Pocket solenoid firererer

[throbscottle]

I want to make a little box with some caps in it to fire a particular type of solenoid (for work, omg what's wrong with me?), which is a 24v 24ohm type, pulling a load equivalent to lifting up to maybe 100 grams. It probably needs about 20 or 30 milliseconds firing time, and being able to hold enough charge for a few go's would be useful. Maybe switch in multiple alternate caps for that.
I was thinking they can be charged in parallel from vehicle battery, then connected in series for use. Or be charged slowly from a couple of C cells, stepped up, maybe.
So, can anyone help work out what value of caps I would need?

TIA

 

[Nigel Goodwin]

I'd like to run some experiments before getting complicated - charge various capacitors to 24V, and apply them to the solenoid - with the solenoid under various loads, including none, and to two or three times the maximum 'anticipated' load.

I'd be inclined to choice one that manages at least double the anticipated load - obviously you can parallel the caps to increase capacity.

Once you've decided how much capacitance you need, then you can start worrying about charging them

 

[throbscottle]

So trial and error then. I was looking forward to doing some theory vs practice tests!

 

[AnalogKid]

What is the minimum operating voltage for your solenoid in this application? Vmin = ?

ec=it If you discharge a capacitor with a constant current (a good first-order approximation for just about anything), the change in voltage times the capacitance equals the constant current times the discharge time. Rearranging:

c = it/e If Vmin is, say, 20 V, then 1A x 1 second divided by 4 V = 0.25 F. So, 250,000 uF will get you 1 second of total operating time. Scale up or down from there.

ak

 

[throbscottle]

That's a good question. I don't know. They are the ones used to release cash drawers so I only ever see them in action when powered from a cash drawer port on a printer or base unit, so normal operating range just isn't something I encounter. I suppose 20v is a reasonable supposition though. The solenoids themselves don't really fail, but it can be a real pita to open the drawers when other faults are present sometimes.
Taking your rough value though, that's a heck of a big cap even supposing I went on 1/5th for a supposed 1 shot version. Maybe I'd be better getting a small 24v sla and keep it charged.
My other thought was to do a high voltage version and connect it through a pulse transformer to drive the solenoid - get more joules in the cap that way... Or is that just getting silly?

 

[AnalogKid]

The total energy in a capacitor is 1/2*CV^2, no matter how you get it in there or out of there. The problem in post #4 is that the delta-V is so low that you are laving a lot of energy in the cap unused (assuming the solenoid will not operate below 20 V). If you follow the cap with a wide-input-range dc/dc converter, then the delva-V can jump from 4 V to 20 V, cutting the size of the cap by 80%. Also, if the solenoid is to be energized for anything over 1 second, you can play a trick that cuts the current once it has pulled in. The holding current always is lower than the pull-in current.

ak

 

[alec_t]

Assuming you can find one (I haven't checked), perhaps use a 6V-to-24V DC/DC converter, run from a 6V sla ?

 

[throbscottle]

Hmmm, that's quite a lot to think about. Don't know if a dc-dc converter would help, it's only being fired for 10's of mS. Actually I just realised I got that wrong in post #1 - probably more like 200 - 300 mS. Duh.

 

[throbscottle]

Hi alec - yes that might be a better idea...
Would be a hefty current draw though

 

[AnalogKid]

Are there size / weight / environmental / cost restrictions on this thing?

ak

 

[throbscottle]

Well, it's a tool for occasional work use, I have no budget so really I'm relying on scrap, and it needs to be as portable as I can make it.
Actually now I think of it, there is a wonderful invention called "mains electricity" which I had forgotten all about! So really I just need a 24v wall wart which can deliver a peak pulse of 1 amp into an inductive load. Get lost in my own head sometimes, honestly!
So I suppose some kind of monostable to give a timed pulse, is the thing. Why the heck did I think it had to be self powered? Honestly. IMA dummass...

 

[AnalogKid]

For the monostable you could use a pushbutton switch and a finger.

ak

 

[Pommie]

If you decide to go portable then you could use 4 AA batteries with a boost converter. To find a suitable module search XL6009 on ebay.

Edit, obviously, the push button would be on the low voltage side.

Mike.

 

[Nigel Goodwin]

So trial and error then. I was looking forward to doing some theory vs practice tests!

I would have thought any theory would just be a total waste of time?.

 

[throbscottle]

Well, strictly to get into ballpark figure territory you understand!
I can't really just rely on my finger for timing, too many human factors involved turn a quick press into a long one (the "Grrrr, work, you git" factor, for one, the elbow factor for another).
Suppose I must just put some parts together and try it
xl6009 - I think it would be pulling at least 4A from the batteries to get 1A out. I guess that goes back around to big caps again...

 

[Pommie]

Pulling 4A out of AA batteries is not too extreme. Assuming alkaline AAs have a capacity of 2Ah and a solenoid on time of 1/3 of a second, they will manage 2*3600*3/4 = 5,400 actuations in an ideal world.

If AAs haven't got the muscles for the job then C size cells should manage.

Also, the xl6009 has an enable pin that will enable it to act as the power switch for your oneshot.

Mike.

 

[AnalogKid]

I would have thought any theory would just be a total waste of time?.

He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast.

- Leonardo da Vinci, 1452 - 1519