# help with capacitor size

Discussion in 'The Projects Forum' started by fancycoconut, Dec 1, 2014.

1. ### fancycoconut Thread Starter New Member

Nov 4, 2014
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0
I am converting my 24v AC supply to 24v DC to power the pump for my espresso machine.
I am using this BR http://www.diyfancylights.com/files/specs/KBPC50005-KBPC5010.pdf
I know I need to add a capacitor (relatively large at that) to smooth the output, but what values should I be looking at. My pump motor is 24v, max 36w, controlled by mosfet.
Ethan

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,070
36W/24V = 1.5A

C(ΔV) = q = it or C=it/(ΔV)

Lets say your motor will tolerate 3V of ripple, so ΔV=3
Assuming you are in a 60Hz country (we would know if you registered for the forum properly), t =8.3ms

C = 1.5*0.0083/3 = 0.0042Farads = 4200uF

3. ### wmodavis Well-Known Member

Oct 23, 2010
739
150
How much ripple is acceptable in your application? You have essentially defined the current the motor requires but 'how smooth the output" you have not stated. Both are necessary to calculate the required filter capacitance.

4. ### MrChips Moderator

Oct 2, 2009
14,291
4,197
If you rectify a 24VAC transformer you will end up with more that 24VDC. What you need is a 18VAC transformer.

5. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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One trade off is to use only a single half-wave rectifier, and that will drop the voltage somewhat...

6. ### wmodavis Well-Known Member

Oct 23, 2010
739
150
But it will also require a larger capacitor to achieve a specified amount of ripple.

7. ### fancycoconut Thread Starter New Member

Nov 4, 2014
8
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Dang, I already ordered my rectifier...
I also ordered a 50v max 15000uF capacitor

Jul 18, 2013
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It is very doubtful you will need a cap, there are many DC controllers in use that use a SCR bridge to control a motor, in your case it will be a rectifier bridge as you do not need control, the only symptom of a unfiltered bridge is a 100/120Hz buzz when the speed is dropped, which does not apply in your case.
So I would say, go ahead and just use the 24v AC supply and bridge, forget the cap.
Edit: I see you are using a Mosfet, is this for variable speed?
If so the higher filtered voltage is no problem as the recommended supply for a DC CONTROLLED motor is at least 10% above the motor rated voltage.
http://electronicdesign.com/electromechanical/power-supply-considerations-servo-amplifiers
Max.

Last edited: Dec 1, 2014
9. ### fancycoconut Thread Starter New Member

Nov 4, 2014
8
0
Yep, the mosfet is for speed control. So you are saying that if I use a full bridge rectifier on my 24V AC source to convert it to 24DC all I need is the FBR? Nothing else? I know some people were saying that this will give me over 24DC