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Odd, I must have looked at it wrong before. Sorry about that.On Wed, 09 Jan 2008 06:43:55 GMT, "Bob Monsen" <rcmonsen@gmail.com
wrote:
It looks like J1 is a dead short to ground. You probably want an N-MOSFET
instead, and you'll need a resistor to pull the gate to ground when the
switch closes.
Regards,
Bob Monsen
No that's a P-channel JFET. It will only conduct when the switch is
open Vgsoff (max) =4V.When the switch is closed the JFET is off. When
the switch opens it provides a discharge path for the capacitor
RDSON*C. Once the cap discharges the JFET drain is at zero volts, the
switch is open so there is nothing for it to conduct. RDSON for the
J176 is 250 ohms.
I didn't take direct current measurement but there is no need. I just
measured the rise time of the cap which is 750uS through the 820 Ohm
R4. The fall time is roughly 1/3 that so RDSON must be roughly 1/3 R4.
It just gets warm (not hot) after repeated switching for testing.
open Vgsoff (max) =4V.When the switch is closed the JFET is off. WhenIt looks like J1 is a dead short to ground. You probably want an N-MOSFET
instead, and you'll need a resistor to pull the gate to ground when the
switch closes.
Regards,
Bob Monsen
No that's a P-channel JFET. It will only conduct when the switch is
It looks like J1 is a dead short to ground. You probably want an N-MOSFETI thought I'd give it one more try.
I used a jfet P-channel to discharge the cap at switch off. It drops
the cap voltage to zero in 230uS not spectacular but good enough. Load
current drops almost instantaneously. I tried a JFET before and it
wasn't fully discharging the cap. I must have had a wire crossed
somewhere because it works well now. With a lower RDSON JFET I could
improve that but for 23 cents for the FET I can't complain.
New schematic.
http://i18.tinypic.com/6z8fygx.png
The load is really a photo mosfet driver not two 1n4001's, R10 is for
current sampling. So the total parts count is 11 and price about 50
cents.
Thanks again
Yes I guess that's what I'll do.Use a 555 one-shot to contol your output transistor.
Replace everything to the left of Q5 & its resistors
with the one-shot.
Ed
Transistor Q6 should have a turn-on resistor (Q6 base to ground), instead ofI built a circuit to deliver a 40mA pulse on top of a 10mA DC offset.
The 40 mA pulse is for 100uS.R4 & C1 set the approximate 100uS time
constant. I'm thinking there must be a way to rapidly discharge C1
when the switch opens using fewer components then R11, X1, and Q6 plus
an RC filter for the SCR gate. I tried just a PNP still takes
milliseconds same with a JFET. This current scheme is the quickest
about 20uS to bring VC1 down enough to turn Q2 off.
The schematic is here.
http://i8.tinypic.com/6lawo5d.png
Any suggestions?
Use a 555 one-shot to contol your output transistor.I built a circuit to deliver a 40mA pulse on top of a 10mA DC offset.
The 40 mA pulse is for 100uS.R4 & C1 set the approximate 100uS time
constant. I'm thinking there must be a way to rapidly discharge C1
when the switch opens using fewer components then R11, X1, and Q6 plus
an RC filter for the SCR gate. I tried just a PNP still takes
milliseconds same with a JFET. This current scheme is the quickest
about 20uS to bring VC1 down enough to turn Q2 off.
The schematic is here.
http://i8.tinypic.com/6lawo5d.png
Any suggestions?