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I need a simple circuit, one or two components at most, that will only allow a certain range of voltage to pass. I.e. If a voltage is below a min, I do not want it to pass. If it is above a max, I also do not want it to pass.

I only want a voltage to pass if it is within a certain range, not below or above.

I have researched this using Zeners, Op-Amps, etc, and just have not been able to figure it out. The hard part has been having only a range between a min and a max.

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    \$\begingroup\$ What do you mean when you say "don't want it to pass" ? Do you mean that the sought circuit should become an open circuit? Do you mean that it should clamp the voltage to min and max (i.e. the circuit outputs max when the voltage output is above max)? What range of voltages and currents do you have in mind? How fast does the voltage change? Last but not least, what are you trying to accomplish? What's the end goal? What's the application? \$\endgroup\$ Nov 3 '14 at 4:49
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    \$\begingroup\$ what is the reason for the one to two component limit ? \$\endgroup\$
    – efox29
    Nov 3 '14 at 5:10
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    \$\begingroup\$ What do you want to happen when the voltage "doesn't pass"? Do you want the circuit output to be low, high, high impedance, or what? \$\endgroup\$
    – The Photon
    Nov 3 '14 at 5:47
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    \$\begingroup\$ Define 'nothing to happen.' At the input you have some arbitrary voltage. At the output, you want the output to follow the input if the input falls within some range. If the input falls outside of the range, what do you want the output to do? Drop to zero? Get clamped to one end of the range? Who cares, but generate and out of range signal that can be used somewhere else in the circuit? What is this voltage driving? \$\endgroup\$ Nov 3 '14 at 8:38
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    \$\begingroup\$ The spec of "voltage not pass" makes no sense. That could mean a min/max function, switching to high impedance, or something else. You had opportunity to clarify and write a proper spec, but since you refuse, the only thing lef to do is to close this mess. Oh, and -1 for the handwaving, and then answering with more nonsense when asked to clarify. We do engineering here. Hand wavers need not apply. \$\endgroup\$ Nov 3 '14 at 13:47
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You can make something like the following, however its not within the two component limit.

It's a window comparator. It produces 11V-12V when the input voltage is within range of the upper and lower vrefs and produces a zero when its out of range. You can then use this output to drive some sort of switch to allow your signal to go through or not. Clearly this is not a 2 component solution. You can however make this count even smaller by buying a window comparator rather than making one.

If you have a relay that can be driven from an comparator/opamp directly then great. Otherwise, you will need a switching transistor in between the relay and the output of the comparator.

Whicever way you go, it doesnt look like you can do this with one or two components.

schematic

simulate this circuit – Schematic created using CircuitLab

Edit

As @EmFields pointed out (Thanks!), the way the schematic was set up would actually not work because the opamps would always be on. Now the output goes low when Vinput is within range, and goes high when its out of range.

Edit 2 See comments.

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  • \$\begingroup\$ Use the common type of comparator with open-collector outputs, and you can eliminate the two diodes. If you count a dual-comparator IC as one part, you're down to 2 components. (Add a freewheel diode across the relay coil, and you're back up to 3) \$\endgroup\$
    – The Photon
    Nov 3 '14 at 5:53
  • \$\begingroup\$ Could use a MOSFET instead of a relay perhaps? I like this solution because it scales well with changes in the required voltage thresholds and load output current. \$\endgroup\$
    – MarkU
    Nov 3 '14 at 6:02
  • \$\begingroup\$ You could run into pain using a single MOSFET in the pace of a relay (depends on the next stages requirements). Among other issues, if it is high current required your average MOSFET won't handle it. Also if it is variable current required -- MOSFETs are (kinda) fixed current sources (and in this case fixed to some unknown value cos Op Amp output connected to gate). But a different element could be used. \$\endgroup\$ Nov 3 '14 at 7:45
  • \$\begingroup\$ Perhaps a CMOS switch would be a better idea than a relay. Also, a relay could take a while to turn on or turn off, and during this time the signal could be out of the window. \$\endgroup\$ Nov 3 '14 at 8:31
  • \$\begingroup\$ Also, this could be more than 2 components - you have to get those reference voltages from somewhere. Could be a couple of voltage dividers, trim pots, DACs, etc. \$\endgroup\$ Nov 3 '14 at 8:36
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Use a pair of zener diodes as clamps - one from the power rail and one from the ground rail. If you use a 6 volt zener between the signal and ground, the signal will be clamped to 6 volts (less than or equal to 6 volts). If your power supply is 15 volts, then using a 15 - 5 = 10 volt zener between the signal and the power rail will clamp the signal at 5 volts (greater than or equal to 5 volts). This won't work if your power rail is less than a diode drop above 5 volts, however.

If separate voltage sources don't count against the component count limit, then you can just replace the zener diodes with regular diodes and provide two reference voltages, one being one diode drop higher than the high end of the window, and one being one diode drop lower than the low end of the window. The diodes will clamp the signal to the window. Many ICs have protection diodes like this connected to all of the pins to prevent the pin voltage from swinging outside of the supply voltage range. Current flows through the diode to the power rail if something tries to drive the pin across the rail.

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Not counting the passives, the schematic below gives a 2-component solution, and the LTspice files for the simulation are here.

enter image description here

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"The Photon" actually pointed me in the right direction, a Window Comparator from here: how to build a circuit that compares two potentials?

and here: http://forum.allaboutcircuits.com/threads/window-detector-circuit.26663/

Both are over the two component limit that I wanted to stay within, but I can work with it.

I thank all for the great responses and good info.

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