World's Simplest Soft Latching Power Switch Circuit ONLY BJTs - Page 1
Forum section: electronics / beginners

author: Paul707
author_rank: Newbie
timestamp: August 08, 2013, 05:31:10 pm
content: Dave mentioned in the video that if we wanted to, we can still use BJT instead of the a mosFET p channel (@ 4:40mins) In my case, cause of the parts I have on hand, I need to make it with just BJTs. I replaced the FET with a 2N3906 PNP and the circuit just stays on and doesn't shut off. When you hold the button, it oscillates but just doesn't shut off! What the heck am I missing? I hardwired the circuit and built a multisim and I still get the same results on both ends... If I could I would attach the multisim file. I can email it to whoever that would like to help to save yourself time from building it. The circuit on multisim worked with a MTD2955E mos_ENH_P. I did what I can to try to solve this with trial and error and theory but did not succeed. Please help

author: c4757p
author_rank: Super Contributor
timestamp: August 08, 2013, 05:49:15 pm
content: 1. Add another 1Meg resistor in parallel with C1 to discharge it. A real 22uF electrolytic leaks a lot more than a SPICE one.2. Add a base resistor, about 4.7k-10k, to Q3. It might work in the simulation without it, but in real life you'll blow it up.In the simulation it seems to be rather sensitive to component values, particularly the base resistors for Q1 and Q3. It took a bit of playing around to get it to work.Keep in mind it's not the world's best circuit (IMHO). Appears to be prone to oscillation, and some load capacitance on the output (decoupling caps on the board you're switching?) can latch it on. I was able to do that with as little as 18 uF. An output diode can correct that, but then you lose 0.7 or 0.4V.

author: Paul707
author_rank: Newbie
timestamp: August 08, 2013, 06:27:50 pm
content: Quote from: c4757p on August 08, 2013, 05:49:15 pmDid what you said on my breadboard but nothing changed.

author: c4757p
author_rank: Super Contributor
timestamp: August 08, 2013, 06:55:41 pm
content: Ahh.... always the differentiator between theory and practice, isn't it?OK, just to check - where exactly did you put the resistor (and how relative to R1), and are you testing it under load?Also, if you've already tested it in real life with your example schematic, try a different transistor, Q3 could be fried.I'll go try it myself.

author: c4757p
author_rank: Super Contributor
timestamp: August 08, 2013, 07:22:10 pm
content: Well, you're 100% correct that it doesn't work in real life (for me, it oscillates when I hold the button, and otherwise stays on). I've tried a whole bunch of stuff and can't seem to fix it, and now I'm out of time. SorryAnybody else?

author: Andy Watson
author_rank: Super Contributor
timestamp: August 08, 2013, 08:13:38 pm
content: The first problem is that there is nothing to limit the base current through Q3 and via Q2.

author: Paul707
author_rank: Newbie
timestamp: August 08, 2013, 11:21:10 pm
content: Quote from: Andy Watson on August 08, 2013, 08:13:38 pmYou're absolutely right Andy. I'm beginning to believe that this can only be done with a mosFET and 2 BJTs

author: c4757p
author_rank: Super Contributor
timestamp: August 08, 2013, 11:42:20 pm
content: You should definitely be able to do it. Probably just a problem of touchy bias. Get out a pencil

author: mianchen
author_rank: Regular Contributor
timestamp: October 21, 2013, 11:42:35 pm
content: I had this exact same problem with 2N3906 and BC559 for turning on/off an LED load. If the load is a 100R - 1M resistor, everything works (1.5M resistor stops it working). Please enlighten me