Transistor base voltage, DMM said (negative) -0.7V ref to a single +12V rail? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: December 29, 2017, 07:34:05 pm
content: I made a little 2 BJT transistor multi-vibrator and was testing my used Tek 2215A that I just started checking out.It was oscillating ~588Hz and I was trying to see exactly what the base does with DC coupling......but could not get this scope to show from zeroed line, the AC waveform with the DC offset, and so I expected the whole waveform to be above the zero line. But it was not, it was still sort of centered above/below the DC zero line (I set with VertPOS on GND coupling 1st like manual said)) then took readings on DC coupling. It also says for Dc instanoeus voltage what to do and again I could not get what I expected on DC coupling (is it my expectations or settings or freq of only 588HzAnd on my quality Brymen 869 DMM with COM connected to 0V (of the 12V wall wart (from a quality fibreop modem) and + on the base, it was saying -0.7V on 1 base, and and other trans was barely on @0.1-0.2V, both trans are 2n3904So it oscillating for sure at the time, what are the caps/switching doing to make the DMM see -0.7V on the base of 1 tran ? But also early in the night I don't remember seeing (-) voltage. Is it due to the switching and the way the caps discharge ?I should hook it back up now, but I just typed 1st anyways

author: lordvader88
author_rank: Frequent Contributor
timestamp: December 29, 2017, 07:59:19 pm
content: What is the breakdown of these circuits ? I made this onehttp://www.electronics-tutorials.ws/waveforms/astable.htmlwith added LED/resistors like thisHave read over of trans Pi-model stuff months ago, but don't know most of it yet so using the less complicated way.With 5k6 on both base, 1k on both collectors, 510 on both green LEDs, 220nF capsI start with the state : 1 trans is off, 1 on, and get the DC collector bias voltage of the off trans from the 12V> 1k > 510> LED loop, and measures rightAnd for the base of the ON trans, I said Vce is just about zero and hence LED is off and said12V>5k6 >Vbe should be a loop and have a max 4.6V across base ResistorBut that step can't be right 12.2V-4.6V-0.6 is not zerowhen 1 trans is on do I say let Vce be about 0.2 (so zero ) (12V-0.2)/1k Rc and then work back to get the base curent and then the DC drop of Rb (ignoring the cap for now)And I don't have a proper picture of what the caps do, but I know the basic equations and reactance

author: rstofer
author_rank: Super Contributor
timestamp: December 30, 2017, 12:15:28 am
content: Quote from: lordvader88 on December 29, 2017, 07:59:19 pmThe caps are everything! There is a very good description of the circuit operation on the linked page. It is better than the one I wrote so I scrubbed mine.I think you're looking in the wrong place. All you care about is the charging and discharging of the capacitors and how that affects the base. You can just assume that the transistor gets to saturation during the time it takes to get the capacitor to charge to 0.6V and turn on the opposite transistor.

author: Audioguru
author_rank: Super Contributor
timestamp: December 30, 2017, 01:01:48 am
content: A capacitor in that circuit charges to about +5.3V by the collector resistor of the turned off transistor and in series with the base of the turned on transistor. Then when the collector of the transistor is driven to +0.2V the capacitor drives the base to -5.1V and it slowly discharges by the 10k base bias resistor.