Need approach: 250mS Current dump into 4 ohm inductor

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stevech
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Posted by stevech: Wed. Jun 4, 2014 - 02:36 AM
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Problem (I've tried but faild so far)

Circuit to run on small batteries - small as possible.
Feed a pulse of current into a 4.5 ohm inductive load that is a solenoid of sorts - moves a cylindrical magnet one or the other way, depending on +/- current flow in coil. Moves very infrequently, e.g., + current, then 10 minutes later - current, then a day or so goes by.

I tried a step-up converter charging a 3300uF capacitor. Then an H-bridge to get the current flow. Just not enough power - don't know if it's too little current or too few watt/seconds. But solenoid doesn't mechanically move the magnet sufficiently.

For drill, I tried a wall wart transformer claiming to provide 1700mA at 12V. Not. It droops to 6V with that 4.5ohm load.

I've not tried an overkill fat 12V battery and the H-bridge as even if it worked, it would be too large.

An ideas?

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Learning and Information, General Electronics
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ignoramus
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Posted by ignoramus: Wed. Jun 4, 2014 - 02:54 AM
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How much current?

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Bronze Markbobgardner
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Posted by bobgardner: Wed. Jun 4, 2014 - 02:58 AM
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Watts is Watts. .25 sec into 4.5 ohms -> watts = I^2 x R 12V/4.5ohms is 2.7 amps. (Batteries need to be able to hump this out for .25 sec). This is 34 watts. A garden tractor battery might work?

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meslomp
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Posted by meslomp: Wed. Jun 4, 2014 - 05:54 AM
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as bob suggested, but not said....
your 12V power supply is not big enough.
12V/4.5Ohm gives 2.7Amp. You power supply is not able to deliver that, so it will immediately drop.
Note that the initial current might be a lot higher than the 2.7Amp.
Q=CU =IT
lets say you may drop 1V in the 0.25sec time span with 5Amp.....
so C=IT/U
(0.25*5)/1 gives a cap of 1F needed, don't think you will find those in 25V version(need to be a lot higher then the working voltage specially in that range) 1F cap is a gold cap and they normally do not go above 5,5V due to mechanical/electrical/chemical restraints.

But You need to get a bigger supply to start with....

regards

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stevech
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Posted by stevech: Wed. Jun 4, 2014 - 02:12 PM
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Battery operation is needed. The wall wart was just an experiment. This solenoid is often used with consumer products that use AA batteries. They must use some sort of energy storage from an switched power supply to be able to dump that much power for a 1/4 or 1/3 or so second. I'll look at the sizes of super-caps like that.

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Chuck-Rowst
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Posted by Chuck-Rowst: Wed. Jun 4, 2014 - 02:51 PM
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So, why don't you get one of the consumer products its used in and reverse engineer its circuitry? If not to copy, at least to understand one way it can be done.

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RickB
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Posted by RickB: Wed. Jun 4, 2014 - 03:52 PM
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A much higher voltage to overcome the lr time constant? I.e. constant current drive for a short time similar to a stepper driver.

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tpappano
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Posted by tpappano: Wed. Jun 4, 2014 - 05:27 PM
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I'm impressed by how much 'supercaps' have evolved, now with some having *very* low internal resistance. A guy on "the YouTube" replaced the battery in his car with six of them in series.

Tom Pappano
Tulsa, Oklahoma

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sbennett
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Posted by sbennett: Wed. Jun 4, 2014 - 05:42 PM
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Jack Ganssle on "Using a capacitor to sustain battery life"
[url]http://www.embedded.com/electronics-blogs/break-points/4430050/Using-a-c...

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sparrow2
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Posted by sparrow2: Wed. Jun 4, 2014 - 05:58 PM
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A small hand held vacuum cleaner often use 3 AA batt. and use about 4 Amps so perhaps that quality of batt's will work.

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sbennett
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Posted by sbennett: Wed. Jun 4, 2014 - 06:12 PM
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The Hunter Industries Node irrigation controller operates off of a 9V alkaline battery for about a year. It switches a low power latching solenoid valve to control a slave irrigation solenoid valve, using the controlled water pressure to drive the slave.

[url]http://www.hunterindustries.com/irrigation-product/controllers/node[/url]

Digging further, I found a similar product with specifications for their latching solenoid valve.

Battery Powered Controllers & Valves
[url]http://www.digcorp.com/irrigation-professional/battery-powered-controlle...

R710DC Solenoid - 3 Ohms
[url]http://www.digcorp.com/professional-irrigation-products/r710dc-solenoid[...

Stan

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stevech
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Posted by stevech: Wed. Jun 4, 2014 - 07:42 PM
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sbennett wrote:
The Hunter Industries Node irrigation controller operates off of a 9V alkaline battery for about a year. It switches a low power latching solenoid valve to control a slave irrigation solenoid valve, using the controlled water pressure to drive the slave.

http://www.hunterindustries.com/irrigation-product/controllers/node

Digging further, I found a similar product with specifications for their latching solenoid valve.

Battery Powered Controllers & Valves
http://www.digcorp.com/irrigation-professional/battery-powered-controllers-valves

R710DC Solenoid - 3 Ohms
http://www.digcorp.com/professional-irrigation-products/r710dc-solenoid

Stan

Thanks... that solenoid spec sheet says 20-100mSec which is what I had guessed having been unable to find such a spec for a different brand. I'll try an alkaline 9V - I didn't think it could deliver 1-2 Amps even pulsed due to internal resistance.
I doubt a supercap is used... to new/high tech to be in such consumer products yet.

One of two I'm fiddling with
http://www.sprinklersupplies4les...

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sbennett
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Posted by sbennett: Wed. Jun 4, 2014 - 08:35 PM
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That sprinklersupplies4less link gives a model number of Orbit58874N. Searching for the Orbit p/n I found this discussion about an orbit valve internals.
-- Ray's Hobby Projects: How to control Orbit 62035 valve
[url]http://rayshobby.blogspot.com/2010/08/how-to-control-orbit-62035-valve.h...

A supercap might be useful if charged immediately before an actuation event, otherwise disconnected from the battery. Also consider low ESR organic polymer electrolytics as an alternate or in parallel with supercaps?
[url=http://www.digikey.com/product-search/en?pv13=65&pv13=77&FV=fff40002%2Cf... Aluminum - Polymer Capacitors[/url]

Stan

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stevech
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Posted by stevech: Wed. Jun 4, 2014 - 08:53 PM
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Ah, thanks a zillion. Those guys are talking about pulsed 24VDC whereas I've been struggling with 9-12V.

The AC valves are of course usually 12V but they are not magnet-memory push/pull.

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ignoramus
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Posted by ignoramus: Thu. Jun 5, 2014 - 12:42 PM
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... tried and failed...

Implies You have a sample solenoid in Your sticky mitt.

The question still stands..how much current is required to energize the solenoid?

Have You measured at what current will the coil be sufficiently excited?

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Bronze Markbobgardner
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Posted by bobgardner: Thu. Jun 5, 2014 - 02:08 PM
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Just post the L and the Ohms and we can calc the time constant etc.

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gchapman
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Posted by gchapman: Thu. Jun 5, 2014 - 06:07 PM
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stevech wrote:
I've not tried an overkill fat 12V battery and the H-bridge as even if it worked, it would be too large.
The following LTO cell has a 30C (39 amp) max 5 second discharge:
LTO 1865 Rechargeable Cell: 2.4V 1300 mAh, 39A rated 3.12Wh (BatterySpace)
A battery can be by soldered cells on a PCB.
There are smaller cells if that 18mm dia x 65mm cell is too large.
"Applications: Ideal for High Rate and High Cycle Life Applications"

"Dare to be naïve." - Buckminster Fuller

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Torby
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Posted by Torby: Thu. Jun 5, 2014 - 10:50 PM
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Grab a handfull of capacitors and an adjustable power supply.

Last time I was fiddling with a solenoid, and told it took 4 amps at 18 volts to operate, but would overheat and smoke if you left the power on and so on, after playing a while, I found I could reliably trip it with a 1000uF capacitor charged to 5v. Then I build a circuit to charge the cap to 5v through a 1k resistor, and an FET to dump the cap through the coil.

So, with a handful of capacitors and an adjustable power supply, figure out if any of them can reliably trip your gadget.

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stevech
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Posted by stevech: Fri. Jun 6, 2014 - 06:31 AM
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I have done some tests, but not enough. I think the average current (excluding the inrush) is about 200mA, and < 100mSec is needed.

The OpenSprinkler schematic shows a super-simple pulse from the MCU to a transistor driver to ring a small inductor and a diode/capacitor to store the pulsed power. It has the MCU's A/D hooked up such that perhaps the MCU pulses until the voltage reaches X. Then it toggles the dump switch to feed the capacitor's energy into the solenoid (with the desired current flow direction). This is to alter the solenoid's resting latch condition (on/off valve).

I pencil and paper looked at using the solenoid's coil as the inductor for the charge pump. Got mired in FET switches. It probably could work.

Well, I'll get time to fiddle with 18 or 24V pulses and see what happens.

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Torby
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Posted by Torby: Fri. Jun 6, 2014 - 12:50 PM
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Cute. They're using a simple boost converter to charge up a capacitor, then dumping the cap through the solenoid. Just need a great big capacitor.

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Posted by david.prentice: Fri. Jun 6, 2014 - 01:04 PM
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Think about a camera flash or a electric stock fence.

The camera flash charges up a BFC and then discharges it through the lamp/tube. I am not sure of flash voltages, currents or energy.

Stock fencers charge a BFC and then use an inductor to create the 20kV pulse. Battery fence energisers deliver less than a Joule. Some mains energisers deliver a few Joules. There are laws that regulate how much energy you can put into the fence. i.e. you don't want to kill the cows.

Oh, a regular AA cell stores quite a few Joules.

David.

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stevech
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Posted by stevech: Fri. Jun 6, 2014 - 07:12 PM
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I hope to fiddle with a simple BFC design this weekend. If I can find a suitable inductor in my junk boxes.

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Posted by david.prentice: Fri. Jun 6, 2014 - 09:05 PM
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The Boost will use an inductor of some sort. The faster the oscillator, the smaller the inductor.

The BFC is generally a good plastic dielectric. i.e. not electrolytic. Electrolytics do not like being discharged rapidly.

David.

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Posted by stevech: Fri. Jun 6, 2014 - 10:41 PM
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david.prentice wrote:
The Boost will use an inductor of some sort. The faster the oscillator, the smaller the inductor.

The BFC is generally a good plastic dielectric. i.e. not electrolytic. Electrolytics do not like being discharged rapidly.

David.

Thanks for the capacitor tip.

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tpappano
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Posted by tpappano: Fri. Jun 6, 2014 - 10:55 PM
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Quote:
The BFC is generally a good plastic dielectric. i.e. not electrolytic. Electrolytics do not like being discharged rapidly.

Really?? How many millions of xenon strobe lights have been sold in the last 60 years or so? They pretty much all used electrolytics to good effect 8)

Tom Pappano
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Posted by Kartman: Sat. Jun 7, 2014 - 11:12 AM
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Strobes have pretty low repetition rates, whereas if it were a CDI unit for an engine, an electro probably wouldn't last too long.

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tpappano
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Posted by tpappano: Sat. Jun 7, 2014 - 03:53 PM
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Quote:
Strobes have pretty low repetition rates, whereas if it were a CDI unit for an engine, an electro probably wouldn't last too long.

Sure, but the OP's application seems to be a sprinkler timer, a very low repetition rate situation. Anyway I just thought is was unfair to say electrolytics don't like being discharged rapidly. Electrolytics *do* like their heating from internal resistance to be kept within acceptable limits so that their design lifetime can be achieved. Rapid charging and discharging could cause excessive heating shortening a caps life, but then one could also consider a physically larger/lower esr part for that sort of application. A film cap for a CDI is a good fit because, among other things, the energy per spark is low. A film cap big enough to operate a solenoid might might take up too much room in the OP's box 8)

Tom Pappano
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Kartman
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Posted by Kartman: Sun. Jun 8, 2014 - 04:52 AM
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Yep, poor ol' electro's get the blame for a lot of things these days! Where would we be without them?

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