i need to switch the transitor (Vce minimal) when input voltage (0 - 18V) is greater than 6.3V. To achieve sharp edge I use a zener diode D1. However, I also need a protect transistor base, so I use a protecting diode D2.

I run into the problem, when D2 needs a high current, but i cannot load the input voltage devider in that way due to destroying the sharp edge so I use a 4.7K.

How to solve the clamping diode D2?

enter image description here


The characteristic should be sharp at least like this:

enter image description here

I tried to change R2 from 4K7 to 1K (to maintain D2 zener current)and it seems the sharpness stay there.

Any idea how to solve this circuit better?


To make it clarify, it will be used in circuit below. It is current limiter with, let say Mosfet protection. In case The Vds is higher than 5V, the mosfet will turn on completely, so it acts like fuse.

Due to this no need to precise comparator (4.5 or 5.5V doesnt matter).

If someone see improvements in schematic, please correct me.

enter image description here

  • \$\begingroup\$ Please explain how you selected the resistor values that you use here. How exactly is D2 protecting Q1? What do you mean by "sharp edge"? What is the desired voltage range for switching? \$\endgroup\$ – Elliot Alderson Jan 9 at 20:33
  • \$\begingroup\$ If your 5V supply is steady and accurate enough you could use a comparator IC like the LM393. Why are you not taking this route? \$\endgroup\$ – TimWescott Jan 9 at 20:54
  • \$\begingroup\$ to answer this question you must specify the threshold tolerances and hysteresis otherwise not possible , then this design may or may never work !!! \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 9 at 20:59
  • \$\begingroup\$ Its use this in circuit, where the tolerance isnt critical. I tried this circuit with LED diode in collector, and it changes from blind to shine as the input changes from 6.4 to 6.8V. Lets say this satified me. The hysteresis is so low, so i dont care about. \$\endgroup\$ – Michal Podmanický Jan 9 at 21:08
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    \$\begingroup\$ So what exactly is the problem? It sounds like your circuit is working. What do you mean by "solve the clamping diode"? \$\endgroup\$ – Elliot Alderson Jan 9 at 21:25

You can improve the sharpness by just moving the parts around a bit. The base current doesn't exceed about 30mA in the below circuit, which is acceptable.


simulate this circuit – Schematic created using CircuitLab

enter image description here

But really, as @Tim suggests, you'd be better off using a comparator and some kind of reference. For one thing, you could easily get tolerance of 1% or better, whereas the zener alone has +/- 5% tolerance.

  • \$\begingroup\$ Thank you. According to precision issue, I clarified it in edits. \$\endgroup\$ – Michal Podmanický Jan 10 at 22:44

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