Release devices

[TomHoward]

He may have re-invented the wheel, but assuming the plastic bracket doesn't cost much, you're looking at about a tenner per release including hook clamp, plus a power supply. I'd say that's pretty good going!

A very simple solution - my kind of engineering!

We machined the perspex up in house, I reckon we're looking at about £200 all in to make 40 of the things. (Edit: Excluding hook clamps - didn't spot you'd mentioned that.) The plastic bracket is cut on a CNC machine so has all the drilled holes cut as well so all that's needed is the fold.

 

Ordered up another 50 solenoids this afternoon for £112. We need 40 of these devices for an upcoming production - so all the off the shelf solutions were going to be thousands.

 

Thanks for that info on the PSU - we had noticed that the opening was fairly gradual, they ease open and then snap closed using a 24V 8A bench supply for testing. We were thinking about hooking them up to two car batteries for operation to give the immediate current but a discharge makes even more sense - the power needed to hold them open is probably less once they are energised as well, and we were worried how quickly they would burn out held.

 

Think I'm going to use terminal / binding post speaker connectors for the power input, as they're cheap and easy to terminate to length.

[dbuckley]

You can over-voltage solenoids quite considerably for short periods of time to get more power.

 

I've done 12V solenoids with 110V half wave AC to give big vibrations, and as they were only on for two seconds all was well.

 

I'd start with a 4x over-voltage.

 

And that's a very nice setup.

[TomHoward]

I think I'll pick the brains of our other technician re the electronics of the capacitor discharge circuit - I can follow that one, but I'm not sure if that discharge circuit is powerful enough to drive 20 of these - and how we'd work out the figures for the caps for something that would be.

 

I've got a couple of 6A AC transformers that should do alright for powering it paired up with that cap circuit - have to investigate how quick it charges again, but I was a bit worried about apply 10A straight onto that old bench PSU of ours, because I could see the thing just going up in smoke, or taking the fuse out.

[cedd]

Would a CDU per solenoid be an option? They'll cost pence to make. It would give reliable firing, would mean cable length was far less of an issue and you could start possibly thinking about using something along the lines of XLR cabling between units. Gnd and pin 2 for power supply to recharge. Pin 3 to trigger a discharge.

You'd have to do some maths and work out the loadings of each one during a recharge and also if you left the fire button pressed a little too long, so it becomes the line current holding the solenoids open rather than the capacitor discharge current.

 

It may be a non-starter. Thinking about it, you're going to want to hold these things open for a couple of seconds. A CDU gives an effective blast of current for a fraction of a second. I don't know if it'll have a benefit when it comes to holding them open. It may still use less current than if it was just being driven normally.

[TomHoward]

They seem to flip open on a pulse - they're not required to be open for any length of time. The arm isn't spring loaded so it just falls down, especially if it's under tension. If the charging current will then hold them open it's an improvement as I am worried about the sudden increase in current on a PSU taking a fuse out, which wouldn't happen as much with the CDU.

 

I was considering either having a CDU before the switch box, or a CDU for each firing group (40 solenoids will be in around groups of 5-10) depending on how quickly the things recharge.

 

For the wiring I think I'm going use terminal posts:

 

http://img842.imageshack.us/img842/2884/imag0021f.jpg

 

As they're cheap and cheerful so keep the cost down per unit, and there's either no connectors at all on the wiring or fork / eye crimp connectors. I will swap them out for an insulated one though if I use a CDU as while 24V shouldn't be a problem the capacitor could give you a fair whack if you where holding one of those when it went off.

[dbuckley]

... though if I use a CDU as while 24V shouldn't be a problem the capacitor could give you a fair whack if you where holding one of those when it went off.

You wont get any benefit of using capacitive discharge if you keep the voltage at 24V, other than reducing the load on your power supply. A bigger power supply will be cheaper than a lot of CDI electronics.

 

My recommendation would be building site transformer and a pair of diodes as a half bridge. Cheap as chips and will work well. Just don't leave the relays energized!

[TomHoward]

You wont get any benefit of using capacitive discharge if you keep the voltage at 24V, other than reducing the load on your power supply.

Forgive my being stupid, but why would a similar capacitive circuit not improve the situation? Do you mean using the components in the linked circuit, or any circuit at all?

[dbuckley]

Any circuit at all.

 

Assume you have a 24VDC power supply that can supply the current drawn by the solenoid continuously. A capacitor charged to 24V is the same thing but can't supply the current continuously, the cap discharges according to the time constant formula.

 

A 24V 0.6A solenoid is going to be roughly (Ohms law, R=E/I, 24V / 0.6A) 40 ohms. A 2200uF capacitor will give a time constant of 88 milliseconds by which time the voltage will be down to 9V.

 

The advantage of capacitive discharge is it can supply a lot of energy quickly. The energy in a capacitor is given by 0.5 x C (in F) x V^2. Note voltage squared, so the energy goes up by the square of the voltage. again taking 2200uF, with 24 volts you have 0.63 Joules, or to use the old units, 0.63 watt seconds. Just plug the solenoid into 24VDC and you have 14.4 watts per second. So the capacitor discharge delivers much less force to the solenoid. To get the same amount of energy using capacitive discharge you need about 120VDC with 2200uF.

 

So I repeat; its hard to improve on just powering the solenoids directly, and take care that the power is only applied for a short period of time. If you need more energy, you can get (lots) more by powering the solenoid from a higher voltage.

 

The electronics mags a few decades back were full of capacitive discharge ignition systems for cars. They always used an inverter to step up the car 12V to something much higher (some up to 400v) and this was then discharged across the 12V coil from some decent sized capacitors. This gave a lot more kick than 12Volts...

 

Back in the day, we had capacitive discharge strobes. Anyone who built a Service Trading strobe from the kit knows that they charged a bank of caps up to 900V which was subsequently discharged through the tube. But these "old school" strobes have been completely outclassed (for entertainment purposes) by modern strobes like the Atomic, which have no caps at all; they just put the tube across the mains.

[TomHoward]

Hmmm. So if I've understood correctly, the capacitors just become big and unworkable quickly?

We're considering using two car batteries to give the high current required for short time as well, although the short energising time supplied to the capacitors seems attractive. I will have to check how long they are rated for.

 

As I say we have a second technician who is more up on the electronics side, I think I'll leave powering them to him. We have a 8A continuous 10A peak bench PSU which should fire about 15 of them, but it doesn't look like something you'd want to entrust your show to.

[madorangepanda]

Got more details on the exact solenoid? It wont be hard to work out the best way to power it given the full specs.

[TomHoward]

Not much more:

 

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?Vie...em=160472101195

 

12-24V, 42R, 0.3A at 12V, 0.6A at 24V. We've been powering them at 24V from a variable bench PSU, they seem to flip open in the arrangement we're using at about 17V.

[cameroncoats]

Engineer something to open them on the car batteries, then switch to the psu to hold them.

Even two switches pressed in rapid succesion should work.

[TomHoward]

Just to round this topic up - we looked into two car batteries, but we managed to find a 15,000uF capacitor and in the end made the model railway CDU circuit with some up-rated components for the increased peak currents (and charging currents) and the capacitor. It fires eight solenoids at a push, and four with a very positive action. PSU is a 24V AC at 500mA with a bridge rectifier - charges to around 35V in 1.5-2 seconds and holds four solenoids open for just under a second.

 

We were also looking at car batteries, but wasn't worth the money, especially with the addition trouble of charging, locating etc, compared to the (hopeful) reliability of the CDU.