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rectifer issues

davelectronic

Dec 13, 2010
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Ive got another transformer issue, ok its center tapped, i want to wire it up to a conventional full wave bridge rectifier package, this might seem straight forward it probably is.
My head wont let me work this one out, so three wires from the transformers secondary, here goes , ? if i reverse bios a rectifier diode from the center tap to the - volts side of the rectifier, and the outer wires to AC rectifier input can i use the full rated current, the voltage is the culmination of both sides of the transformer, ive tried, but can i get full rated current, as if i was to use the conventional diode rectifier configuration for a centre tap transformer IE the two diode configeration, or is the above idea, although ive incorporated the center tap via a reverse diode not allow full current, as the voltage is the maximum for the output.

My mind is saying no, not full current, only by using the two diode configuration, if this is the case how can i use a conventional bridge rectifier package on a center tap transformer to get full current and half the maximum voltage. :)
 

davelectronic

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What no one knows the answer to this, it must be trickier than i thought, ahhh. :(
 

davelectronic

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Guess i will have to load a transformer and see, test like, if it survives it can take full load current configured my way, if it dies then the answers no it cant handle full load current configured my way. :(
 

(*steve*)

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Draw a circuit diagram for us Dave, It's hard to figure out exactly what you're asking.

It kinda sounds like a 2 diode full wace rectifier using a centre tapped transformer. With that you get the full current from the transformer, but only half the voltage (because you're only using one half of the centre tapped winding for each half cycle)
 

davelectronic

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Hi Steve. yes it uses a full wave bridge rectifier package, but due to the three wire secondary, i used a single diode for the centre tap to the - volts of the rectifier reverse biased, so the power flows back into the windings, the outer two wires of the secondary go to AC on the rectifier package, the voltage reading is the full transformers output voltage, but i wanted to know if i can use full rated current. i will try and draw a diagram later. Thanks for your reply.
 

(*steve*)

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If you have a centre tapped trasformer connected to a bridge rectifier, the centre tap of the trnsformer becomes a centre tap on your DC voltage. Typically then you have regulators between the centre tap and both of the DC connections to create a regulated double-ended power supply.

However there's no reason you can't use other topologies. None of them, to my knowledge, have any diode in the centre tap.
 

davelectronic

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I have 4 connections on the bridge rectifier, i connect the two outer wires to the rectifiers AC input, i connect the center tap lead to - volts on the rectifiers DC out put via a reverse biased diode on the - volts terminal of the rectifier, direct connection causes a short, as it the rectified output, i cant connect the center tap to either AC terminal as this is also a short, so i connect the tap to - volts with a reverse biased output to avoid a short, or could the tap go straight to ground.
 

davelectronic

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I will draw a diagram later, explain better what i mean, i am on my old cheapo phone this morning, i will try drawing a diagram later. Thanks for helping me out.
 

davelectronic

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So ive drawn a diagram of what ive tried, i got full voltage from the transformer, but can i get full load current this way, or can i remove the center tap diode and put this tap straight to - volts, the answer was no, it created a short, in a word i want use a full wave bridge rectifier on a center taped transformer, incorporating all windings and get full load current using a full wave bridge rectifier package. :)


Taped_bridge_rectifer1.jpg
 
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(*steve*)

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You do it like this:

14nzdro.gif
 

davelectronic

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So i guess not full load current, thanks for posting the schematic, so my way is of no benefit ? :)
 

BobK

Jan 5, 2010
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I don't see your problem. If you want a single voltage that is the voltage of between either winding and the center tap, you can get the full rated current. You simply connect a diode with the anode to each end of windings with the cathodes tied togethen, and the center tap is your ground. Steve's circuit is showing a dual supply, which will only get half the max current, as would a single supply with a bridge across the two ends and no center tap, and this configuration would give 2x the voltage at half the current.

Bob
 

jackorocko

Apr 4, 2010
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You do it like this:

14nzdro.gif

what would happen if you float the center tap and connect V- to ground?

Dave, is it your point to get V- V+ power rails?

edit: bobk answered me a minute before I posted my question. 2x the V half the current
 
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jackorocko

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You simply connect a diode with the anode to each end of windings with the cathodes tied togethen, and the center tap is your ground

I get it, the connection where the two cathodes meet is the power rail. only one side conducts per half cycle of the AC
 

davelectronic

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I know what your saying BobK, and Jackorocko, i know the easy answer is two diodes configured as you say, and ive done that many times, i could put this through a second bridge rectifier package, but i would lose a volt or a little more, what iam looking for is not half the current at full voltage, i want full current at half the voltage, but utilize a 35 amp full wave bridge rectifier package, and a single supply from this, but my diagram gives full voltage, so i dont think i can get full current from that, and as you say Steve's diagram is a dual supply and half current with full voltage.

For me its like this, three wires from the transformers secondary, a conventional center tap, rectifier package four terminals two AC two DC + / - volts, how can i connect up the full wave bridge rectifier package for half the transformers voltage, and get full load current on the DC output, three wires, four terminals, i cant see a way. :)
 

BobK

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Okay, now I understand. Just put the two AC sides of the bridge to the two end windings, and then take the + output of the bridge is your + and the center tap is the -. Leave the - output of the bridge unconnected! Then you have the same 2 diode circuit I suggested in my last post.

Bob
 

davelectronic

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So if ive got this right, the AC bridge rectifier input will take the two ends of the transformers secondary, ok so far, + volts DC on the rectifier is obvious, so ok there, know ive got a center tap left, and - volts DC on the rectifier, so i leave the - volts DC of the rectifier unconnected, instead i use the center tap as my - volts DC, do i have it right this way. I think i must have, as your explanation is as would see it. :)
 

BobK

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Yep. If you draw a diagram you will see that you are just using 2 of the rectifiers in the bridge, and the other two are just connected (internally) back to back, so no current flows through them.

Bob
 

davelectronic

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I can visualize what the rectifier is doing, thanks for your help BobK, and Steve's diagram and Jackorocko for looking in. :)
 
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