# Kill off voltage below a certain level

Hi I'm looking for a component that does the following: if the voltage is below 1.5 V, then you need to make it 0 V. I'm looking for a high pass filter but with a certain voltage value. Does this exist?

Yes, I want to keep everything above 1.5 as is, everything below 1.5 V to be 0. I'm working with the power pin on a USB connection. I want to cut off all power coming from the USB. Currently, I'm issuing a command in the terminal which changes the USB power to 1.5 V through a USB hub, but I need that 1.5 V to go to 0 V.

• So you're trying to shift a signal down 1.5V? – Matt Young Aug 3 '13 at 0:18
• It sounds like he wants voltages above 1.5V to be as-is, and voltages below 1.5V to be clamped to 0V. – JYelton Aug 3 '13 at 0:19
• Is this a power supply or a signal? – Dave Tweed Aug 3 '13 at 0:27
• Would a simple comparator (comparing against a 1.5V reference) driving a relay work for this? – us2012 Aug 3 '13 at 1:12

Took me a while to realize what you mean by "High Pass Filter". When engineers say HPF, they usually refer to frequency domain, therefore HPF in context of your question is confusing.

Now, what you want to do is to provide a low impedance path to the ground when the voltage on some USB pin gets below 1.5V, right? I'm sure that there are many ways to do this, but one which is the most obvious for me:

simulate this circuit – Schematic created using CircuitLab

You will have to find appropriate transistors based on your application (currents, voltages, speed).

When $V_{in} \geq 1.5V$ the comparator outputs low voltage:

• NMOS is closed (effectively behaving as an open circuit)
• PMOS is open (effectively behaving as a short circuit)

When $V_{in} < 1.5V$ the comparator outputs high voltage:

• NMOS is open (short circuit). The voltage of $V_{out}$ will be pulled to ground.
• PMOS is closed (open circuit). This will ensure that $V_{in}$ itself is not shorted to ground.

NOTE: I assumed here some generic comparator which can output both high and low voltages. I can't suggest any specific part without additional info, therefore it's up to you to find an appropriate one. However, as Johnny B Good indicated in his comment, if you want to use an IC comparator (one chip solution), you need to make sure that it can switch rail-to-rail. This is essential because many comparators employ "open collector" output which goes "high impedance" instead of "high voltage" (these can be made a rail-to-rail by adding additional external circuitry). This is not an issue with comparators built around generic op-amps.

• Just in case OP wires this circuit and wonders why doesn't it work: An important thing to note here is the output circuitry of the comparator. Most cheap comparators come with an open collector output. So, they can either have a GND or a floating output. – Jonny B Good Aug 3 '13 at 8:02
• @Jonny B Good, thx for your comment. I assumed a generic comparator that does the job (either IC or built around op-amp or something else), but it is essential to warn OP against the basic pitfalls. – Vasiliy Aug 3 '13 at 8:15
• An open collector output on a comparator doesn't mean it's "cheap", and a simple solution (in fact, the intended solution, and what makes open-collector outputs valuable, not cheap) is to put a pull-up resistor on the output. – Phil Frost Aug 3 '13 at 11:56
• @PhilFrost: Sure, an OC output doesn't mean the comparator is cheap - but the fact remains that the vast majority of jellybean and off the shelf comparators is OC. Just like a square is a rectangle but not every rectangle is a square. – Jonny B Good Aug 3 '13 at 12:20
• @PhilFros, what do you mean by "body diode of M3"? Are you referring to Source-to-Bulk diode? – Vasiliy Aug 3 '13 at 12:25