In a scenario of sudden power loss, my latching valve will be still open by default. That valve operates with 5V, and needs 0.5A current with duration of 0.1s to be shut off. How to do that in a cost effective and safe way?
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2\$\begingroup\$ 1 - What voltage is your solenoid? 2 - Please define "cost-effective". $1? $10? $100? 3 - In line with 2, what does your valve control? That is, what do you lose if the valve stays open? 4 - Are there size constraints on your backup? \$\endgroup\$– WhatRoughBeastCommented Apr 4, 2015 at 1:38
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\$\begingroup\$ 24V? 48V? AC? DC? You might be able to achieve that much current with a large capacitor. \$\endgroup\$– Hot LicksCommented Apr 4, 2015 at 2:17
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\$\begingroup\$ Would it be better to use a valve that required power to stay open and would automatically close when power is removed from it? \$\endgroup\$– Dan D.Commented Apr 4, 2015 at 2:48
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\$\begingroup\$ Sorry for the confusion. It is actually a latching valve operating in 5V. BOM is preferably a couple of dollars, but safety is also a big concern. \$\endgroup\$– shvaCommented Apr 4, 2015 at 4:19
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\$\begingroup\$ Why are you using a latching valve in the first place? Could you post a link to the latching valve that you currently have? \$\endgroup\$– Nick AlexeevCommented Apr 4, 2015 at 4:51
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3 Answers
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A supercapacitor or a small battery should provide adequate energy storage. Most supercapacitors have too great of an internal resistance, but some low ESR devices are available, e.g. something from the AVX BestCap range. A 100 mF device ($11 USD) would drop from 5 V to 4.5 V after a 0.5 A, 100 ms discharge. If that final voltage is too low, you could use a 200 mF supercap (I would not recommend charging the capacitor greater than 5.0 V, since its rated voltage is only 5.5 V).
You'll obviously need a switching device (e.g. power MOSFET) and logic to detect the power failure and enable the FET to supply power to the valve.
If this sounds too complex and expensive, and you can accept a greater current draw when the system is normally powered, perhaps you could use a spring-loaded normally-closed valve that has to be held open with power. To make the consumption (and heating) less egregious, you can use a special circuit that uses a higher current to move the valve and a lower current to hold it open. TI even makes an IC just for that purpose.
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\$\begingroup\$ In some safety critical applications, that the normally closed, energized while active valve could be mandatory. \$\endgroup\$– jbarlowCommented Apr 4, 2015 at 8:16
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I'm assuming from the way you word your question that you have a latching valve. One pulse opens the valve, a different pulse causes it to close.
This is a standard problem for latching valves and there are standard solutions. All involve storing energy in a capacitor and using that energy to close the valve when required.
I don't have any links handy right now but I'm pretty confident that a quick Google search will show you some of the different techniques used.
One of the products that we build uses latching solenoid valves in hazardous locations (Class 1 Div 2). We use, I think, 10,000 uF at about 6 Vdc to open and close the valves. This amount of energy storage works extremely well.
answered Apr 4, 2015 at 4:06
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I'm no genius but how about keeping on a relay, when that relay loses power than a capacitor and resistor setup powers your valve, it would be passive and should not consume that much power. Oh and connect your capacitor to your power
