How to discharge the capacitor?

Thread Starter

samy555

Joined May 24, 2010
116
Hello
From: http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P2.html

I've built the following circuit:




In fact I used a 200K ohm resistor instead of 100K and a 1000uF capacitor and I had a delay time of about 3.5 minutes.

The problem is when the main power off the 1000uF capacitor maintained its charge for long time and if the main power returned the circuit is ON immediately without any delay. How to discharge the capacitor?

Thank you very much
 

ronv

Joined Nov 12, 2008
3,770
Hello
From: http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P2.html

I've built the following circuit:




In fact I used a 200K ohm resistor instead of 100K and a 1000uF capacitor and I had a delay time of about 3.5 minutes.

The problem is when the main power off the 1000uF capacitor maintained its charge for long time and if the main power returned the circuit is ON immediately without any delay. How to discharge the capacitor?

Thank you very much
You could try a schottky diode across the 100K resistor with the anode to the + of the cap.
 
Last edited:

MikeML

Joined Oct 2, 2009
5,444
Even better; ditch the Darlington and replace it with a regular NFET (not logic level); That makes it possible to use a much smaller timing capacitor, and a much higher value for the timing resistor (now 100K). You can eliminate the 330K and the other resistor tied to the base of the Darlington, too.
 

ian field

Joined Oct 27, 2012
6,539
Hello
From: http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P2.html

I've built the following circuit:




In fact I used a 200K ohm resistor instead of 100K and a 1000uF capacitor and I had a delay time of about 3.5 minutes.

The problem is when the main power off the 1000uF capacitor maintained its charge for long time and if the main power returned the circuit is ON immediately without any delay. How to discharge the capacitor?

Thank you very much
Just put a diode in parallel with the 100k resistor - pointing upwards so its reverse biased when the capacitor is charging. When you switch off and the supply rail falls, any voltage left on the capacitor forward biases the diode and discharges through the other circuit elements.
 
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