transistor only switching with oscilloscope probe - Page 1
Forum section: electronics / projects

author: ToBeFrank
author_rank: Regular Contributor
timestamp: August 30, 2011, 09:14:15 am
content: I'm trying to diagnose why my transistor is not switching. I've used this circuit in several other boards with no problems. I'm using it the same way on this new board, which is to invert an input signal and level translate it. The input signal is high at ~7.2V and low at 0V. The circuit is below. If I probe at the R4 pad I see 1.29V for high. I also see 1.29V when I probe the R5 pad. This is as I expect from the voltage divider. When I probe the base of the transistor (pin 1), I see 690mV. The odd thing is when I probe the base of the transistor, it starts switching as it is supposed to and the circuit works. As soon as I remove the probe from the base it stops working. What does this indicate?

author: Psi
author_rank: Super Contributor
timestamp: August 30, 2011, 09:28:12 am
content: First thing i'd check is that R5 is grounded properly.Quote from: ToBeFrank on August 30, 2011, 09:14:15 amWhere exactly are these two pads in relation to the circuit?Also, when its not working, does the transistor stay on or off?

author: Gall
author_rank: Frequent Contributor
timestamp: August 30, 2011, 11:34:34 am
content: Did I understood correctly that you measured 1.29 V at the OUTPUT of the divider (which goes to point 1) and different voltage at point 1? Most likely, cold joint (bad soldering) at transistor's base. A npn transostor parallel to the lower voltage divider's resistor (like this circuit) would limit the divider's output to 0.6 V if working properly. Broken transistors with bad internal joints do exist, too.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: August 30, 2011, 01:37:15 pm
content: Quote from: Psi on August 30, 2011, 09:28:12 amYep my first though was bad grounding and the probe is providing the ground connection. Measure the resistance from base to ground should give you some clues.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: August 30, 2011, 03:08:37 pm
content: I checked the ground at R5 and it is ok. I then took out the soldering iron and touched all three joints of the transistor. It's now working fine. As Gall pointed out, it was a cold joint. Thanks!

author: Gall
author_rank: Frequent Contributor
timestamp: August 31, 2011, 11:32:01 am
content: Just to prove it, try to measure voltage at R5 now. It should be around 0.6-0.7 V, limited by the transistor and not by voltage divider.A rule of thumb: every Si transistor always has 0.6 V base-to-emiter voltage (Darlingtons have 1.2), and every Si diode has 0.6 V when open (red LEDs 1.2 V, blue LEDs 3 V). If you see more than 0.6 V on a resistor between base and emitter (like R5), this resistor is not really connected between base and emitter of a transistor. Either it is not really connected (due to cold joint) or this is not a transistor (maybe fried transistor)

author: ejeffrey
author_rank: Super Contributor
timestamp: August 31, 2011, 08:37:31 pm
content: QuoteIndeed. This sort of thing is the most important skill for debugging and troubleshooting. When some damn circuit just doesn't work, none of the voltages make any sense at all, and you can't begin to figure out what could be going on, you have to find a place to start. Believe what you know about electronic components, and when you find a part that is doing something 'impossible', don't ignore it because it doesn't make sense. A transistor with a reverse biased base-emitter junction and 1 amp of collector current, a diode forward biased by 10 volts, or a 1/4 watt 1 ohm resistor with 1 volt across it that is cool to the touch are impossible. If you think you see it, you are not measuring what you think you are measuring. It could be a dead component, but far more likely you are measuring the wrong thing due to a bad joint or incorrect connection.