Capacitor size for actuating two latching solenoids

[schnell]

Hello,

We are wanting to utilise two latching solenoid valves in our domestic water system.

We wish to switch both of them in one second, i.e., switch one , then the other one, within one second.

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We wish to assert which is the lowest value of capacitor that we can use for actuation like this…..but the datasheet of the latching solenoid does not appear to help us…………….
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Anyway, we must actuate these valves using the energy in a capacitor initially charged up to 12V by a SEPIC converter with a 5V(in) input.

I.e.
We charge the cap up to 12V, ‘hit’ one solenoid, then charge the cap back up to 12V, then ‘hit’ the other solenoid……
…we can’t have the SEPIC charger running while the solenoid coils are actually being actuated.

Solenoid actuation circuit:
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http://i56.tinypic.com/zkivqa.jpg

Here is the part of the latching solenoids datasheet that concerns electrical parameters
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http://i53.tinypic.com/jg5v1i.jpg

The datasheet does not give the DC coil resistance, but we measured it at 30R.
The datasheet does not give the coil inductance, but we put a step input of 12V into it, and measured the rise of current………….from this test, we calculated that the coil inductance was 43mH….but we are unsure if this test is bogus?.......because doesn’t the inductance of solenoid valve coils vary widely throughout the actuation process?



Power source
The power source that we have for valve actuation is provided by using power from our 5V rail which we already have in our product.
(i.e. we have a 5V rail present which is provided from a 5W Buck converter.)
Our SEPIC charger uses this 5V as its input)

Anyway,
We are thinking if sizing the energy storage capacitor so that it can maintain a voltage across the latching solenoid coil of above 6V over a 20ms actuation period.

Is this correct?

 

[VEBill]

The answer seems to be on the order of 10,000 uF. But standby for others to chime in.

Whatever the exact final answer is, you should probably double it (at least) to ensure reliability. Or take the opposite approach and calculate the maximum value you can tolerate for recharging in the one second gap.

PS: I notice that solenoid #2 is connected in a bridge arrangement as would be expected, but solenoid #1 isn't. It almost looks like "a typo" in the schematic, on the right hand terminal of solenoid #1.

 

[schnell]

thankyou VE1BLL,

it is a typo, it should be connected as a bridge

 

[schnell]

here is the proper schematic for solenoid actuation.

http://i55.tinypic.com/1z3nz9e.jpg

 

[OperaHouse]

Having the converter running while the solenoid id on can't be any worse than charging a really large discharged cap! Seems like a lot of circuitry just to drive a solenoid.

 

[schnell]

You're right, but the SEPIC charger will be heavily current limited anyway, since we can't draw too much power from the 5V rail, so it probably won't make much difference if we leave the SEPIC running during the actuation anyhow.

-The fact is, we are slotting these latchign solenoids into an existing product which has a 5V buck converter in it.....we don't want to have to adjust that to make it able to supply higher power....so we are going to be making the SEPIC charge the cap very slowly....in about 480ms.