EMRFD Message Archive 1221
Message Date From Subject 1221 2007-12-09 15:55:04 Mike Czuhajewski Automatic power off, started out as [emrfd] KA7EXM microWattmeter Here's another possibility for an auto-shutdown circuit, which I
rediscovered recently. Designed by Chuck Olson, WB9KZY, inspired by a
QST Hints & Kinks item several years ago, you can find it on his
Jackson Harbor Press web site. He sells an inexpensive kit for it, or
you can build your own since both the manual and schematic are
available for download (as they are for just about everything he has).
Push the button and it stays on for over ten minutes until it times
out, depending on what value resistor you use.
73 DE WA8MCQ
1222 2007-12-09 16:43:27 Nick Kennedy Re: Automatic power off, started out as [emrfd] KA7EXM microWattmete There's a version on the NEQRP site by Bruce, N1RX.
There's a link to click for the JPG below the text.
I played with it a little. My notes on the circuit:
Hooked up a 180 ohm load at 9 VDC to give 50mA. No problem getting
the full 50mA with only about 0.25V drop across the channel while the
gate was high. For the MOSFET I tested, the switch is full on at
3.99V and just beginning to turn off at around 3 VDC. At 2.6VDC, the
channel is mostly off and at 2.16V, it is fully off. So there's about
0.83 volts from on to off.
With a 1 M resistor to ground to drain the 47uF capacitor (time
constant = 47s), it took 62 seconds for the current to begin dropping.
With no resistor at all, the current is still at full value 2 hours
later. (Capacitor is a general purpose aluminum type.)