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What can be uses as an adjustable Zener?

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kinarfi

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Untitled.png This is a simple saw tooth wave form as part of a PWM design that I am working on. I can set the the max voltage with the pot, but the min voltage is a problem, in this SIM, I am using 8 1N916 diodes in series.
I can make an adjustable current limit out of a LM317 and a constant current out of a JFET, J310, but I don't know how to make an adjustable zener. Any one have any suggestions?
Thanks,
Jeff
 
Thank you Jony130 for that almost instant feed back, I've been trying it on Spice and it doesn't seem to work for this circuit, it would need to handle the ~50nsec, 150ma discharge spike of the capacitor. What does seem to work is the use of an LED between the cathode and ground + a diode or schottky for finer tuning.
 
Can't you just use ground as the minimum voltage of the saw-tooth?
 
LM317 is an adjustible voltage regulator
 
This is a simple saw tooth wave form as part of a PWM design that I am working on.
I've seen much much simpler sawtooth circuits, but then they may fall short of your design requirements. So, it would be worthwhile telling us your design requirements, and perhaps we can help come up with an acceptable circuit rather than concentrating on a single detail (the adjustable zener) that may ultimately be unnecessary.
 
I was mistakenly leery of using resistors in the cathode leg of a programmable unijunction oscillator circuit, fortunately, it works great which allows me to set the min and max of the sawtooth wave form which is one of the two inputs to a comparitor to establish the pulse width modulation I want.
The sawtooth wave is contstant and the other input is a relatively flat dc voltage, the J310 JFET acts as a constant current source to give me a straight ramp as opposed to just an RC curve for the rising part of the ramp. The dc input varies from ~ 5 to ~ 3 so the sawtooth needs to be adjustable to < 5 and > 3 to provide a PWM with a duty cycle of 0 to 100%. The end design looks like this:Untitled.png
I have used several other oscillators and this one seems to be easiest to adjust and have the most range of the oscillators I have tried.
Thanks for the help,
Jeff
 
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You can generate a voltage with a resistor/zener, a higher voltage than you need, stick this into a trimmer to get you a variable voltage, then use a transistor/resistor as a voltage clamp.
I've used this on reasonable power smps's with success.
 
How about this one: The peak value is exactly 2/3*Vcc, or in this example. 6.67V

64.gif
 
There are often situations where a very precise voltage reference in high volume with factory laser trimming to 5 significant figures. However variable Vref chips are dependant on DIY R ratio tolerances or worse yet a potentiometer. The fundamental building block is a bandgap reference diode and a rail to rail op amp with a ratiometric gain.

The cost of such devices is proportional to the cube of the number of significant figures for accuracy and stability in ppm/ deg C.

The number of applications is only limited by our imagination and experience.
 
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