Why circuit no worky? - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 07, 2017, 06:10:18 pm
content: The resistors get warm that are hooked up to the transistors. I'm giving it 9 volts. When I hook it to the power supply it drops from 9.1 volts to 8.9 so its pulling current.Transistor is a bc557b the circuit calls forPNP Transistor, P/N 2907A, qty: 2Resistor, value 470 Ohms, qty: 2Resistor, value 100k Ohms, qty: 2Capacitor, 10 uF, qty: 2LED, Qty: 2I tried both with 10 uf and ten pf caps , and no lights.Is this not a compatible transistor?

author: ataradov
author_rank: Super Contributor
timestamp: February 07, 2017, 06:30:33 pm
content: It appears that you have emitters and collectors of the transistors switched.This circuit should not pull enough current to make any sensible power supply drop voltage.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2017, 06:50:35 pm
content: I agree. The transistors are wired backwards. See the datasheet and compare it to what you've built.http://www.farnell.com/datasheets/296678.pdfIt's very likely the transistors have been damaged so, when you correct the wiring, discard them and replace them with new ones.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 07, 2017, 07:06:25 pm
content: No, the transistors are wired properly. If they were PN2907A (EBC) they would be backwards, but you are using BC557B which are CBE.The Farnell data sheet is kind of screwy in that it has the pin _numbers_ backwards from standard TO-92 pin numbers, I think.https://eva.fing.edu.uy/file.php/585/materiales/HD/PN2907A-D.pdfhttp://pdf1.alldatasheet.com/datasheet-pdf/view/11554/ONSEMI/BC557B.html

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 07, 2017, 07:49:35 pm
content: Check the value of your base resistors. The schematic calls for 100k ( brown black yellow). It appears that you have much lower value resistors here. And the LED resistors should be 470R, Yellow Purple Brown.I've breadboarded the circuit, a simple multivibrator, and it works fine using BC556B, and 4.7 uF caps, at 5 volts supply.If you are using BC557B, then you DO have the transistors connected properly, with Emitter (right most pin, #3) connected properly to the positive rail. However you may have blown the transistors and/or the LEDs by using the wrong resistor values.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 07, 2017, 07:52:39 pm
content: Quote from: alsetalokin4017 on February 07, 2017, 07:49:35 pmLooks like the LED resistors are fine, but the base resistors look like 100 Ohm.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 07, 2017, 10:02:37 pm
content: Hmmm.... when I sub in 100R for the base resistors, the multivibrator action stops and both LEDs stay on brilliantly. The transistors survived though; once I go back to 100k the thing works properly again. (5V supply)And-- when I reverse the transistors so that C is connected to the positive rail, the multivibrator action still works but with much shorter flashes. The transistors survive.(100k for base resistor and 5v supply). Again subbing in the 100R for base resistors, stops the multivibrator action with both LEDs on brightly. But the transistors still survive and when put back to the schematic configuration, it works properly again.So there must be something else wrong with the OP's breadboard circuit.(ETA: Yes, with 9V supply and 100R base resistor, the resistor gets _extremely hot_ pretty fast. Both LEDs shine brilliantly. The transistors survive though, at least for the few minutes of my test.)

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 08, 2017, 01:30:36 am
content: Yes the only thing I noticed was the resistors get hot. I do think they are the wrong value. These transistors are backwards from the diagram but they are supposed to be be (trap for young players). That was a great response from all !!!

author: xavier60
author_rank: Super Contributor
timestamp: February 08, 2017, 01:49:52 am
content: The capacitors' polarity should be reversed. Not certain if this would stop it from working.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 08, 2017, 04:15:41 am
content: Quote from: raspberrypi on February 08, 2017, 01:30:36 am1. The base resistors on your breadboard are definitely the wrong value. They should be 100k, Brown Black Yellow. (1, 0, 0000). But of course different high values will also work here. Try 47k, 68k, 100k, 220k etc. and see how it affects your circuit's performance and current draw.2. Your BC557B transistors ARE WIRED CORRECTLY on your breadboard. Looking at the flat side, the leads are Collector, Base, Emitter from left to right. All the data sheets, even the Farnell one, agree on this. The only problem with the Farnell data sheet linked above is that the pin _numbering_ is screwy. But the functional assignment is correct, even on the Farnell data sheet. Your transistors are _not_ backwards from the diagram. It is just that the PN2907 is EBC and the BC557B is CBE, looking at the flat side. Yes, except for the pinout, these transistors are pretty much equivalent as far as this circuit goes (they are being used as saturated switches.)3. For such tiny currents, low voltages and relatively large capacitances, the electrolytic cap polarity doesn't matter much. Obviously, since my breadboard works and I followed the schematic's polarity indication. The capacitors control the rate of flashing, tiny caps like 10 pF are useless here unless you want flipflopping at ridiculously high rate (so fast you can't see it visually.)Have you got your LEDs in properly? The Cathode is usually indicated by a little flat area on the base of the LED, and the Anode lead is usually a little longer than the Cathode lead (unless it has been trimmed).

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 08, 2017, 08:44:24 am
content: This was a difficult one. I changed out the resistors to the proper value and still didn't work. So I measured the leds with the DMM, seemed fine, but it turns out the leds were too close to my high voltage thing I made awhile ago and it cooked them. I put in new leds and it works fine. It blinks faster when you change out the 470 resistors to smaller ones. So what is the purpose of the 100k resistors? When you jump down in cap size to pf they just get dim because they are switching so fast.

author: mikerj
author_rank: Super Contributor
timestamp: February 08, 2017, 08:50:28 am
content: Quote from: alsetalokin4017 on February 08, 2017, 04:15:41 amVery hard to see what the caps are on the OP's circuit, but certainly not electrolyitics. Possibly tantalums, or could even be ceramic caps which would be way lower than the 10uF specified.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2017, 08:52:51 am
content: Quote from: raspberrypi on February 08, 2017, 08:44:24 amThe 100k resistors change the frequency. The LED series resistors will affect the frequency too but not as much as the 100k.A more detailed description of the circuit can be found on Wikipedia.https://en.wikipedia.org/wiki/Multivibrator#Astable_multivibrator

author: tautech
author_rank: Super Contributor
timestamp: February 08, 2017, 09:04:18 am
content: My kids built these at school and when one of them got his first car set 10mm red and blue clear bright LEDs in his grille and switched on the flasher circuit with his rear window heater switch.Little bugger had great fun sneaking up behind his mates with his headlights off and with those red and blue lights flashing that for all the world looked like the cops were after you in a rear view mirror.Until a cop spotted him from a unmarked car coming the other way.The cops saw the funny side a tiny bit but still made him rip them out on the spot.Please don't try it, you mightn't be so lucky.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 08, 2017, 11:18:07 am
content: Quote from: mikerj on February 08, 2017, 08:50:28 amWhy not electrolytics? They are 10uf electrolytics. I put in 220uf electro lytics and it blinks really slow. So now I'm going to change the 100k to smaller.

author: ebastler
author_rank: Super Contributor
timestamp: February 08, 2017, 11:24:53 am
content: Quote from: raspberrypi on February 08, 2017, 11:18:07 amWhat mikerj said is that the capacitors in the photo you posted are not electrolytics. The photo probably shows the 10pF caps you had tried alternatively. (As mentioned before, these are way too small to obtain oscillation periods that are visible to the eye.) Electrolytic caps are fine in this circuit.

author: Caio Negri
author_rank: Contributor
timestamp: February 08, 2017, 03:08:36 pm
content: Quote from: raspberrypi on February 08, 2017, 11:18:07 amThe orange disk capacitors in your photo look like ceramic capacitors, those are usually in the order of picofarads. Check those again carefully to verify their capacitance. Here's a chart of capacitor types and their simbols:http://www.electricaltechnology.org/wp-content/uploads/2013/03/Types-of-Capacitors-Polar-and-Non-Polar-Capacitors-with-Symbols.jpg