amp meter needed ?

[gigster]

does the amp meter need to be in this circuit ? for it to work? its a 12v battery charger

 

[Nigel Goodwin]

No it doesn't, although it's a VERY good idea to include it. You should also use a special transformer designed for battery charging, these are designed to current limit - a normal transformer may well destroy itself on a flat battery. There's also no need for the capacitor in a crude battery charger!.

 

[gigster]

so a 120v \ 12v 5amp transformer wont work? I made the charger and had a volt meter hooked up to the battery leads but when I plugged it in the diodes blew.I would buy a charger but where Im at (not in usa) they want 100+ dollars for a manual 6a charger.
Is there something I can add to protect the trans.?

 

[Nigel Goodwin]

Add a current limiting resistor - it will need to be a high wattage one!.

 

[ericgibbs]

hi gigster,

One of the reasons the diodes failed is because they are only rated for 1 amp.

If you have a 12Vrms 5amp transformer, you should have at least 5amp diodes, I would goto 10amp, if possible.
Connect a 5amp slow blow fuse in the output from the charger to the battery. [say, in +V lead]

The charger should also be mains powered from a fused supply, 3A fuse should be OK.

I would also ground the -V supply from the charger to the mains ground, just in case the charger transformer develops a fault.

As chargers are sometimes used outdoors, it would be advisable to use a RCCB in the mains supply line.

EricG

What type of battery are you going charge??

 

[RODALCO]

And provide adequate cooling for the 10 Amp diodes.
At 6 Amps they will dissipate around 4.2 Watts each.

Also have the series resistor on standoffs because it will get hot.

 

[gigster]

Ok thanks! I think I got it now.And automotive batteries to be charged

 

[gigster]

the current limiting resistor goes in the hot main wire? if so Im calculating with 120v and 5amps? how many ohms resistor ? 5X24=120 but its not a 24ohm res and is it the kind thats in like a ceramic rectangle?

 

[RODALCO]

5 Amps at 120 Volts is 0.5 Amps at the primary side of the TX.

A 12 Volts lead acid (car batt) requires between 13.7 and 14.4 Volts to fully charge.

At your TX secondary you should aim for an output voltage within that range.

A good value to start with is 10hm: which will give you a 5 volt drop at the primary and about 0.5 V at the secondary.
Rating should be 2.5 Watts. Get a 7 Watt ceramic wirewound resistor or put smaller ones in parralell. 3 of 30 hm: 2 Watts each (makes 10hm: )

You can do a bit of trial and error here. Be carefull with mains wiring and put on well insulated standoffs.
A batt charger is self regulating anyway. Initial high charging current when batt is flat, voltage of batt increases, hence less charging current, till a status is reached that charger volts equals charged batt volts.

 

[gigster]

I got some 10 amp diodes they have 4 pins I just use the + & - pins correct?

 

[Nigel Goodwin]

the current limiting resistor goes in the hot main wire? if so Im calculating with 120v and 5amps? how many ohms resistor ? 5X24=120 but its not a 24ohm res and is it the kind thats in like a ceramic rectangle?

I would fit a resistor in the secondary side, NOT the mains side - either in one of the leads from the transformer to the bridge, or in the positive lead to the battery.

Before I actually got round to buying a battery charger (which cost very little anyway), I used to 'throw something together' whenever I needed to charge a battery. A simple and easy current limiter is a bulb, try a headlight or foglight bulb in the positive lead and see what current you get - if the battery is dead flat the bulb will even light to tell you!. I used to use an old front foglight, which was a damaged one I saved - this always worked fine.

 

[fingaz]

sounds like you got 10A bridge rectifiers. If there are 4 pins, 2 should be labelled with + and - and 2 should be labelled with ~

The ac voltage goes to the two pins labelled ~ and the two pins with + - will provide rectified DC voltage

 

[gigster]

well is this what a higher amp diode looks like?