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Choosing a capacitor


#1

A basic electronics question from a softy.

I have a solenoid valve that i need to keep cool so it doesn’t deform and start leaking. The manufacturer offers a suggestion involving using a capacitor and resistor in parallel wired to one of the valve leads. See the link for a description of the problem and solution and schematic of a suggested circuit. A table in the .pdf suggests using a capacitor rated 1764 uF with a 68 ohm resistor for my valve (P60) running at 12VDC. So i go to DigiKey and see quite a selection of capacitors, and, of course, none at exactly 1764. They are also rated with a voltage. So how do i decide what capacitor to use? What type of capacitor is practical? Do i need exactly 1764 uF? If so, can i just use appropriately rated capacitors wired in parallel so they add up to 1764? What about the voltage? Do i use capacitors rated a bit higher than 12V to allow for start-up, say about 16V? Thanks for your help!

Matt

schematic:

http://valcor.com/PDF/Power_Temp.pdf


#2

@ Matt5 -

I doubt that you need an exact match. In fact the document referenced sort of states that.

Another often-used method, and one that we prefer, is to power the valves on and then go down to what is called a ‘HOLDING VOLTAGE. Assume a 12VDC valve is energized with an unregulated voltage (approximately +25% of rated voltage). Once applied, the valve changes state. As soon as this occurs, go to a voltage of between one-quarter to one-third of the rated voltage (3 to 4VDC).

You should be able to make a test setup and mix/match a RC Circuit that gives you a voltage of between one-quarter to one-third of the rated voltage.

Most, but not all, RC circuits are not that critical


#3

Thanks, willgeorge. Good point about the variance allowed. What about the voltage rating and type? Should i be looking for something a bit higher than 12V? Any type of capacitor, e.g, aluminum, should do?


#4

@ Matt5 -

Electrolytic capacitor (usually polarized). Large values are normally electrolytic capacitors.

The capacitor working voltage must be at least that of the power supply.

Maybe something here will help you.
http://www.allaboutcircuits.com/vol_1/index.html


#5

Thanks, willgeorge. Appreciate your help. And your link looks like a decent electronics primer.


#6

there’s a somewhat rule of thumb that you should always spec the capacitor voltage at twice the expected voltage. So for nominal 12v, you’d want a 25v cap. Higher voltage isn’t a problem, the cap just won’t charge to that maximum level.


#7

@ Brett -

Good point! I agree… I should have stated that…


#8

A 2000 uF cap will work just fine. Error on the larger side. The cap is there to look like a short circuit for the pull in time of the valve and then charges to the voltage drop across the resistor. The lager the cap the longer the pull in time.


#9

Thanks, guys. Helpful.