Can I use this configuration of diode to cut the voltage when above 5V?

Thread Starter

yaantey

Joined Oct 7, 2011
48
Hey,
Attached is an amplification circuit. I want to know whether I can use the configuration of the diodes to cut-off any voltage above 5V. The output from amplifier is connected to microp ADC. So can anyone let me know if I can use this configuration? If not can you direct me to a circuit I can use.
Thanks.
 

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SgtWookie

Joined Jul 17, 2007
22,220
D1 clamps the output at 5v + the Vf of D1, or ~5.7v.

If you want to clamp it at 5v, then you'll need to use something like a 4.7v or 5.1v Zener from ground to the output signal.
 

Thread Starter

yaantey

Joined Oct 7, 2011
48
I tested with with a 6.2V zener diode to clip off the voltage. I connected the zener diode in the configuration as shown in the link (http://www.electronics-tutorials.ws/diode/diode_7.html) under "single zener diode clipping" and used a function generator to input a AC signal and an oscilloscope to see the ouput. The voltage was cut-off at 7.25V. So, my question is can I use a lower rated zener diode in the same configuration to cut-off voltage at 5V?

Thanks
 

Thread Starter

yaantey

Joined Oct 7, 2011
48
One more question is, what does the power rating of zener diodes indicate? Which power rating zene diode would be good enough for me? Will 500mW do?
 

PaulEE

Joined Dec 23, 2011
423
One more question is, what does the power rating of zener diodes indicate? Which power rating zene diode would be good enough for me? Will 500mW do?
yaantey,

If you only need to limit the output at +5v, you do not need two diodes. Two diodes establish limits at +/-V, where "V" is a difference of V volts across the diode terminals. As drawn, you'd limit your output to +5.7v (approx.) one way, and the bottom diode wouldn't do anything because your circuit cannot generate an output voltage less than common (ground). If, however, your circuit had split supplies on the op-amps, THEN the bottom diode would limit the output to -0.7v.....

Basically, if you want to limit your output voltage to +5v (approx.), use a +5.1v zener diode as the bottom diode (cathode to output of amplifier, anode to ground), and remove the top one.
 

ErnieM

Joined Apr 24, 2011
8,056
Hey,
Attached is an amplification circuit. I want to know whether I can use the configuration of the diodes to cut-off any voltage above 5V. The output from amplifier is connected to microp ADC. So can anyone let me know if I can use this configuration? If not can you direct me to a circuit I can use.
Thanks.
Typically microcontrollers give you a little "wiggle room" above Vdd and below Vss. The exact amount depends on the diodes they place on the inputs just as you are doing in your circuit. Do check the spec for the part you are using to determine this voltage and that there are really diodes inside the micro where I say they are. Some newer devices eliminate these diodes on some pins. They are the traditional ESD protection diodes.

It is common to use the internal diodes for protection; you have a nice big 10K resistor to limit the current going thru these diodes. Should you wish to use external diodes use schottky types for their lower diode drop.

Don't use a zener, it will affect the accuracy below 5V. They leak you know, even below the knee.
 

mcasale

Joined Jul 18, 2011
210
Hey, here's a dumb idea: drive the op amps with a 5V supply!

If you need the outputs to go all the way up to the rail, there are others that will do that.

Just a thought.
 
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