Continue to Site

# Passive circuit or component to eliminate low voltage

Status
Not open for further replies.

#### neazoi

Hello,
I would like to do something like a relay does, but using non-mechanical but still devices. The functionality I want is this:

When voltage is below a certain level, the output of the circuit/component must be zero volts.
When the voltage is above a certain level, the output of the circuit/component must be that level.

How can I accomplish this using passive non-mechanical devices?

A comparator (perhaps with a filter on its input, depending on your needs) driving a power mosfet ?

When voltage is below a certain level, the output of the circuit/component must be zero volts.
When the voltage is above a certain level, the output of the circuit/component must be that level.

How can I accomplish this using passive non-mechanical devices?

In simulation, use a Switch with a VCVS, something like this:
G_Switch n1 n2 VCVS PWL(1) c1 c2 0V,0V Vcl,V(n1,n2)
... where Vcl is your certain level voltage.

For explanation of the other Switch parameters, s. this snippet:

Last edited:

Hello,
I would like to do something like a relay does, but using non-mechanical but still devices. The functionality I want is this:

When voltage is below a certain level, the output of the circuit/component must be zero volts.
When the voltage is above a certain level, the output of the circuit/component must be that level.

How can I accomplish this using passive non-mechanical devices?
You can't do that with just "passive" parts (resistors, capacitors, inductors).
You need a solid-state relay.
What are the voltages and currents you want to switch?

neazoi

### neazoi

Points: 2
I’m a little curious to know the reason behind the constraint that imposes the use of passive components. Unfortunately the range of components that can be classified as passives are not so wide.

On the other hand, if you want to do that with a minimum amount of components, a possible solution - not so accurate – is to use a PNP BJT transistor in series having its base biasing defined by an input resistor in paralel so that Vbe*R matches the desired voltage.

neazoi

### neazoi

Points: 2
Place in series voltage barrier composed of zener diodes and a current through this barrier to a resistor will flow only at higher voltages. You can go as low as knee voltage of ordinary diodes.

neazoi

### neazoi

Points: 2

I am not willing to guess what you want to do with this gate. Rephrase your first post please.

I am not willing to guess what you want to do with this gate. Rephrase your first post please.
Yes you are right,
I am trying to see if I can find a way to minimize the fan-out problem. Because logic 0 is not 0v in this gate, but about 0.6v. If you connect this gate in series to another one, logic 0 would be 1.2v and so on.
So a way (if it exists) to limit the logic 0 to low voltage, but pass the full voltage at logic 1 is needed.
Obviously is has to be some ingenious simple circuit, not an amplifier of any kind. IF it can be done.

Do you have actually a problem or you are asking generally about diode-resistor gates?

Do you have actually a problem or you are asking generally about diode-resistor gates?

Well, this problem is known to this king of gate, I am trying to build one that eliminates this problem, for hobby.

Well, this problem is known to this king of gate, I am trying to build one that eliminates this problem, for hobby.
You can't without adding more active devices.

neazoi

### neazoi

Points: 2
You can't without adding more active devices.

I have done that using an output relay, but the relay IS a "saturated amplifier" (switch) and it's mechanical. I was thinking if it can be done with an ingenious combination of zeners.

I have done that using an output relay, but the relay IS a "saturated amplifier" (switch) and it's mechanical.
In other words, you already found out that an amplifier solves the problem.

Do you want us to prove why it's not only sufficient but also necessary?

In other words, you already found out that an amplifier solves the problem.

Do you want us to prove why it's not only sufficient but also necessary?

I believe you, not necessary

I think of a neon bulb as a device which is high impedance until you apply a certain voltage, then it goes to low impedance. It is Off until you apply 60V. Then it lights up suddenly, and its resistance drops. However once it lights, it stays lit below threshold voltage. So this is not the component you're looking for.

You also probably want to operate at lower volt levels. The next similar thing might be a unijunction transistor, or scr. However once they turn on, they stay on until voltage/current drops to zero, so they are unworkable.

neazoi

Points: 2