Capacitor size to actuate two latching solenoids?

[eem2am]

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

Here is the part of the latching solenoids datasheet that concerns electrical parameters
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https://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?

 

[Johansen]

this is one of those trial and error problems, due to so many variables that cannot be simulated.

in addition, you need a safety margin for the capacitors. how long do you want this product to last/perform as intended.

 

[erikl]

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….

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.

A simple estimation (approximation) :
1. With V=12V , the max. coil current of I=400mA can be reached in less than 2ms
2. During the 20ms pulse, you'd want to keep the capacitor voltage above 6V

Hence your voltage decrease should be δV/δt ≤ 6V/20ms = 300 mV/ms. Better choose 250 V/s , then you get
C = I/(δV/δt) = 400mA/(250 V/s) = 1.6 mAs/V = 1600µF.

I'd take at least a 2200µF, better a 4700µF / 16V low ESR Al electrolytic capacitor.