RE: More Stupid Transformer Questions.

John,

Not that any of this is new information, I'll add my 2 cents worth.

They build transformers in configurations like (12V 0V 12V) for an interesting reason. If you need 12VDC (RMS), you may employ such a transformer in a full wave configuration using only two individual discrete rectifier diodes, and in this configuration the total diode voltage drop is half the total diode voltage drop compared to a four diode full wave bridge rectifier (4 leeds on the single full wave bridge device). When you cut the total voltage drop across the diodes in half, you cut the wasted power consumption across diodes in half. Saving wasted energy in diode drops is why the center tap was brought out of transformers. It doesn't have to be used as previously discussed.

The 12VDC configuration of a (12V 0V 12V) transformer is that 0V is tied to ground, and each 12V transformer wire (2 each) is connected to the Anode of each of two diodes. The Cathodes of the two discrete diodes are then tied to each other and the voltage out at this junction is +12VDC (RMS). Typically in a power supply, the +12VDC is then connected to a large capacitor (+) positive and ground is tied to (negative) of that large capacitor. If you reverse the two diodes, you must reverse the capacitor polarity and you end up with positive ground. To obtain a 24VDC supply with a (12V 0V 12V) transformer, you must use a full wave (four diode) bridge rectifier and the certer tap is not used as previously discussed and you must live with two diode drops.

An interesting point about all this is that a (24V 0V 24V) transformer makes a good +24VDC power source in the same manner and for the same reasons. When this configuration for a 24VDC supply, is used over a plain 24VAC transformer (with or without a center tap), you will save a diode drop of wasted energy in the conversion process. Let us say that a diode drop is 0.60 VDC. If you are drawing 10 Amps, then one diode drop is using up 6 Volt-amps to get the rectifier job done. A full wave bridge rectifier requires 2 diode drops whereas a center taped transformer allows you to get by with one diode drop. You do not have an option with transformers that do not have a center tap, or transformers that do not produce the voltage you want from the center tap to either of the other two windings.

More often than not, we build power supplies around the transformers that we can easily obtain. If we need a 24VDC supply and we can find a transfomer in (24V 0V 24V), then we might choose this over a (12V 0V 12V) transformer and a full wave bridge rectifier to save a few volt amps.


David R. Howland dhowland@...




-----Original Message-----
From: John Ross [SMTP:johnr@...]
Sent: Tuesday, August 24, 1999 1:23 PM
To: CAD_CAM_EDM_DRO@onelist.com
Subject: [CAD_CAM_EDM_DRO] More Stupid Transformer Questions.

From: John Ross <johnr@...>

If I have a transformer that puts out 24 volts at 5amps and is center taped
like " 24 0 24 ". can I use both 24v lines to get 10 amps like using 2
separate 5 amp transformers? Or does this cause a problem with phasing?

John Ross




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