# Transformer / Rectifier question

Please see the attached drawing… my question is: Is there any difference between the two circuits?

I traced the current flow paths for each half cycle and found no difference, yet my “gut feeling” is telling me that there IS a difference (specifically, that the top circuit provides all the transformer current where the bottom one only provides half).

This is one of those things that I’ve been looking at for too long and thinking about too much and I’m blanked out.

So, what’s the verdict?

Thanks!

– Roger

(specifically, that the top circuit provides all the transformer current where the bottom one only provides half)

Yes that is it.

They are both full wave rectified but the top one will provide twice the current of the bottom one.

The voltage are a little bit lower from the top one.
Powerloss in the rectifier are twice .

Current are lower for the bottom one, if not compensated with thicker winding wire

Pelle

Pelleplutt:
The voltage are a little bit lower from the top one.
Powerloss in the rectifier are twice .

Current are lower for the bottom one, if not compensated with thicker winding wire

Pelle

The answer suddenly came to me… looking at my own drawing. The bottom circuit could be done identically with 1/2 of a bridge rectifier. This implies that a “mirror image” circuit (another capacitor on the other side of the bridge supplying a negative voltage) is available, whereas the top circuit has no “unused negative side” (if this makes any sense).

So, as Mike said above, yes the top circuit does indeed provide twice the current (i.e. all the transformer current) because it has no “unused negative side”.

For what it’s worth, the reason I’m doing this is that I’m building a high current (10 amp) adjustable linear power supply and I intend to use a relay to switch between 9 VAC and 18 VAC depending on the setting of the output voltage (to limit regulator dissipation at low output voltage settings).

In other words, if I get about 12.7 volts DC peak from the 9 volt winding, I would do something like this:

Output voltage - Winding used
1.2 - 9
3.3 - 9
5 - 9
6 - 9
9 - 18
12 - 18
15 - 18

Somewhere around the 6 to 7 volt output range, the regulator input would switch from “low voltage” to “high voltage” (with hysteresis of course to prevent a “bad spot” where the selection can’t make up it’s mind and it chatters).

This whole project is just for fun… it’s going to use an Arduino controller, a stepper motor and 10 turn pot to set the output voltage, a graphic VFD display for voltage, current, regulator temperature, an LM-317 plus PNP power BJT’s for the regulator, Arduino controlled fan, etc…

Completely useless… just something to play with.

Grumpy_Mike:

(specifically, that the top circuit provides all the transformer current where the bottom one only provides half)

Yes that is it.

They are both full wave rectified but the top one will provide twice the current of the bottom one.

Yup. I see it now. Why it eluded me until now I have no idea, but now I get it.

Actually somewhere around 1.4 times the current. The losses in the secondary
windings are proportional to I-squared so having half the winding resistance allows
root-2 times more current for the same losses in the secondaries.

However the primary losses are also important as well so the factor is probably less
than 1.4

MarkT: Actually somewhere around 1.4 times the current. The losses in the secondary windings are proportional to I-squared so having half the winding resistance allows root-2 times more current for the same losses in the secondaries.

However the primary losses are also important as well so the factor is probably less than 1.4

Well, I wasn't worrying about the nitty-gritty of transformer winding resistance or the loss across diodes in a bridge... I was thinking that somehow I wasn't using 1/2 of the transformer's capability.... and as it turns out Grumpy_Mike was right and then I saw it as plain as day.

Yup the bridge rectifier will contain 4 diodes...

Krupski: Well, I wasn't worrying about the nitty-gritty of transformer winding resistance or the loss across diodes in a bridge... I was thinking that somehow I wasn't using 1/2 of the transformer's capability....

Not using 50% of the transformer's capability is exactly the nitty-gritty of secondary resistance and current. Transformers are usually thermally limited for constant load (they also have a max transient current limit due to magnetic saturation, but that's complicated as it involves the difference in amp-turns between primary and secondary.)