The AM Forum

i have a question about rectifiers.

lets say i have a 12vac transformer without ct. and i use a bridge rectifier how much dc current is it good for if say the transformer states 3a ac output.

then if i have a 24vct transformer that is good for 3a output. and i use full wave 2 device rectifier on it to get about same dc output as the 12vac transforemr. would its dc current available be higher?

reason i ask i have 2 transformers with similar currernt rating but one is center taped and the size differnt is pretty big.

The 12vac 3amp transformer would give you about 3 amps at 12vdc +- depending on what kind of filter you had if you use a bridge rectifier.

The 24vac 3amp transformer would give you about 6 amps at 12vdc +- (also depending on the filter) if you use a 2 diode full wave CT circuit.

Dave

We touched on this subject recently in another topic called “primary and secondary KVA on plate iron”  
http://amfone.net/Amforum/index.php?topic=31603.0.  
Near the bottom of that thread I posted a file on transformer secondary RMS currents versus the DC output load current for various rectifier configurations.  The pdf file is called Amveco rect notes1.

The secondary current for a transformer with full wave bridge is 1.8 times the DC load.  So for the 12 VAC transformer your maximum DC load is 3 A / 1.8 or 1.67 A DC.

The secondary current for a transformer with full wave center-tap is 1.3 times the DC load.  So for the 24 VAC center-tapped transformer your maximum DC load is 3 A / 1.3 or 2.3 A DC.

An interesting experiment would be for you to tack together each circuit with a 1.67 A load and measure/compare the primary ac currents.

It depends on what type of xfmrs they are.  Let us know what type you're referring to. Model numbers would be a help.  If they are both filament xfmrs you can load them to the full current rating across the full secondary.

As Dave and Tom have pointed out, with a FW (2 diodes) rectifier you'll need 24VCT to get 12 volts.

Using a 12VCT 3A xfmr with a FW rectifier will only give you 6 volts, but not 6A load.  The VA load in the primary may not be exceeded but the 6 amps may be too much for the size wire used in the secondary.  This is true for filament xfmrs.  OTOH if the 12VCT xfmr was designed to be used in a power supply for 6 volt using a FW rectifier then 3 amps would be the limit.

Let us know what these xfmrs are.

Fred

I just re-read your post, I see you are talking about a 24VCT xfmr.

Again it depends on what type of 24VCT xfmr it is.

Fred

I always understood the convention to be that when N v. CT was specified for the secondary, that meant N - 0 - N  or 2N v. across the entire secondary.   

This convention (if I have it right) gets broken often so when buying transformers on eBay and elsewhere you have to be careful.

A month or two ago I saw nice vintage plate iron with a CT on the secondary incorrectly listed on eBay for FWB and I thought anyone who buys it and does that will destroy it.  I was tempted to get it just to rescue it but didn't.

Rob

No that's not correct.  1000vCT  would be  500v-0-500v

Fred

Fred, if there is a right way and a wrong way you can count on my trashed memory to get it wrong.  thanks for the correction.  All my old handbooks always use N - 0 - N as the secondary voltage designation method.  That way it's impossible to make a mistake.

Typically choke input isn't used at the low voltages discussed in the original post but for higher voltage transformers the current capability is different depending upon the type of filter (choke versus capacitor) used.  The high peak charging current with a capacitor input filter and its resultant heating require reduced ratings for vintage iron and for a lot of modern iron (i.e. within the Hammond line) there are dual ratings based upon filter type.

Rather than developing a brain short just look at it very simple.

A transformer is rated to deliver X voltage at Y current. XY=Watts.

However you configure the DC the Watts cannot exceed the original XY. The perfectionists call it the VA rating.

Carl