Transformer - Rectifier question

If this was occuring at 12V then more than likely you have a 24VAC CT @ 1.5A and your full winding current is actually 1.5A. In other words, if you ran it at 24V with a bridge rectifier across the full winding you would only have 1.5A available, while having 3A available in the 12V Full Wave Grounded Center Tap config.

Now if you run it with a bridge rectifier across each 1/2 of the winding you would have 12V @ 0.75A on both because each 1/2 of the winding is now being drawn from CONTINUOUSLY rather than having a 50% duty cycle like they do in Full Wave Grounded Center Tap config.

As to why it states 3A on the transformer itself, more than likely they spec'ed the available current at 12V. Seeing as how they also spec'ed it as a 12V-0V-12V rather than a 24VCT this would make sense.

Why do you need DC to test a halogen lamp? The will operate fine from AC. Just use the AC directly from the transformer.

crutschow said:

Why do you need DC to test a halogen lamp? The will operate fine from AC. Just use the AC directly from the transformer.

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That was my question.

A 12 Volt 35 Watt Halogen (or any other light bulb) will draw about 2.9 Amps and a 12 Volt 60 Watt will draw 5 Amps. If this is a standard Halogen bulb it will run just fine on AC directly off a transformer sans DC rectification.

Ron

I used a halogen bulb just for testing. But I primarily need this to output DC.

maharajan18 said:

Currently I am using a 12 V 35 Watts halogen bulb. And I used a multimeter to measure the current. It was around 2.5 A.
When I used a 12 V 60 Watts bulb the current was around 3.3 A but the voltage started dropping below 10 V

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OK, then I can buy that. The 12 volt 35 watt bulb would draw about 2.9 amps but 2.5 is reasonable. That assumes 3 amps available. The 12 volt 60 watt bulb would draw about 5 amps which is about 2 more than you have and the voltage folds over and drops. I assumed the lamp(s) were to be the regular load not just for a test.

Ron

maharajan18 said:

I used a halogen bulb just for testing. But I primarily need this to output DC.

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In your first post you said you were using the power supply to test automobiles bulbs, so that is the assumption we used. If you are using the bulbs to test the power supply, rather different.

The attached image gives you options of what you have. I assume you have a 24 volt center tapped transformer rated at 3 amps. Now you can have 24 volts @ 3 amps or you can have 12 volts @ 1.5 amps but you can't have 12 volts at greater than 1.5 amps. Not with your existing transformer and a simple rectifier as I see it.

There are transformers with dual secondaries where for example two 12 volt secondaries can be placed in series or parallel with each other but that is not what you have.

If you want 12 volts at greater than 1.5 amps you are going to need a larger transformer. That is how I see it anyway. I may be wrong and it would not be the first (or last) time.

Ron

Transformer are rated at their winding current. This is 24vac @ 3 amps.

Now to your original question, you are cutting the voltage tight. Quite often the transformer is 25.2 vac C.T.. If so it will help.

Let's assume it is only 24.0 vac. With full wave, grounded center tap, arrangement you will have 12 vac sinewave on either end to ground. This will give you close to a peak voltage of 1.414 x 12v = 16.96 volts peak going into each diode. Depending on your selection of rectifier you could have 0.7v to 1.4 vdc drop across rectifiier. Better if you use a Schottky diode as it will give you less voltage drop, at 6 amps forward current in order of less then 1.0 vdc. So you now have a peak DC voltage of 16.96 -1.0 vdc = 15.96 vdc.

Without a filter cap, the r.m.s DC voltage will be 11.3 vdc @ 6 amp load. If you want to raise that up a bit you can add an electrolytic filter cap. You would need about 12,000 uF to bring the r.m.s. up to about 14vdc @ 6 amps. For less current, the r.m.s. voltage will raise toward the 16 vdc peak as there will not be as much voltage slump with the large cap.

Be a little careful on lighter loads as the transformer voltage will be higher and the drop across rectifier will be less. You could end up with 18 vdc with no load.