Choosing a capacitor

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

@ 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

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?

@ 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

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

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.

@ Brett -

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

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.

Thanks, guys. Helpful.