Transformer - Rectifier question

[maharajan18]

I have a 12-0-12 V, 3A Center tap transformer. Now i can get 12V 3A DC by using a full wave rectifier on the outer taps or a bridge rectifier on the center tap and one of the outer taps.

Is it possible to use two bridge rectifiers on each outer tap with the center tap common between them and connect the two DC outputs in parallel to get a 12V 6A DC output?

Or will this parallel connection cause a short circuit.

I will be using this to test automotive bulbs so I need more current than 3A

Thanks in advance

 

[Neal]

You will be shorting out the secondary winding.
Why not use an ohmmeter to check the bulbs?

 

[maharajan18]

Thanks for the reply. Have to look for a bigger transformer.

 

[Jon Wilder]

More information needed here -

Is 3A the full winding current? Or is the full winding current 1.5A and you get 3A in a full wave grounded center tap config?

 

[maharajan18]

More information needed here -

Is 3A the full winding current? Or is the full winding current 1.5A and you get 3A in a full wave grounded center tap config?

I get 3A in both full wave and bridge configurations. So I think 3A is the full winding current.

 

[Jon Wilder]

Where are you coming up with the 3A spec? Is that what the actual source current rating of the transformer is? Or is that just what your circuit is drawing?

Also, do you happen to know what the make/model of the transformer in question is?

 

[maharajan18]

Where are you coming up with the 3A spec? Is that what the actual source current rating of the transformer is? Or is that just what your circuit is drawing?

Also, do you happen to know what the make/model of the transformer in question is?

The output 3A current rating is written on the transformer. The current draw will be changing based on the load.
Its just a generic 12-0-12 volt center tap transformer. Dosen't have any make/model names. Probably Chinese made.

 

[Jon Wilder]

OK...for a minute there it sounded like you had load tested and measured it.

If 3A is the full winding current, you should be able to get DOUBLE that in full wave grounded center tap rectifier config due to the 50% duty cycle of each 1/2 of the winding in this config. You also get less of a voltage drop across the diodes in FWCT config as well. The tradeoff is that the winding has to be spec'ed for double the voltage and 1/2 the required current across the full winding.

With a bridge across each 1/2 of the windings, you'd have two outputs that could supply 1.5 amps. Same goes for a bridge across the full winding with a grounded center tap.

 

[Reloadron]

What you likely have is a 24 volt center tapped 12-0-12 transformer. If the transformer says 3 Amps then you have two 12 volt 1.5 amp outputs. If you measure across the secondary and see 24 VAC (omitting the center tap) then that is what you have. They are a very common center tapped transformer. I doubt you would get 3 amps between the center tap and either line side of the secondary. Like Jon says.

Ron

 

[Jon Wilder]

What you likely have is a 24 volt center tapped 12-0-12 transformer. If the transformer says 3 Amps then you have two 12 volt 1.5 amp outputs.

That makes absolutely no sense. Both 1/2s of the winding are essentially in series with each other from start to finish. Current is not summed in series circuits (in regards to your statement about having 2 x 1.5A outputs).

Furthermore, the 3A rating is CONTINUOUS current. In a full wave grounded center tap config current flow is NOT continuous through each 1/2 of the winding. Each 1/2 of the winding has a 50% duty cycle in this arrangment, which allows both halves of the winding to source double the current, but at 1/2 the full winding voltage.

 

[Diver300]

It depends how long you need to run the transformer for. If you are only testing for a few seconds, then a 2 times overload on a transformer isn't too big a deal.

The transformer will eventually overheat at much more than its rating, but it takes time.

 

[Ratchit]

maharajan18,

Perhaps this article will give you some ideas. It came from a journal a long time ago in a galaxy far, far away.

Ratch

 

[Jon Wilder]

On a typical 50 watt tube amplifier you have a transformer with a center tapped secondary that has a rating of 690VAC @ 150mA. This type of transformer is used in a full wave center tap config, which would give us 345VAC.

Well, if what Ron said were true, we'd have two 75mA outputs from the secondary. Well coupled with the 345VAC center tap voltage, this would only equate to 25.875 watts! This just does not add up at all.

However, if what I say is true, we would have 345VAC @ 300mA. This would translate to 103 watts...which would more than account for the 70% loss you get in the output transformer and hence would make much more sense.

 

[Reloadron]

That makes absolutely no sense. Both 1/2s of the winding are essentially in series with each other from start to finish. Current is not summed in series circuits (in regards to your statement about having 2 x 1.5A outputs).

Furthermore, the 3A rating is CONTINUOUS current. In a full wave grounded center tap config current flow is NOT continuous through each 1/2 of the winding. Each 1/2 of the winding has a 50% duty cycle in this arrangment, which allows both halves of the winding to source double the current, but at 1/2 the full winding voltage.

So from CT to either leg of the secondary what would be the maximum current available on a 24 V CT 3Amp transformer?

Ron

 

[The Electrician]

I have a 12-0-12 V, 3A Center tap transformer. Now i can get 12V 3A DC by using a full wave rectifier on the outer taps or a bridge rectifier on the center tap and one of the outer taps.

Is it possible to use two bridge rectifiers on each outer tap with the center tap common between them and connect the two DC outputs in parallel to get a 12V 6A DC output?

Or will this parallel connection cause a short circuit.

I will be using this to test automotive bulbs so I need more current than 3A

Thanks in advance

If you use a two rectifier configuration with center tap you can get 4.24 amps @ 12 volts.

 

[Jon Wilder]

On a single polarity full wave grounded center tap rectifier you should be able to pull double the full winding rating but at 1/2 the full winding voltage. This is possible due to the 50% duty cycle of each winding (i.e. each winding is only passing current for 1/2 the AC cycle) so there is a "cool down" period.

Also...each 1/2 the winding has 1/4 the reflected impedance of the full winding (reflected impedance ratio = turns ratio squared).

On a full wave bridge setup, the winding is passing current CONTINUOUSLY...i.e has a 100% duty cycle. This along with the fact that the full winding has 4x the impedance of each 1/2 of the winding drops the available current that the secondary can pass.

In either scenario, the available power is the same -

24 Volts x 3 Amps = 72VA

12 Volts x 6 Amps = 72VA

The primary doesn't know the difference as it's passing the same primary current either way. This of course assumes that you have full load applied to the secondary.

 

[Reloadron]

Jon Said:

In either scenario, the available power is the same -

24 Volts x 3 Amps = 72VA

12 Volts x 6 Amps = 72VA

Thank you as in my post I failed (epic) to look at the VA and subsequently screwed it up.

Thanks
Ron

 

[maharajan18]

On a single polarity full wave grounded center tap rectifier you should be able to pull double the full winding rating but at 1/2 the full winding voltage.

Can you explain a bit more about "single polarity full wave grounded center tap rectifier".

Currently I am getting almost the same current under load using either a full wave rectifier or a bridge rectifier.

 

[Jon Wilder]

Full Wave Grounded Center Tap, as in -

**broken link removed**

Define "under load". What is the load you're using?

 

[maharajan18]

Define "under load". What is the load you're using?

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