Normaly closed relay with trasistor

[mazur50]

How can I make a normally closed relay with transistor. what I am getting at is I want to turn a circuit on when the power is taken away. I Have the Power to Power the transistors. I just want them to Close when the power is taken away from the base.
let me know if you have any ideas.

[AndyOne]

Look for a depletion mode MOSFET, they are pretty rare though.

Andy.

[rmteo]

Or use a double throw relay and connect your circuitry to the NC (normally closed) contacts.

[Gary Warner]

Or use a double throw relay and connect your circuitry to the NC (normally closed) contacts.
This gets my vote. Simple, cheap and effective.

[Stef]

If you need to control the relay with the transistor a simple inverter circuit will fill the bill remember to put a small diode across the coil to protect the transistor.

[mazur50]

What i am is looking for a transistor to act as a NC relay.

[rmteo]

In the context of what you are trying to do, a transistor is basically a switch that requires a voltage (actually current into the base) to turn it ON. So if you do not have a means to turn it on (no power) it will stay OFF.

[AndyOne]

Look for a depletion mode MOSFET, they are pretty rare though.

Andy.


A depletion mode MOSFET is always on until voltage is applied to the gate.

Andy.

[rmteo]

Sorry, I missed that. He said he was wanting to use a transistor.

[Odysis]

How much current is this thing going to draw in the 'closed' posistion? And how much are you willing to have drawn during the 'open' phase?

What I'm thinking is, as said earlier, an inverter. From the + line, whack a resistor then the collector, emitter onto the earth line. Another resistor goes on the base, through which the input signal runs. The output is taken off the joint between the resistor and transistor.

[Brandano]

It might be more interesting to know what is the final aim of the circuit. There might be simpler ways to achieve the same goal

[mazur50]

I have power to drive the Circuit i just want it to be triggered when there is no power.

[Fixion]

You could use a PNP transistor in a common-collector configuration. Connect the collector to ground, connect your load to the emitter and connect a shunt resistor from the transistors base to ground. This way, if there is no power to the base, the transistor is conducting, there-by grounding your load. If your input voltage to the transistor (at the base) is high, then the transistor cuts off the current flow, "disconnecting" the load.

What is your load? What kind of input signal are you expecting?

[VasMan]

Possibly a SCR (2N5060 or similar) or a MOSFET (2N7000 or similar) might do the job. They'll be closed while powered on, but will open when power is cut.

Vas

[Fixion]

Possibly a SCR (2N5060 or similar) or a MOSFET (2N7000 or similar) might do the job. They'll be closed while powered on, but will open when power is cut.

Vas

The SCR will remain closed (conducting) even after the input signal is cut, unless the load current drops below the holding current. But I guess you can also use two n-channel MOSFETs or two NPN transistors to do the job.

[westfw]

You can bias a transistor of essentially any variety with a resistor to the base/gate that is small enough to turn the transistor solidly on (using the power you have to drive the circuit), and then have your control circuit provide the opposite voltage via a smaller (or zero-ohm) resistor. When the control circuit goes unpowered or is disconnected, the bias resistor is unchallenged and turns the transistor on.

[oldcoot88]

How can I make a normally closed relay with transistor. what I am getting at is I want to turn a circuit on when the power is taken away. I Have the Power to Power the transistors. I just want them to Close when the power is taken away from the base.
let me know if you have any ideas.
If you use a double-throw relay you don't even need a transistor. The normally-closed contacts stay closed until the relay coil is energized.

oc88(Bill)

[oldcoot88]

WOOPs. Missed post # 3 :o