12v charger - polarity ?

[Pyromaniac]

I have a firing system with a 12v SLA battery. As most things Chinese it came with no instructions or charger.

It charges via a socket - round hole with a central pin.

No labels tell me which is the + Pos or - Neg part of this socket, so I am loath to plug anything into it as the rest of it is pcb/digital - easily fryable !!
and I cant afford to destroy it this side of Nov 5th.

I have a screwfix freebie digital multimeter - never used one in my life though.

I guess it is not going to be up to much - but will it tell me which part of the socket is negative and positive, so I can decipher what sort of charger to get ? if so - how !

Also - charger - how would I know what polarity a charger is, for example - I have a multi headed charger 3 - 12 v here for mobile phones and suchlike, can I find out the polarity of these too ?

In fact - is there an online multimeter basics guide anywhere ?

Many Thanks

Gerry

 

[electronicsuk]

Working out the polarity of the charger is easy, there is almost always a diagram on the transformer, and it almost always shows the centre pin as being positive. If not, put the meter into the 20v DC range (or DC autorange), connect the probes as indicated in the user manual, and then put the red probe to the centre tip of the plug and black to the outer sleeve.

When the transformer is plugged in you should see a voltage reading, if a - symbol is displayed before the reading then this indicates reverse polarity, i.e. the centre pin is negative rather than positive. This does, of course, rely on you having the red and black probes connected to the correct sockets on the meter! Usually black to 'com' and red to 'V', but varies between manufacturers.

Working out the polarity of the input on the firing unit is not so easy, but chances are it will be centre positive. Do you even know what voltage you need? It will certainly be >12v for charging a 12v SLA.

 

[Pyromaniac]

Thanks for that - I will take the case apart in a while and have a closer look at the charging socket, see if I can trace it back to the battery - I have a feeling it goes through a pcb though and wont be as easy as following 2 wires !

It is definately a 12v battery though !

 

[electronicsuk]

You might find that the negative wire to the battery goes straight to the negative side of the charging jack, so before taking the firing unit to bits, it might be worth using your meter on continuity test setting to check this.

 

[Pyromaniac]

Ok - after a fight to get into the box - shearing off 2 screws in the process ( they superglued them in )

Battery puts out 12.4 v DC according to the multimeter.

Battery BLACK lead is + POS
Red lead is - NEG

Black lead goes into circuit board and stops - no continuity to the charging socket. ( There is a keyswitch that might alter this which I did not put on )

RED lead goes from battery to a connector on the circuit board - then out from another connector to the charging socket.
what happens between the 2 connectors I have no idea but there must be something connected in between them.

At the charging socket if I put a lead into the open front of the socket (not the back where the wires are connected) and touch the outer ring (not pin) and the battery - NEG at the same time I get a buzzer from the multimeter.

So this means I am looking for a 12v DC charger with + POS center pin connection ?

I get no readings from the charger socket looking for + or - voltage reading as in 1st reply.

 

[Spark123]

Very Very strange having a red -ve and black +ve connected to the battery - are there any markings on the said battery?
Going by your username I take it the system is for pyrotechnics, I doubt you will need the firing key in to charge the battery.

 

[Pyromaniac]

I thought it very strange and had to think about what I was typing above quite hard as I automatically wanted to type red = + POS !

Battery has a moulded ' + ' with what looks like red resin around the terminal - to this is soldered the black wiring . . .

Negative is a moulded ' - ' and black resin - red wiring . . .


Yes - is a firing system for firework display use, with key in I can actually pass current to fire ignitors, with key removed I have power to test for continuity and to program times into the sequencer but no power to fire ignitors.

Gerry

 

[Spark123]

It is sounding like you need the centre pin positive type charger to me as you say the bat -ve is connected directly to the socket outer shell.
I am a bit dubious about the idea of having to have the key installed to charge it up tho, also having worked with small pyrotechnics before (and knowing another forum member who almost took his hand off with one) would be a bit dubious about using a low quality device.

 

[Pyromaniac]

I have no idea if the key needs to be inserted to actually charge the battery.
I would have thought not, but didn't want to try charging the battery until I knew what charger polarity to use.

Once i have a charger connected - is there any way to test if current is going into the battery ?

There seems to be no LED to show it being charged, but there are ones to show charged and low charge.

What setting should I have a multimeter on and to what should I connect it to see if charging current is going to the battery ?

Once I can figure out if I have current going into the battery I can try it with and without the key to see if ( as I hope ) the key being inserted has no function in the charging process.

 

[Spark123]

Once you have identified a suitable SLA battery charger adaptor, when you plug it in try connecting the your multimeter across the battery. You should see the battery voltage rise to about 13.4 - 14 volts.

 

[Pyromaniac]

Ok - will do that - many thanks for all the help - it is greatly appreciated.

Gerry