caps. I cant find any other 230v ac anywhere else on the board at the mo. The 5 amp fuse in the plug top was also blown

Edit: I think it's a 6.3 amp medium blow fuse. www.ue-soft.de/oilfield/techtabs/electric/fuses001.pdf

think I'll try a 13 amp just to test it.

 

[Paul Baldwin]

:fail:

Lot's of magic smoke and a nice flash of light resulting in one nicely melted fuse! Several of the copper tracks have removed themselves quite effectively from the bottom of the board too.

Never mind, the heatsinks will come in handy and theres a fair few other components that can go in the parts bin

Time to retire to the relative safety of the pub

 

[jhellwig]

Usually on circuit boards if the fuse is soldered in it shouldn't be replaced cause something bad has happened as you found out.



FYI If you ever get a new capacitor for anything, The polarized ones usually have one lead longer than the other. I think it is the positive that is longer. Just something to keep in mind if you ever by one and wounder what way it goes. Someone correct me if I am wrong.

 

[Paul Baldwin]

I was aware of that with the capacitors thanks, it was the fact this one had no markings that threw me. I decided on a poop or bust approach with this thing and now it's definately bust lol. I have got away with this sort of thing before, the last time was when a friend decided to knock 3/4 pint of vodka and orange down the back of my tv while it was switched on! I let it dry out for 3 weeks or so, cleaned up the insides with switch cleaner, replaced fuse and all was good, for a while anyway! My old Aiwa amp also blue an internal fuse for no obvious reason. I replaced it some 2 years back now and it still makes the room shake. Win some, lose some