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Sonorous tester of short circuits. please, urgent!

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gothmog_es

New Member


Hi. Im from spain and i'm having trouble with this circuit. I have to make this circuit for a project and at the time of
mounting it (all is checked and is mounted ok), any of both transistors get
burned (maybe i got burned 4 or 5 of each class :cry:).

If you could facilitate some aid on the matter to me or, at least, say to me as they are the causes of which it does not work correctly.

Thanks for all and sorry by my poor english :roll:
 

kinjalgp

Active Member
I think the missing resistors at the base of the 2 transistors is causing the problem.
 

Gene

New Member
When IC1A outputs, both transistors turn 'on' at the same time. Seems like this creates a short circuit between +V1 and -V2 ? With a short circuit, excessive current flows. When that excessive current is inside your transistors . . .
 

isaacj

New Member
That was my first thought as well.
 

bogdanfirst

New Member
i think that it should be connected so that when the output is high you gen one transistor conducting and when it is low you get the other transistor conducting
 

gothmog_es

New Member
kinjalgp, missing resistors? Any idea of resistor's ohm value? Anyway i'll probe tomorrow.

bogdanfirst, im almost surely transistor configuration is ok because they are configured as astable (not sure about this translation, i hope which will be ok)

gene, if you are ok, im in very trouble because all the circuit will be wrong, isnt it? In this case, I know that I am abusing but, do you know some form to fix the circuit and get it work fine?

thank you very much for all :)
 

Gene

New Member
OK. Let's talk. I regret that I don't know what the circuit is supposed to be doing. If the idea is for one transistor to conduct when pin 1 goes + and the other to conduct when pin 1 goes -, then (assuming the IC is in one state or the other) there would always be conduction. But, when T2 turns on, there is no input available from T1 (because it is off).

You mentioned astable operation. Fine. But if you are trying to pulse the speaker for an audio tone, lets go to a multivibrator (2 transistors) or a 555 IC.

Maybe you could give us a description of what you are trying to do here and some of the components you are using.
 

gothmog_es

New Member
Utility that detects short circuits doing to know it in sonorous form.
This utility turn on when the resistance between the test
ends is of the order of the miliohmios. IC1A is connected like astable
multivibrator. R1 and C1 approximately fix their frequency of
oscillation to 1kHz, assumming that in IC exists a virtual mass
(potential of zero volts). T1 and T2 amplify the exhaust stream of
IC1A and inject a square signal of 20 volts tip to tip of amplitude to
R4. This causes that 100mA flows a current by resistor saying of +/-.
R5 assures that the tension in the end never is superior to +/-0,1V.
IC1B controls the tension in the test ends. When the ends are in
opened circuit, the IC1B gain equals to relation R4/R5, which causes
that the exit of both amplifiers is equal. This does that the
loudspeaker, as is connected to the two exits, does not sound. If, on
the contrary, the ends are in short circuit, the exit of IC2B becomes low, causing that the oscillations of IC1A
arrive at the loudspeaker taking place therefore the sound.

I hope you can understand it :?

Components are this:

Resistors

R1=R2=R7=100kW 1/8W
R3=10kW 1/8W
R4=100W 2W
R5=R8=1W 1/8W
R6=680W 1W

Capacitors
C1=22 nF disc
C2=100 mF 25V electrolytic

semiconductors

IC1=LM148
T1=2N3904
T2=2N3906
 

Phasor

Member
Translation: A continuity tester with audible output :lol:

The problem seems to be the value of R5 - 1 ohm. If a 20V signal is applied, when the probes are shorted, that equates to 20A current... That is WAY too much for a little 2N3904/3906! :p

The rating of the 3904/3906 is 200mA, and 40V Collecter-Emitter. Even without the probes shorted, this circuit runs too close to the limits. Also, when the NPN (3904) blows, the PNP is destroyed by a too-high base-emitter voltage.
 

gothmog_es

New Member
arg, online translators sucks :lol: thanks for translation phasor, i wonder you understand better now circuit function :)

Uhmm, you mean R5 value must be greater? How much? Maybe ok with transistors with more rating but same type? How could i look for its name?
 

Phasor

Member
For the transistors, I would suggest BD139/BD140 (I think they carry about 1A), and increase R4 to 1000 ohm, and R5 to 10 ohm. Also, maybe you can use a lower supply voltage.
:idea:
 

gothmog_es

New Member
sorry for answering too late, but the faculty occupies much of my time. I will try with the new transistors and new resistors and probing anything else. I'll tell you anything on monday.

bogdanfirst, i cant change the circuit now, because is for a project. Anyway, thanks for your help
 

gothmog_es

New Member
hi again, i've a new question. If i put a resistor on transistor's shared base, which value would you suggest?

Thanks again for all
 

kinjalgp

Active Member
The value of base resistor depends upon the type of transistor and your supply voltage. If you are operating transistor in saturation, any resistor in some safe range would do. I think in your case 4.7k resistor in the base of each transistor will be ok.
 

bogdanfirst

New Member
just a simple question, is it a buzzer of a speaker?
if it is a speaker with low resistance i dont think that you can hear anything
 

kinjalgp

Active Member
Why Bogdan? :roll: Why will it not sound with speaker?
If the oscillator frequency is set within speakers range then the speaker should work. Also transistors used are capable of driving low impedance load.
 

gothmog_es

New Member
Well, bad news (for me at least :cry:).


I tried putting a resistor in each transitor's base but the base-emitter voltage is the same :?: and got burned again. I tried changing transitors with bd1xx with dissipators and R4, R5 values. They didnt get burned but the speaker (it's a speaker, not a buzzer) didnt work. If i hold R4, R5 values, the same result. Reducing power supplies didnt work too.

Someone have said to me the problem maybe on capacitors, really? Which is exactly their function?

and what happens with R6, bogdanfirst? should I suppress it?
 

Gene

New Member
While enjoying the topic, I regret that I don't get it. Apparently the circuit is intended to make a sound when there is a short circuit. OK. A battery and a buzzer, right? On the other hand, this circuit seems to drive a pair of transistors when IC-A turns on. OK. But, when the IC is on, what have you got? The transistors do not make a multivibrator in this configuration. Even if they did, there is no driver/amp stage for the speaker. Seems like the most you could hope for would be a "click."

I do hope you get your project operational. If you remove the two troublesome transistors and short the circuit test leads, do you get a voltage at the IC-A output where there was none before? If you do, then that could be used to drive a sound generator (two transistors in an astable multivibrator - easy) BTW, did you design the circuit or did you find it in a book somewhere?
 
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