Reed switches, Hall sensors, trigger coils... discuss

[dieter]

I currently have some issues in determing a perfect power switching system with a magnet sensor, for use in pulse motors.


Reed switches:  They are great in that they are passive, so they don't waste any energy, and they are quick, causing a true square signal, but they tend to burn out quickly. Although their livespan can be increased by several principles:
-don't let the back emf go trough the reed switch.
-don't let any high power go trough a tiny switch .
See also here:

www.reed-sensor.com/Notes/Protection_Circuits.htm

Also, big Reed switches for watts are expensive and they also attract the rotor, and did I already say that they burn out on a regular basis?

.........

Hall sensors, ideal one might think. I have just designed a pulse motor with a hall sensor. It works, although the problem is:  the hall sensor needs 5 V in, so I need to lower the supply voltage, wasting maybe 15mW already. Then I have to use a transistor for the hall sensor output. I use a tip 2955, works well, but is easily burned when the coils induction or the supply power or the heatsink is wrong. Also, the turn off isn't as sudden as in a reed switch, which is bad for back emf harvesting. This whole circuit, together with a 1.6 kOhm pulse coil, dissipates 300 mW, but the coil alone only about 25mW... a lot of circuit loss.

So I wonder, did anybody ever try to build a long living reed switch, eg. one that is filled with oil to suppress sparking??

Regards

[TinselKoala]

MHOP:
http://www.youtube.com/playlist?list=PLml9VdOeqKa8F1PebS_EX7AX2aA_ZZtb9

Long-lived reed switches are mercury-wetted (not oil) and are protected from arcing by external capacitors etc. They are the simplest way to produce fast risetime pulses but as you note they are delicate and expensive and do fail in various inconvenient ways.

You may not be using the "best" Hall sensor for your application. Try Allegro Microsystems Hall sensors: Unipolar non-latching switch, and ratiometric types.

[conradelektro]

I like trigger coils very much, because they are simple and can easily be wound at home. Also coils from relays can be used as trigger coils (because they have many turns of fine wire).

The best idea concerning trigger coils came from TinselKoala who uses an OpAmp to increase the sensitivity of a trigger coil.

I attach a circuit from TinselKoala (TL082, which he published in this forum) and a circuit which I developed based on his idea (LM311, for a low power pulse motor).

A very good OpAmp would be the MAX931, but I never got around to use it myself.

Greetings, Conrad

[dieter]

Thanks a lot for your answers, some interesting information.

[conradelektro]

A method of driving a pulse motor just with the drive coil interested me for some time. The drive coil can also be the sensor or trigger coil.

http://www.youtube.com/watch?v=OqQSJjRJ6EQ  I made a video of such a pulse motor and also show the drive circuit in the video.

As far as I know this circuit was invented by a person with the nick name DadHav. Look at the INFO of the video where I put a link to DadHav's video.


I also attach a very simple pulse motor drive circuit (which needs at least one drive coil and a trigger coil) and the photo of a ring magnet spinner (diametrically magnetized) driven with this circuit. All this has been discussed in this forum (but I have forgotten in which thread). The coils were taken from relays.

The coils need a very high DC resistance (very many turns of very thin wire). The coils I use had a DC resistance of ~4 K. If the DC resistance is smaller you have to use a different transistor.

Greetings, Conrad