Building a power supply from second hand parts

[Splint]

Hi,
I would like to build a power supply to opperate some 60 volt DC servo motors which will be controlled by means of pulse width modulation. The controller boards are rated at 100 volts and 20 amps maximum **broken link removed** . I have in my garage the electrical componants from several photo copiers (a great source for small stepper motors bty) which are loaded with usefull looking parts. For my first question, I have noticed numerous parts labeled as noise filters, are noise filters the same as capacitors? I will need to rewind the secondry windings to get an output of 60 volts rectified, I have heard that rectifying ac to dc will lead to a voltage increase on the dc side, if this is correct can someone tell me how to calculate what the change will be? How do I limit the amps going to the controller boards to 20 without affecting the 60 volts dc (the motors use 3.5Amps continuous and 27amps peak)? I have seen on the net various diagrams of rectifiers which have a componant which looks like a centre tapped transformer, I have also seen a diagram of an automotive alternator which is simpler design comprising of a basic circuit with 4 diodes, is it ok in this situation to run with the simple alternator type system? I have also heard that it is ok, or even desirable to have more voltage available to the servo boards than the rated 60 volts of the motor, is this correct and can problems arise if the motor is being heavily loaded with excess voltage available at the board and how much extra voltage is safe?
Thanks
Splint

 

[0mega]

I have heard that rectifying ac to dc will lead to a voltage increase on the dc side, if this is correct can someone tell me how to calculate what the change will be?

No this is not correct. Using diodes to rectify ac to dc usually takes away voltage (there is a small voltage drop across diodes, usu. ~0.6V).

Look on the net for "power supply design". What kind of current are you trying to drive?

 

[Nigel Goodwin]

No this is not correct. Using diodes to rectify ac to dc usually takes away voltage (there is a small voltage drop across diodes, usu. ~0.6V).

Sorry, but you are wrong!.

Transformers are rated in RMS volts, when you rectify and smooth the output the voltage increases considerably.

A 12V transformer will give roughly about 18V DC after rectification and smoothing. The actual ratio is 1.414, minus the voltage drop across the diodes - but bearing in mind the regulation of the transformer, and the load you are drawing from it.

With any transformer system using a regulator is essential if you require any kind of stable accurate voltage.

 

[0mega]

Oh yeah of course! i was thinking peak volts, cos i've just been doing it in physics. Yes, nigel is right. I bow to your intellect

 

[Splint]

I think that would be the case where a centre tap transformer were used but what about where a simple bridge rectifier were used, there would only be the resistance of the diodes correct? I found this site which explaines the different types, the bridge system looks appealing to me for its simplicity. Any thaughts?
Thanks
Splint

 

[Nigel Goodwin]

I think that would be the case where a centre tap transformer were used but what about where a simple bridge rectifier were used, there would only be the resistance of the diodes correct? I found this site which explaines the different types, the bridge system looks appealing to me for its simplicity. Any thaughts?

It makes no difference, except that with a bridge you lose two 0.7V as you have two diodes in circuit at all times.

Bridge rectifiers are probably more commonly used than full wave ones, there's no problems with them.