Simple Low voltage cut-off with Zener and pnp transitor. I cant get it to work in real life.

Thread Starter

Brett Rietveld

Joined Jul 23, 2017
2
So at the Source of my circuit there is actually going to be a super capacitor, and my load is a bluetooth module. the bluetooth module runs fine from 5v to around 3 and half volts during the discharge of the capacitor. after that it goes into some sort of "protection mode" and wont turn on even if the voltage is 5 again, unless it's been either disconnected completely or the voltage in is less then 1.5.

So I tried making this circuit https://imgur.com/a/atTh8Py
In the above example i used circuitLab to test it and it worked how i was hoping. The zener would conduct through the base of pnp and allow power to my load only when it was above the zener's voltage. Keep in mind that I do not care for efficiency in this project, and as long as the voltage is mostly cut off from my load then i will be happy with the results.

I used an s8550 pnp transistor in the real life test here https://imgur.com/a/CY9LIrU
At the higher voltage of 5v it works as expected of course, but at lower voltage of 2.6 it is still conducting enough through the zener to keep the transistor active.

I'm not sure what else to try, but if anyone has any advice or ideas why this doesn't work in the real world, i would love to here them, because i am still very novice with electronics! Thanks!ZenerDiodeCircuit.PNG ZenerDiodeCircuit.PNG ZenerDiodeCircuit.PNG IMG_20190214_163815.jpg IMG_20190214_163815.jpg
 

Bordodynov

Joined May 20, 2015
2,389
Low-voltage zener has a very flat characteristic. Add a Base-Emitter resistor to the junction (for example, 1 kΩ). This will prevent the transistor from turning on at low voltages. The resistor may have to pick up.
 

crutschow

Joined Mar 14, 2008
23,314
Here's a circuit using the TLV431 which is a low-power version of the TL431.
Pot U2 allows adjustment of the trip point.
R4 adds positive feedback for hysteresis so it switches cleanly.

upload_2019-2-17_9-13-35.png
 

Thread Starter

Brett Rietveld

Joined Jul 23, 2017
2
I was trying out the ideas with the resistors, and was still inconstant results personally. I really all the knowledge you guys are giving me, i am going to order some tlv431s and try that out as well now
 

crutschow

Joined Mar 14, 2008
23,314
The TLV431 circuit has several advantages over the Zener/transistor circuit:
  • Has accurate/stable voltage reference
  • Trigger voltage is easily adjusted with a pot
  • Switching point is rapid and precise.
 
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