Minimum and Maximum Voltage Regulation

[corvese210]

Hello,

I have a voltage source that will be outputting voltages between -20V and +20V. I need a circuit that will regulate this voltage between +0.5V and around +14V. Anything below 0.5 must stay at 0.5 while anything higher than 14 must be kept at 14...all voltages between must be their actual values.

I know I can use a zener diode regulator circuit to keep it below 14V but I am not sure what to do with the minimum voltage value, espically once it goes negative. Any negative value must be held at 0.5V.

Any suggestions????

Thanks for your help!

 

[mneary]

How accurate do those 'clamp' voltages need to be? A 14V zener diode wired to limit at +14V will also hold negative voltages to about 0.65V (close enough to 0.5?).

Of course all of the voltages mentioned depend on tolerances and current levels.

 

[corvese210]

Thanks for your help...0.65V could do it but I am looking to get a little closer if it is possible with a fairly simple circuit...Is there a way to do it to get exactly 0.5V?

 

[ericgibbs]

Thanks for your help...0.65V could do it but I am looking to get a little closer if it is possible with a fairly simple circuit...Is there a way to do it to get exactly 0.5V?

hi,
The only problem with the zener is that the 0.65V will negative -0.65V.

How simple is 'fairly simple'.?

 

[corvese210]

I was thinking if it was possible with a voltage clamping circuit, but if their are other ways I am open to any suggestions...Thanks!

 

[ericgibbs]

I was thinking if it was possible with a voltage clamping circuit, but if their are other ways I am open to any suggestions...Thanks!

hi,
Can you give us more details of the project,
What is the source of the 'varying signal'and its form, what are are driving with this clamped signal,etc.

 

[MrAl]

(This has been deleted by author)

 

[corvese210]

I have two transducers measuring PSI (0-100) and outputting in volts (0-5), so each volt represents 20 psi. I have an AD620 (instrumentation amplifier) which takes each transducer voltage as an input, finds the difference, and scales it by 20....this way i have the difference of the two transducers with a 1:1 voltage to PSI scale. I only care about a difference between 0.5V and 14V so if the output of the amplifier is less than 0.5V I need it to stay at 0.5V, and if the output of the amplifier is greater than 14V I need it to clamp at 14....all values in between must be the actual voltage...this clamped signal will be driving a load with an impedance of 10Kohms...

Thanks for any help you could give!

 

[MrAl]

Hello,

I have a voltage source that will be outputting voltages between -20V and +20V. I need a circuit that will regulate this voltage between +0.5V and around +14V. Anything below 0.5 must stay at 0.5 while anything higher than 14 must be kept at 14...all voltages between must be their actual values.

I know I can use a zener diode regulator circuit to keep it below 14V but I am not sure what to do with the minimum voltage value, espically once it goes negative. Any negative value must be held at 0.5V.

Any suggestions????

Thanks for your help!

Hi there,


Assuming you dont need super accuracy and you want super simplicity, you can clamp
the voltage to +14v with a zener and resistor. For the +0.5v clamp you could use a
+1.2v power supply with a diode (anode connected to the +1.2v supply) and other
end to the zener and that will clamp the output to approximately +0.5v if it tries
to go negative or even below +0.5v.
Since you probably dont have a +1.2v supply sitting around you can use, you can
make one with a +5v supply and a resistor and two diodes. The two diodes get
connected in series and also in series with the resistor. The cathode of the end
diode goes to ground, the resistor to +5v. The voltage at the second diode anode
will be roughly 1.4v, not 1.2v, so that means the output will actually be clamped to
about 0.7v instead of 0.5v. If that's not good enough, then use three Schottky
diodes instead. Two in series with the resistor to construct a +1.0v supply, and
one diode connected to the upper diode and the zener. That will clamp close to
0.5v but of course it wont be perfect.

If you need really accurate clamp points we'd have to design a more precision
circuit...please advise. You should at least also tell us what output impedance
you need so we can determine if these kinds of solutions will even work at all
or you need a more robust output control.

 

[corvese210]

in a perfect world the output impedance should not exceed 100ohms...

 

[corvese210]

Any suggestions for this circuit?

 

[MrAl]

Hi,

Did my post make any sense to you?

 

[corvese210]

It made sense but I was wondering if the output impedance limitation had any effect on that particular type of design?

 

[MrAl]

Hi again,


Yes it does, are you saying that you want the output impedance to be 100 ohms or less then?

 

[corvese210]

Yes...also,

How accurate do those 'clamp' voltages need to be? A 14V zener diode wired to limit at +14V will also hold negative voltages to about 0.65V (close enough to 0.5?).

This limitation at 0.65 would be good enough...would this function as i need it if i were to wire it as that quote suggest?

 

[MikeMl]

Yes...also,

This limitation at 0.65 would be good enough...would this function as i need it if i were to wire it as that quote suggest?

How about this:

 

[corvese210]

thank you very much...i will try it out, if it works like the graph it should be perfect.

 

[corvese210]

So I implemented your circuit design and the concept is there but it is not clamping at the correct voltages...it is very close though. The upper limit is clamping around 13.1V in this circuit.

There is also more than a unity gain. I am inputting 1V into the circuit, which should give a 1V output, but instead I am measuring an output of 1.4V.

What resistors can I change to adjust the gain and the clamping voltages?

I also discovered that the output of the first stage of my circuit will not be higher than 14V...therefore I only need a circuit to keep negative voltages above 0.5V.

Is it easier just to implement this part?

 

[MikeMl]

What is your supply voltage? How well regulated is it? What type of opamp are you using?

The two clamping voltages V(H) and V(L) are derived off the supply voltage. If it is different than 15V, you will have to adjust the voltage divider consisting of R2, R3, and R4. Note that the final clamped voltage is one diode drop above V(H) and one diode drop below V(L). Measure V(DD), V(H) and V(L)

Since the opamp voltage follower buffer has to swing from ~0.5V to 14V while being operated from Vss=0V and Vdd=15V, the opamp must be rail-to-rail inputs and rail-to-rail outputs. This precludes many common opamps, or you may have to use a dual supply with both positive and negative voltages.

 

[corvese210]

I made the circuit using a dual supply +/-15V on the op-amp. My supply voltage is just coming from a lab power supply now to test the circuit. It will be coming from the output of a instrumentation amplifier though, which is why it will never be above 14V anyway, as I've discovered.

I will measure the resistance of my resistors because they could be a little off...