Solenoid for Special Effects

[J Pearce]

A capacitor discharge unit can be a handy way of ensuring they fire cleanly whilst ensuring they won't get overdriven if the control circuit remains on.

[dbuckley]

Indeedy, but I have found that sometimes the capacitor discharge is a little short, and then the amount of C required gets rapidly out of hand :) But if CD can do the job, it does provide a pretty much guaranteed way of not melting solenoids.

 

E2A it was whilst developing something along these lines that I discovered what happens when one vaporises PCB tracks with excess current. Clive, on his website has a nice demo of this, but I can assure you that when it happened unexpectedly in the workshop I really thought my number was up!

[bigclive]

Another simple option for hard firing of solenoids, but then reducing the current, is to use an electronic PTC fuse. It has to be matched properly to handle the full current for as long as the actuation period, but then trip faster than the heat in the coil builds up. The PTC thermistor fuses start with a low resistance but then self heat due to current flow to the point they start to increase in resistance, and from there the heat/resistance thing avalanches until they reach a stable state with the current dramatically limited. To reset, simply remove the power for a short time to allow the thermistor to cool again.

 

Like the degaussing circuits in old CRT displays. Basically a big coil in series with a PTC thermistor, hooked across the mains.

[Andrew C]

Another simple option for hard firing of solenoids, but then reducing the current, is to use an electronic PTC fuse.

And I've got a box of 2A* ones if anyone can find a use. They weren't fast enough for my application. (postage cost only)

 

 

 

*I think, I'll check Monday

[bigclive]

PTC thermistors are something that have to be experimented with for an actual application. You really need something like a motor stall current to be high enough to guarantee a trip quickly without the normal running current being an issue.

[TomHoward]

There is an older topic on makeshift Kabuki - we build a 32-circuit system for a show which was based on solenoids, snap shackles, and a capacitor discharge power supply. Triggering was doing via a Milford Instruments relay box, to trigger solenoids from a capacitor discharge power supply. I can supply some photos if it helps.

 

http://www.blue-room.org.uk/index.php?showtopic=31397