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Topic: What size capacitor needed to power solenoid? (Read 4 times) previous topic - next topic

redtank

I have an Arduino based project I'm doing for work. The project uses a 12vdc automotive starter solenoid to power a wire cutting shear. To the best of my calculations, the solenoid draws somewhere between 35 and 45 amps @ 12vdc. (coil resistance is .2 to .3 ohms). I'm using a solid state relay to energize the solenoid, but sadly, the relay is only rated for 8 amps; I'll have to find another. Solenoid engagement time is <50ms. Power supply for the project is a dc to dc converter: input 24v, output 12v, 3.8amps. I'm wanting to supplement the power supply using a capacitor, perhaps a bass capacitor used for stereo systems. I'm not sure how to calculate how many farads of capacitance I'm going to need, any suggestions?  Also, can someone suggest a source for dc to dc SSR's (hockey puck based)? I thought Mouser would have them, but instead, they have DIN mount style units that are expensive. Right now, I have this project up and running on a test basis only, using the 8 amp SSR and a 20ah SLA battery, so far, my relay has performed well.

Thanks,
Cris

Magician


Approximately :  t = RC

C=t/R = 0.05/0.2 = 0.25 Farad.
That is pretty big value for capacitor, 250 000 uF.
Electrolytic  type will works, just limit charging current with resistor.
Probably, current will be less than you expect as V/R, as load inductive,
so less capacitors needed.

redtank

Magician,
Okay, thanks for the formula. Now, can I use a motor start capacitor or maybe one used to start air conditioner compressors? I know these are rated for AC use, but are they not constructed in the same manner as bass capacitors? Their cost is much less...

Thanks,
Cris

Terry King

Hi,
Gigundo Capacitors are widely available: http://www.sonicelectronix.com/cat_i6_-capacitors.html etc etc

You could use this kind of power FET board with 4 - IRF540 FETS ($13.50) and just parallel the channels for extra margin: 
http://arduino-direct.com/sunshop/index.php?l=product_detail&p=60

And they are optically isolated, so you could mount this as close to the solenoid / capacitor combination as possible and keep the Arduino farther away...


DISCLAIMER: I mentioned stuff from my own Shop...  But I like finding solutions 
Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

Magician

I guess you mean capacitor in air conditioner that help to start three phase motor when
it plug on one phase?
Afraid its not good, 1-10 uF what I've seen so far.
250 000 uF should not be costly, as you need low voltage, let say 16V, and
it even better if 5-10 smaller one connected  in parallel.

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