Re: Drawer latches, nitinol, solenoids, etc.

From: Jim Granville (no.spam_at_designtools.co.nz)
Date: 06/29/04 

Date: Wed, 30 Jun 2004 09:18:54 +1200

Mike Turco wrote:
> I'm lookin' for a drawer latch that I can drive with a mosfet. What I have
> is more-or-less like a cash register drawer. I'd like to find something
> inexpensive -- or at least less expensive than designing my own latch and
> having to have it sent out to be made. Quantities are low, probably ten or
> twenty a month.
>
> I'd like to avoid bringing in a 12V line for a solenoid, although (I
> suppose) a dc-dc converter is possible.

Solenoids do not have to be 12V. It is the amp-turns that matter, so
normally, you get the solenoid wound to suit the application.

The most energy efficent, and mechanically simple solution is a
latching solenoid. This uses a holding magnet, and needs a bipolar
drive.

>
> I was hoping to keep the current for the entire product below 100ma, and the
> electronics only pull a few mA. I was thinking that maybe I could charge up
> a capacitor or something and then use that stored energy to kick open the
> solenoid. Off-hand, that doesn't sound practical, but I thought I'd mention
> it.

This is used, but normally in production volumes an A-T matched solenoid
is better.
If you need to use standard parts, in lower volumes, then a charge pump
is a valid solution - keep in mind what happens on a sticky drawer tho.

>
> I've also been thinking about making (or finding) a nitinol latch. It
> resolves the issue of the 12V, and the whole mechanism will probably be less
> expensive than one driven by a solenoid.
>
> Another, possibly inexpensive way to go about this would be to use a small
> motor and a few plastic gears to drive a pin.

There are 'linear motor' actuators, that are a low cost
motor+nut+threaded rod.
-jg