Current regulator

[Thunderchild]

I am going to make a current regulator for a car battery charger. any reason why a BDX53 should not do the job ? if I was to use a fet (or mosfet) which one should I choose and what diagram should I build by. it will be four amps
Thanks
Simon

 

[Nigel Goodwin]

I am going to make a current regulator for a car battery charger. any reason why a BDX53 should not do the job ? if I was to use a fet (or mosfet) which one should I choose and what diagram should I build by. it will be four amps

Should be fine, bear in mind you will probably need a good size heasink!.

However, you don't really need a current regulator to charge a lead-acid battery? - a simple resistor (large wattage!) is all that's needed, and monitor the voltage across the battery. There are plenty of circuits about for car battery chargers, generally they switch to trickle charge once it's fully charged.

 

[Thunderchild]

well actually as I have two transformers (identical) of two older identical chargers I actually wanted also something to stop them from shorting out as the transistors will act like diodes and replace the resistors alowing a current limitation to preven under any circumstance a short circuit. say one transformer is 0.1 volt higher than the other directly connecting them together will make a big bang but a trasistor on each will do the trick unfortunately it means two rectifiers but never mind at least I can have different arrangments for example 24 volts

 

[Nigel Goodwin]

well actually as I have two transformers (identical) of two older identical chargers I actually wanted also something to stop them from shorting out as the transistors will act like diodes and replace the resistors alowing a current limitation to preven under any circumstance a short circuit. say one transformer is 0.1 volt higher than the other directly connecting them together will make a big bang but a trasistor on each will do the trick unfortunately it means two rectifiers but never mind at least I can have different arrangments for example 24 volts

To put them in parallel simply use a rectifier on each, and join the outputs of the rectifiers, that prevents any current flowing back.

But as long as the two transformers are identical then you can normally parallel them directly with no ill effects.

 

[Thunderchild]

you sure about that ? yea you are right about directly joining the rectifiers together but that means that if I still want to use a regulating tr then will have to pass 8 A. but two transformers in parallel looks dodgy to me what if there is just 0.001 volt difference with the negligible resistance of the wires that will be a huge short circuit wouldn't it not ?

 

[Nigel Goodwin]

you sure about that ? yea you are right about directly joining the rectifiers together but that means that if I still want to use a regulating tr then will have to pass 8 A. but two transformers in parallel looks dodgy to me what if there is just 0.001 volt difference with the negligible resistance of the wires that will be a huge short circuit wouldn't it not ?

In theory perhaps so, but not in practice, as long as the transformers are identical they will have the same number of turns and any differences will be too slight to have any effect.

Don't forget, the windings have resistance as well, so they will 'self balance' to a small extent.

 

[Thunderchild]

ok I will try that thanks it will save buying another rectifier as only one is good. will a BDX53 take a continuos 8 amps ? or should I choose a bigger tr like the 2N3055 hoping not to get done

 

[Nigel Goodwin]

ok I will try that thanks it will save buying another rectifier as only one is good. will a BDX53 take a continuos 8 amps ? or should I choose a bigger tr like the 2N3055 hoping not to get done

A BDX53 is only rated at 8A maximum, and only 60W maximum, I would certainly not advice trying to use only one BDX53. Two on very large heatsinks might be OK?, it really depends on the input voltage to them.

I wouldn't like to put 8A through a single 2N3055 either!.