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'[EE] Transistor or FET or ??? to Protect Mechanica'
2007\07\05@215817 by Brooke Clarke


I recently purchased a Self Winding Clock that had the winding motor contact
points burned out by applying 12 V instead of 3 V.  I was able to replace them,
but expect in the future it will be impossible to get replacement points.   For
more on that see:

What I'd like to do is protect this clock and my other similar clocks be adding
a circuit that would be driven by the switch closures.  A simple NPN or PNP
transistor switch would seem to fill the bill.  But there are concerns:

1) this is a "vibratory" motor consisting of a couple of electromagnet coils
that pull up on an armature and then release it to fall down.  The coils
switching on and off are going to generate some voltage spikes, there is a 68
ohm resistor across the coil to minimize the spikes. the resistance of the
combined resistor and coils is 5.6 Ohms.  Implying a DC current draw of 540 ma.
  I haven't yet looked into the dynamics of the inductance and motor speed.
It would be desirable to use a transistor that has a large VCE and VCB and
shunt it with a Transil to limit the actual voltage to some same value but
still let the coil collapse as fast as possible.

2) with the fresh batteries at 3 volts total and more like 1.8 volts near end
of life what transistor can turn on hard with a low Vcc?

Have Fun,

Brooke Clarke

2007\07\05@222138 by Jinx

face picon face
> 2) with the fresh batteries at 3 volts total and more like 1.8 volts
> near end of life what transistor can turn on hard with a low Vcc?

You might look at Zetex

Very good for IR transmitters too. Low saturation, good switching
times and high currents in small packages

2007\07\05@224111 by Spehro Pefhany

picon face
At 08:59 PM 7/5/2007, you wrote:
{Quote hidden}

You could consider a MOSFET such as the FDS6064N7 ($1.50 or so one-off).
6m ohms with 1.8V drive. SOA looks okay for this application. Keep the thermal
resistance reasonable, it could dissipate a couple hundred mW worst case
(when the battery is dying).

With no suppression the voltage could spike as high as ~40V, which is double
the rating of 20V, so you'll want something like TVS to catch that energy,
or alternatively you could cut the resistor value to ~27R, but that wastes
battery energy.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"             Info for manufacturers:
Embedded software/hardware/analog  Info for designers:

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