Soft latch #262 help understanding what i'm doing wrong! - Page 1
Forum section: electronics / beginners

author: jfigge
author_rank: Contributor
timestamp: October 25, 2019, 05:42:38 pm
content: I'm just starting out with electronics and have built several Arduino project connecting modules with software (i'm a software engineer), but now I want to get more involved with the electronics side of the build. I recently watched Dave's video on the World's simplest soft latching power supply and wanted to follow along with each stage of his design. This meant I tried to recreate the latch circuit starting with the initial 2 transistor solution.Based on the whiteboard diagram @3:26 I tried to mimic the circuit but wasn't able to make it work without some changes - only I don't understand why I needed the changes. I'll do my best to explain the circuit, but please see the attached images...For the top transistor I used a 2N3906 and for the bottom transistor I used a 2N3904. I played with the resistors a bit and had something working using 100K for each, but really it didn't seem to matter what resistor values I used. I also used a 9v battery for the power supply and a green LED with a 1K resistor in series for the load.So here's the problem - Whenever I connected the based of the lower transistor to Vout, using a resistor ranging from 330 ohm to 1M ohm, the circuit immediately turned on? So first off I don't understand why there's a current flowing to Vout when the base of the top resistor is zero? Could this be a bad/cheap Chinese transistors problem? If I used a 1M ohm resistor the current should be in the micro-amp range so way below the required current to turn on the gate, right?To make the circuit work I could move the resistor connected to the base of the lower transistor to the far side of the load LED. I guessing this is because the LED requires 2+ volts before it conducts and so unless it's on then there's truly no current flowing.Any help to understand why the 'Always on' circuit wasn't able to toggle on and off as Dave suggested would be very much appreciated.

author: Dave
author_rank: Super Contributor
timestamp: October 25, 2019, 05:50:15 pm
content: Here's an experiment for you:Try disconnecting the LED from the circuit and probing the voltage across it with a multimeter. Try covering it with something dark and then try shining a flashlight on it.Handing you the answer straight away would rob you of the "aha!" moment, so I'd like to lead you to the answer instead.

author: jfigge
author_rank: Contributor
timestamp: October 25, 2019, 07:15:01 pm
content: I'm at work so can't test right now, but googled around thinking this was some kind of hazing the new guy thing and came across this article:https://www.quora.com/Can-LED-lights-be-used-to-generate-electricitySo it seems my "load" is acting as a power source and feeding back enough current to trip the gate -> Oh my god... I have so much to learn (traps for young players!)Thank you!

author: Dave
author_rank: Super Contributor
timestamp: October 25, 2019, 09:53:38 pm
content: Correct!You might also want to tie a resistor (10k should be fine) between the emitter and base of the PNP transistor. It will prevent any stray currents from triggering the circuit when you don't want it to.