Switch question

[dogsofwar]

Is there a switch that will stay open when power is supplied to it, then close when the power is halted?

 

[kjennejohn]

If by 'switch' you mean a relay, then...

yes. Relays can be had with 'normally closed' contacts. This means with power off. Once power is applied, the contacts become open. Kill power and they become closed again.
You can also get relays with both kinds of contacts, one open, the other closed. Apply power, they change state. Kill power, they go back to their original state.

Hope this helped.
kenjj

 

[dknguyen]

FET Switches

I think you can get a NMOS-type MOSFET transistor to do this too. It is a 3 terminal solid-state device (it doesn't physically move to do what it does). The source(S) and drain(D) terminals go in series with whatever you want to switch on and off, but the voltage across them has to be above a certain level (it shouldn't be much of a problem, it depends on the transistor but its usually just 5V I think). The gate(G) is where you apply the on/off signal. If the G-to-S voltage is below a certain level (say 0V), it "shuts off" and if the voltage is above a certain level (say 5V) it turns out. It works more cleanly and quietly than a relay, but there is a very small leakage current when it is shut off. It also uses much less power than a relay to keep it turned on.

To get the switch behavior you want, I think you can use a pull-down resistor in the gate.
I am unsure how it will behave if power is connected to S & D but G is left floating...


D
|.....................................Power switch with other end connected to +V
|........................................|
|........................................|
-------G ---------------------|
|........................................|
|.....................................Pull down resistor with other end connected to 0V (or -V)
|
S

 

[Roff]

I think you can get a NMOS-type MOSFET transistor to do this too. It is a 3 terminal solid-state device (it doesn't physically move to do what it does). The source(S) and drain(D) terminals go in series with whatever you want to switch on and off, but the voltage across them has to be above a certain level (it shouldn't be much of a problem, it depends on the transistor but its usually just 5V I think). The gate(G) is where you apply the on/off signal. If the G-to-S voltage is below a certain level (say 0V), it "shuts off" and if the voltage is above a certain level (say 5V) it turns out. It works more cleanly and quietly than a relay, but there is a very small leakage current when it is shut off. It also uses much less power than a relay to keep it turned on.

To get the switch behavior you want, I think you can use a pull-down resistor in the gate.
I am unsure how it will behave if power is connected to S & D but G is left floating...


D
|.....................................Power switch with other end connected to +V
|........................................|
|........................................|
-------G ---------------------|
|........................................|
|.....................................Pull down resistor with other end connected to 0V (or -V)
|
S

Yeah, but that's the opposite of what he wants. Reread his post.

 

[dknguyen]

Hmm yeah. My mistake. You want a PMOS-transistor instead of NMOS to do that (and I think you have to flip some stuff upside down).

 

[Roff]

Hmm yeah. My mistake. You want a PMOS-transistor instead of NMOS to do that (and I think you have to flip some stuff upside down).

Normal enhancement mode MOSFETs (N or P) will not be on with no power applied. Depletion mode MOSFETs are on with no applied gate bias, but to turn them off, you have to apply a voltage that is the opposite polarity of the drain-to-source voltage. Therefore, two power supplies are required.
Depletion mode MOSFETs are pretty rare, and I don't know if you can even buy one that will handle power switching. JFETs are all depletion mode. I found one company - **broken link removed** - that makes power JFETs. There may be others. But, again, for DC switching, a negative supply is required to turn the transistor off. They have an app note showing how to drive them without a negative supply, but a minimum switching frequency must be specified.

 

[mstechca]

Is there a switch that will stay open when power is supplied to it, then close when the power is halted?

how about an inverted buffer (like the 74xx125).

data flows from input to output, provided that the latch controlling input is -ve. You might need a resistor to pull the logic down.

Now if what you mean by "Halted" is off, then I think a relay is the way to go.

but before thinking about the relay, determine what the switch is used for.
If you need it in a high speed application (oscillator, high speed data transfer, etc), then I would highly go against a relay, because relays are mechanical devices. Semiconductors (IC's transistors, etc) are more suitable for higher speed applications.