Help with circuit... - Page 1
Forum section: electronics / beginners

author: Arion02
author_rank: Contributor
timestamp: October 11, 2012, 02:59:38 pm
content: Greetings everyone,I have a simple(I think) problem that I just can't figure out. I have two goals. The first is to become knowledgeable with transistors. Does anyone know of a good video/book that I can learn the basics of transistors from? I really appreciate any help.The second goal is to build a circuit that has two LED's and tests for an open/closed in a cable. Schematic is attached. I have a cable with 6 separate conductors that I want to test whether there is an open or short in. I want a green LED to light when there is a short and the red LED to light if the wire is open. I thought I had a nice little circuit but it didn't work on the breadboard. It did, however, work in the spice simulator. I am not sure how to calculate the base resistor values for transistors, so I'm sure that adds to my problems. On the breadboard, one LED lights just fine, but when I create an open at the base of the first transistor, both LED's light. I am assuming that the base-emitter junction in the second transistor is letting enough current through to light the first LED. In the schematic, the cable under test would be at the base of the first transistor (on the left) with 9v going into one side of the cable and the other end to the transistor base. I want to learn as much as I can from this project. Again, thank you all for any input/suggestions you can provide. It is much appreciated.

author: marcosgildavid
author_rank: Contributor
timestamp: October 11, 2012, 03:22:09 pm
content: Hi,this handouts were somewhat useful to understand some concepts. Take a look at the examples:http://www.ittc.ku.edu/~jstiles/312/handouts

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 11, 2012, 06:50:35 pm
content: Quote from: Arion02 on October 11, 2012, 02:59:38 pmSo increase the value of R1 and see what happens

author: Arion02
author_rank: Contributor
timestamp: October 11, 2012, 07:36:08 pm
content: Quote from: marcosgildavid on October 11, 2012, 03:22:09 pmThanks for that link!Quote from: PA0PBZ on October 11, 2012, 06:50:35 pmI tried it yesterday with a 10k resistor but I will try a higher resistance. Thanks!

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: October 11, 2012, 09:01:46 pm
content: Somebody link this here a while back ,http://benkrasnow.blogspot.com/2011/05/how-to-design-transistor-circuit-that.html

author: robrenz
author_rank: Super Contributor
timestamp: October 11, 2012, 11:04:14 pm
content: http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

author: onlooker
author_rank: Frequent Contributor
timestamp: October 12, 2012, 01:09:33 am
content: Adjusting R1 will work only if one assumes the hfe of Q2 is large enough. A preferred solution should not have the bias current of Q2 going through D1 at all. You may try to move both D1 and D2 down to the emitters of Q1 and Q2 and replace R1 by a diode.

author: notsob
author_rank: Frequent Contributor
timestamp: October 12, 2012, 01:47:16 am
content: try pcbheaven.comGeorgios has written a whole series on transistorshttp://www.pcbheaven.com/wikipages/Transistor_theory/http://www.pcbheaven.com/wikipages/Transistor_Circuits/

author: Arion02
author_rank: Contributor
timestamp: October 12, 2012, 03:46:52 am
content: Hi all,I found a solution that works. I just moved the LED that would be green (indicating a short in the cable under test) to the base of Q1. Works great on the breadboard. When the base of Q1 is open, Q2 turns on which illuminates the red LED. When Q1 is turned on, Q2 turns off which turns off the red LED and illuminates the green LED. Is the way that I have it working proper?Thank you all. I have quite a bit of reading to do with all the resources listed! THANKS!

author: PA4TIM
author_rank: Super Contributor
timestamp: October 12, 2012, 06:28:07 am
content: Most leds need a few mA to light, you need a very big transistor that can sink a few mA in the base. If it works like this you probably have the transistor wired the wrong way or it is dead allready.I do not understand the schematics, where you insert the cable under test ?If You connect the tranistor permanent to the positive rail it will be allway on ( being npn)

author: Arion02
author_rank: Contributor
timestamp: October 12, 2012, 02:00:21 pm
content: Quote from: PA4TIM on October 12, 2012, 06:28:07 amI just tried two other transistors and they worked in the same manner as the previous two. They are 2N2222a transistors.Quote from: PA4TIM on October 12, 2012, 06:28:07 amSorry about that... I should have put a switch in to represent an open or short in the cable... it would be between R1 and +9v rail.Where can I find how much current the transistor base will sink? Would that information be in the datasheet? thanks.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 12, 2012, 02:20:41 pm
content: It probably will not kill them straight away. The base can survive 200mA peaks but it is a small signal transistor and you normally do not bias them with 20mA or so.If you make it like it was the problem is that when the switch is open you have a floating base. They do not like that so also tie a resistor from base to ground.Second problem is you light the led in the first schematic the same way as you dit now. The current flows through R3, R1 and the base of Q2 to ground. So both constructions are not good.

author: Arion02
author_rank: Contributor
timestamp: October 13, 2012, 06:32:29 pm
content: Quote from: PA4TIM on October 12, 2012, 02:20:41 pmOk, thank you for your input. I guess I will scrap that whole idea and start fresh. What seemed like a simple problem is turning out to be a much harder problem but I am learning a lot. Thank you.

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2012, 07:23:06 pm
content: Quote from: Arion02 on October 11, 2012, 02:59:38 pmYour circuit is close. First try adding the pull-down resistor on the base of Q1 as shown in the attachment below.For another experiment, try repositioning the LEDs as also pictured. This is not a very elegant or efficient circuit, but you could see if it works and have a think about what problems it may have.One more idea is to consider PNP transistors in circuits as well as NPN. When you have a circuit that has to respond in opposite directions, the polar opposite behavior of a PNP transistor can sometimes be useful.

author: Arion02
author_rank: Contributor
timestamp: October 14, 2012, 03:17:07 pm
content: Quote from: IanB on October 13, 2012, 07:23:06 pmThank you. I will keep working on it then.There is so much to learn. I just started my electrical engineering degree this year. As an electronics tech in the US Navy, I have been inspired by the design of electronic circuits that I have to work with every day. I don't want to just troubleshoot somebody else's design anymore. I want to be the guy designing them. It's a long road, but I will get there. Again, thank you all for your assistance.Sent from my Nexus 7 using Tapatalk 2

author: Arion02
author_rank: Contributor
timestamp: October 17, 2012, 09:28:49 pm
content: I hope I got it figured out this time. I have attached the schematic below. I found that I can use the transistor as a current limiter so I removed the current limiting resistors for the two LED's and calculated how much current the transistor will pass with a beta of 100. It should pass 15mA or so for each LED. Is there anything wrong with this design?Sent from my Nexus 7 using Tapatalk 2

author: PA4TIM
author_rank: Super Contributor
timestamp: October 17, 2012, 09:52:18 pm
content: In theory, you are right. In practice it is a different story. It is "not done" to design beta dependent.The beta is not a real constant. It is most times a typical value. It is also not a very steady and contant value.

author: Arion02
author_rank: Contributor
timestamp: October 17, 2012, 10:06:39 pm
content: Quote from: PA4TIM on October 17, 2012, 09:52:18 pmOk. So it would be safer and more practical to run the transistors in saturation with a current limiting resistor with each LED? Thank you for your input!By the way, does the beta value typically vary drastically?Sent from my SPH-L710 using Tapatalk 2

author: PA4TIM
author_rank: Super Contributor
timestamp: October 17, 2012, 10:16:58 pm
content: Here you see the result on collecter current of a few uA more base current.Vbe is dependend on temperature and collector current , it increases, so if that changes, collector current does also, resulting in more heath, more current and a higher Vbe and so on. When using enough power it can result in thermal runaway.

author: Arion02
author_rank: Contributor
timestamp: October 19, 2012, 11:31:35 pm
content: Quote from: PA4TIM on October 17, 2012, 10:16:58 pmThank you for the info. What type of instrument is that in your picture?I will keep working on a better design.Sent from my Nexus 7 using Tapatalk 2

author: PA4TIM
author_rank: Super Contributor
timestamp: October 20, 2012, 07:55:50 am
content: That is a Tektronix 576 curve tracer. It is for testing tranistors, FETs, diodes, VDRs, zeners, capacitors and anything else you want to see its current vs voltage behauvior. You can even use it for testing tubes ( but not standard, you need an extra powersupply ( heather) and make some adapters)If you want to learn more about designing circuits, a very good book is "the art of electronics" from Horrowitch. This gives starts from scratch but goes to a rather high level. One of the the best books I have read.And with a lot of practical tips, analog troubleshooting from Bob Pease is also nice.