Hi All,

This is my fisrt post, glad to join your group.

I have built a circuit to sense an input voltage threshold based on a PNP transistor + zener as seen below:

https://www.circuitlab.com/circuit/b34595/transistor-zener-10v-voltage-sensor/

the sensing level is given by:

Vs = [(R2+R1)/R2] *Vz + Vbe (SAT)

whereas the output is:

LOW : Vout = Vin - Vz - Vce(off) = Ico*R3
HIGH: Vout = Vin -Vce(sat) = Ic*R3

The output of the circuit is limited to ~5.1V by using a resistor-zener (5.1V) combination. Therefore the circuit produces ~5.1V (HIGH) and ~ 0.2V (LOW) according to the sensing level. Vin can be anything from 0v -> 40V.

Although the circuit works as expected , I noticed that for input voltages >20V resistors R3, R4 were getting hot.. I used 1/4W Resistors which is probably the case as I hadnt thought about the power dissipation..

So I need some guidance regarding the power dissipation on the resistors as I am not sure: According to my calculations, R3 max dissipation is 1.6W (max Ic 40mA), whereas changing R4 to 200Ω will dissipate 7W. R1, and R2 will stay 1/4W.

Any ideas/help will be appreciated.

George

ps: had to limit the header to 20 chars..

 

Why so high an input voltage? Welcome, by the way! The learning curve is much easier if we do things with 9V batteries etc, to keep the pesky power ratings out of the way...

I find it annoying when I try to operate circuits either above 14 or so volts, and down to maybe 3 volts. I say over 14 or so volts, because then your resistors do begin to dissipate quite a bit more than you may think! A little smoke is a nice learning thing, tho, ha ha.

You should check the max rating of the zener....I think it's like 1W, and I bet you are close (higher V in could fry you). And the transistor may not have high enough rating to handle the current you may pass thru it.

You may try getting LT Spice - it's free (you'll find the Linear Technologies site via any search engine) - and learn to simulate these circuits, so right away you can see what currents/power you have going thru circuit legs. The easiest thing to do (for me, anyway!) is to get something up and running, it's great, and then you overload it because it wasn't clear how much dissipation would be happening, he he.

Good luck!

 

Either do not use that much voltage or buy larger resistors.
If you decide to use larger resistors, be aware of the zener diode and the transistor over heating.

 

This is supposed to be a kind of level translator circuit ( 0 ->40 input ( <5V Low, >10V High) to 0V , 5V ). The 0->40 input Voltage is according to requirements..Zeners are indeed 1W whereas the transistor can handle up to 600mA of current..I also simulated the circuit with CircuitLab (online tool), I just needed a second opinion from experts like you guys as I havent been involved with analogue electronics for almost a decade..I work in embedded SW..So I guess I should use higher Watt resistors as you suggested for the output! I will try LT spice as well, thanks for the suggestion!

 

One more idea: increase the resistance of R3 and R4 to lower the current flow. If they were both 10,000 ohms, this problem with heat would disappear.

 

Correct..dont know why i didnt do that from the beginning...

 

Increasing the resistance is so obvious that I assumed you could not allow that or you would have already tried it. My mistake.