25VAC to 41VDC(Should be 36VDC)

[Degrom]

Hi.

I built a power supply to be used with a HoppyCNCPro kit.
I started with a 25VAC x 2 at 3.2A toroidal transformer. Linked it up in Parallel and made sure its in phase.

Next I installed a bridge rectifier(35Amp max) and then a set of capacitors to smooth out the spikes.

I used the Gecko formulast to calculate the uF I needed.
14'630uF = (80'000 x 6.4A)/(25VACx1.4)

I used 4 x 4700uF capacitors in parallel ending up with a total of 18800uF.
(Also added a resister to de-charge the caps when you switch the PS off)

I know the capacitors are about 4200uF over the needed amount.(is that a problem?)

If I look at the DC output I get between 40v and 41.2v...

Why is this? (It should be 36V,right?)

Any advice would be appreciated.
Thanks.

[Al_The_Man]

Are you sure you actually have exactly 25vac at the secondaries?
Or is this the labeled voltage?
As often there is difference due to the pri. voltage being slightly high or the sec open-circuit voltage can be a bit higher.
The extra ufd will not affect it.
Al.

[keebler303]

Is your transformer outputting exactly 25VAC?
What is the value of the resistor you have for discharging the capacitors?
If there is no load on the circuit, it will tend to charge the capacitors to a higher voltage. When there is significant load, it will be where it should be. A quick fix would be to add a large power resistor to draw enough current to keep the voltage in line. Another way would be to take a few wraps of wire off your transformer to stay within spec.

Matt

[Degrom]

Thanks all... I will have a look at what the exact output of the secondaries are. I measured it at one point and saw a 25v but I can't remember if that was before the bridge rectifier or after the it. (It was before I added the capacitors and resistor.)

The resister I used was a 2.2k 1watt resister.(Work really well to get the volts down after switching the power supply off)

[Degrom]

Originally Posted by Al_The_Man Are you sure you actually have exactly 25vac at the secondaries? Or is this the labeled voltage? As often there is difference due to the pri. voltage being slightly high or the sec open-circuit voltage can be a bit higher. The extra ufd will not affect it. Al.

Al, right you are... My secondaries is reading between 27 and 29 VAC

Originally Posted by keebler303 Is your transformer outputting exactly 25VAC? What is the value of the resistor you have for discharging the capacitors? If there is no load on the circuit, it will tend to charge the capacitors to a higher voltage. When there is significant load, it will be where it should be. A quick fix would be to add a large power resistor to draw enough current to keep the voltage in line. Another way would be to take a few wraps of wire off your transformer to stay within spec. Matt

The large power resister you are talking about,where will I put it?
(inline between the PS and the stepper drivers? Will a 2.2kOhm 1W resister do?

Thanks for helping.

[Al_The_Man]

If the higher voltage presents a problem, the best way is to remove turns, usually with Toroidal Tfxrs the secondaries are often wound bi-filar (together) so in the case of parallel windings you need to reduce both secondaries equally, this can only be done satisfactorily if they are in fact wound this way, typically 2turns/volt.
The problem of adding a power resistor to reduce voltage can be counter-productive to the effect of the capacitance.
Al.