Capacitor Selection

[Shiju]

I have a circuit which is used to control a 12 V dc ,3W ,stepper motor using PIC.The input supply is 120 V ac,60 Hz which is then reduced to 12 V using capacitor as impedance.
The electrolytic capacitor used after the bridge is 470 mfd,25 V.

Now I have decided to run the motor by taking 12 V supply from an electronic transformer.As a result the electrolytic capacitor needs to be changed since the ripple frequency of the transformer is very high close to 40 KHz which can cause inherent heating of the capacitor.

Could you please advise me how the selection of the capacitor has to be made,Electrolytic /Film ,also how to arrive at the value.

 

[m3]

Hi Shiju,

Use a Tantalum capacitors or low ESR (Equivalent Series Resistence)electrolytic capacitors. When use a Tantalum capacitors, put a several devices in parallel, this capacitor not have a large capacitance.

Regards,

M3

 

[Shiju]

Dear M3,

Thank You very much for your suggestion.

Regards
Shiju

 

[buzzp]

Will be interesting to see if these tantalums can smooth out the DC bus. I am sure they can but how many will you need. I am not sure of the max capacitance in tantalums but I do agree with M3 that they do not come in large values like aluminum electrolytics. Although, I dont imagine you will need a large capacitance in tantalum if you leave in the large capacitor.

 

[nbucska]

3W 12V = 1/4 A.
The drop in 1/60 sec:
i*dt/C = .25 * .01666/450e-6= 9.26 V. (i think it is too much. I would use much larger cap)

If 1/20 sec ( 2 way rect ) then half of it.

Use Al cap -- Ta is too expensive.


<nbucska@pcperipherals.com>

 

[sdmays]

I agree with nbucska, a large value capacitance will help a great deal! To help with the higher frequency harmonics (you mentioned a 40kHz ring) you could use some additional smaller value capacitors connected in parallel to your main electrolytic capacitor(s). I suggest a 0.1uF ceramic capacitor. Whether you add the 0.1uF capacitor or not, using several thousand uF of capacitance would be a step in the right direction.

By the way, Shiju, what is output voltage of your transformer after you have rectified and filtered it's secondary? If the secondary of your transformer is full wave rectified, or as nbucska described it, &quot;2 way rect&quot;, then the resulting rail could be as high as 17VDC. Based on the motor specs you gave above, the coil resistance is approximately 48 Ohms. 17VDC dropped accross 48 Ohms is 354mA as opposed to 250mA, as nbucska calculated above.

Just some things about which to think.

 

[Shajan]

dear all,
its a full wave bridge rectifier that i've used .
i measured the voltage across the capacitor with both the supply voltages i.e. at 12V 60 Hz, and 12V from the electronic transformer,I couldn't note much changes in their voltages.
In the first case , the max voltage was 15.2 V and min 12.8,PK-PK 2.4V
In the second case the with fast acting diodes in the bridge ,the voltage across the electrolytic capacitor was
14.4 V max and 11.6 min with PK=Pk of 2.8 V.
So can I proceed with the same electrolytic capacitors.Please advise.Any way by which i can attach the waveforms.

 

[lcsjk]

Since you do not mention a regulator, I assume you are operating without one. If you have a rectifier, you may need to do some additional measurements.

Based on your latest measurements and the motor data:
A 12V, 3W motor has a coil resistance of 48 ohms.

Using the 470uF cap, you measured a dV of 2.8 volts.
With a full wave rectifier, dT is 1/2 cycle at 60 Hz or 0.00833 seconds.
Using I=CdV/dT = (470E-6)(2.8)/.00833 = 158mA
Using this as your motor current, you have to determine how much ripple you want. If you want to keep ripple to about 0.5 volts, use the same equation and solve for a new C.
C=IdT/dV = (.158A)(.00833sec)/0.5V = 2632E-6 or 2632 uF.

If you calculate with the rated current of 250 mA, the capacitance for 0.5 volt ripple will be
C=IdT/dV = .250(.00833)/0.5 = 3956 uF

Choose a 4700uF/25V aluminum cap with a ripple rating of 0.25amp or better rating. (Most are rated at over an amp.) Always use a 0.1uF cap at the output of the supply or near the motor to reduce the high frequency transients if there are any. You can use parallel capacitors of (4) 1000uF or (2) 2200 uF.

As more capacitance is added, the dV will decrease. The minimum voltage will become higher and the maximum will become slightly lower.

Note that without a 12 volt regulator, you will always have some significant ripple, based on the capacitor you choose.