depletion mode transistor/mosfet part

[bonanz]

Hi all,
I've been googling around for a bit, and am having some trouble finding a suitable part for my needs. I need a transistor/mosfet that is "normally closed", or lets current flow usually, and will act as an open circuit if a TTL level signal is applied to the base. It also must be able to handle relatively high currents (about 3amps peak). Maybe I'm in the wrong direction totally with the transistor idea, but I basically need something in series in a power line that is normally closed, but that if a TTL signal is applied to it, it open circuits and shuts off the power, then once the signal ceases the current flows normally again. If anyone has any ideas/suggestions i would love to hear them.
Thanks,
Bonanz

 

[Sebi]

For AC power line You can purchase a solid state relay (SSR) with "normally closed" output stage. This SSR open the AC circuit when the TTL signal connected to input. But - if i understand correctly - You need hold the two output state without any input signal?

 

[bonanz]

I'm sorry let me clear it up, the source will be a regulated 5v or possibly 12v (not sure yet) from a power supply that can put out up to 6 amps but we'll only be having possible bursts up to 3 amps max. so its a DC "power line" but at any given time i would like to be able to open circuit it with a TTL signal from a controller and essentially shut off the power to a certain portion of the system. Normally it will be low, but if i want to cut the power it will output a 1 for as long as the power should be shutoff and hold it off. then when power is to resume it will return to 0 allowing current to flow and return power to the part of the system that was "shut off."
Thanks for your replies
Bonanz

 

[k7elp60]

A simple solution to your problem would be to use a PNP transistor.
Connect the emitter to the + line. Connect the base to the TTL output, and connect the collector to the load. As long as the TTL output is low the transistor will be on. When the TTL output goes high the transistor will turn off. I would use a heatsink on the transistor, and a pull-up resistor on the TTL output to the + 5V line. 1 kohm should work. This circuit will only work on TTL. But if you decided to use 12V as a power source you could easily use a opto-coupler driven by the TTL and the output to drive the PNP transistor.
Good Luck
Ned

 

[bonanz]

thanks a lot,
can you (or anyone) suggest a part number for a transistor that will handle that much current normally.

 

[Sebi]

O.K.understand. I recommend if You need a 6A DC power supply and want to build one, apply an SMPS IC with shutdown capability.

 

[Sebi]

del.

 

[k7elp60]

Transistor #

I would use a darlington type such as a TIP106 or NTE2344
Ned

 

[Sebi]

In this case no need darlington. For about 200mV voltage drop better this configuration.

 

[k7elp60]

Sebi, has a good point. If the control transistor(NPN and the emitter grounded) is from an optocoupler, then the circuit would work on TTL or even with a 12 power source and could still be controlled with a TTL signal.
:lol: Ned