Are my calculations correct?

[shellfritsch]

I am wiring two identical 24vdc 4A transformers in parallel to attain 24vdc @ 8A.

This is being used to power an electromagnet that maxes out at 24vdc @ 7.5A

I was told to add some power resistors to each transformer output to balance the currents drawn (0.1ohm/5W or less).

Is there a way to control how much current the magnet pulls? If i used stronger resistors would that keep it around 7.5A or a little below? I just don't want to fry my electromagnet.

Also, I was thinking about using regular incandescent wall dimmers for variable control of the AC input to the transformers- is this cool? Am I going to break something?

Any help is greatly appreciated

 

[lats]

i guess your electro magnets would require 24v 7.5A DC instead of AC so incandescent wall dimmers wont work cause they work on AC only. you'll have to use diodes >10A rating to rectify your AC to DC.

 

[shellfritsch]

My plan is to put the wall dimmers before the transformers in the circuit.
something like this:
AC mains > wall dimmer > AC to DC transformers (wired in series) > power resistors > electromagnet

 

[Alliegro]

Wall dimmer of 5-10A would be fine. The wall dimmers are always put in series with the load( in this case the transformers). I would assume that the transformers are identical as sharing the current would otherwise be a problem. Regarding the resistor (balast ) for balancing. here are few tips.
If the resistor is on the secondary of each transformer, the current is around 4 amps.
So voltage drop on resistor would be = IXR = 4X.1= 0.4V
The wattage of resistor as per calculation is IXIXR= 4X4X.1= 1.6W
But ideally it is recommended that you use 5 times the calculated wattage to keep the resistors cool enough to function. use a 10W to be on the safe side.

 

[PaulHolland]

Hi, You can't put a dimmer before a transformer will not work. If you want to combine two DC power supplies who are identical use two high power diodes to connect them otherwise you can't. Current sources you can put parallel but NOT in serie and voltage sources you can put in serie but NOT in parallel. !!!.

Paul.