tranformer rewinding

[Mosaic]

Hi:
I want to get 3 common ground voltages from an 800W microwave transformer.
5VAC, 15VAC and 35 VAC.

If I wind 3 coils, one on the other,biggst coil 1st. Then tie the start of all coils together as a single termnal, leaving the 3 other ends separate. Would i be able to rectify via Fullwave bridges to have 3 sets of VDC all with a common ground?

thx

 

[crutschow]

No. You need to have the windings isolated from each other.

 

[tcmtech]

You do not need to isolate them rather just make one winding for 15 VAC and tap off of it at the voltages you need. That way you will have a single common to all the outputs.

 

[crutschow]

You do not need to isolate them rather just make one winding for 15 VAC and tap off of it at the voltages you need. That way you will have a single common to all the outputs.

That will not work if you want to use bridge rectifiers as the OP wants. Bridge rectifiers operate in push-pull fashion and can not have the transformer outputs grounded. Thus for bridge rectifier outputs to have a common ground, the transformer windings must be isolated.

 

[Mosaic]

Actually I'd like to end up with a common DC ground for the 3 voltages....
How shall I approach that?

 

[tcmtech]

Wind it with a single multi tap configuration where your common ground would be the center point of the winding but do it with taps going opposite of each other then use simple dual rectifiers on each opposing tap points to get your multiple DC outputs.

Think of it as multiple half bridge rectifiers for the different voltages but they still share the common middle point as the common ground. Also by doing it that way its possible to add a second set of rectifiers(full wave configured) on each tap to create plus and minus compliments of each other. -15, -5, -3, +3, +5, +15.

 

[Mosaic]

That's neat, but I think it compromises the current delivery which is important to me.

Could I do a single winding with taps at 5VAC, 15VAC and 35 VAC?. This would mean that all the FW bridges are referenced to a common Tap.

Also, can I double the magnet wire (a parallel pair) as I wind so that I halve the internal resistance of the windings? All the Taps would pertain to both windings.

 

[tcmtech]

With an 800 va transformer that works out to theoretical current of around 160 amps at 5 volts or 53 amps at 15 volts or 23 amps at 35 volts!

How much power do you realistically need any way from each voltage level?

Yes its common to wind transformers with multiple strands together to get an equivalent of a larger value.

 

[crutschow]

Actually I'd like to end up with a common DC ground for the 3 voltages....
How shall I approach that?

If you have separate windings for each voltage then you can connect the common output of each bridge rectifier to a common DC ground.

 

[Mosaic]

Carl..that's awesome. Makes windings a bit easier, especially regarding gauge of wire based on req'd current.

 

[Sceadwian]

Mind you that the total combined output can't exceed the ampacity of the main wiring, even if all three branches are drawing less, the start of the loop is going to be handling the TOTAL current of the entire output with a tapped transformer, this is why tapped rectifiers are de-rated for current vs their voltage loads and why a VA rating is given to transformers. No matter how you play the magnetics you still have to deal with bulk ohmic resistance.

 

[crutschow]

What Sceadwian is saying it that the sum total VA of all the output windings can not exceed the VA rating of the transformer (800 for the transformer you have).

 

[Mosaic]

Id like to do some voltage doubling off a 16V RMS winding as shown here:
http://www.creative-science.org.uk/multipliers.html
This should deliver around 40+ VDC off the output cap , peak.

Also I'd like to produce a 20VDC+ regulated DC output, 16V RMS calcs to perhaps 22 VDC using the same winding.

Also, I want to produce a separate regulated 5VDC off the transformer (perhaps using an 8VAC RMS center tap?) and maintain the -ve side of the C2 output capacitor as a common ground.
Any advice on the approach?

 

[Sceadwian]

The voltage you listed are totally useless without current requirements.