converting circuit from relay to something else ?

[exxos]

Hi all,

A simple switch uploaded here
http://www.aqtq39.dsl.pipex.com/temp2/switch.jpg

Bascially 100VDC unregulated supply rail. Drops 72V via a zener to power a
24V relay. Capacitor just to add some small smoothing to prevent the relay
from buzzing.

While the relay is turned ON, it connected the output to the 24V supply via
a 100K resistor.

When the relay turns OFF it shorts the output down to ground.

Sounds really simple, though I have been trying for the past 2 days to
convert this to a simple transistor circuit. To which after several attempts
given up so decided to ask here before I use the relay verson to see if
anyone can suggest a simple circuit which would do the same.

Chris

 

[Dave]

I would suggest an OPTOISOLATOR chip.
Dave

 

[exxos]

I would suggest an OPTOISOLATOR chip.
Dave

Been looking for a PNP version, will have another look, its the inverting
thing which is the problem...

Chris

 

[CWatters]

Been looking for a PNP version, will have another look, its the inverting
thing which is the problem...

No the tricky bit is getting a low impedence output when there is no power
on the circuit to turn on a regular transistor. Try a depletion mode FET.

 

[exxos]

No the tricky bit is getting a low impedence output when there is no power
on the circuit to turn on a regular transistor. Try a depletion mode FET.

Yeah low output impedence with no power is the fundimental problem, not
heard of a fet like that, just googled for them and doing a bit of reading
now

Cheers,
Chris

 

[exxos]

No the tricky bit is getting a low impedence output when there is no power
on the circuit to turn on a regular transistor. Try a depletion mode FET.

several pages latter I quote..

"A depletion-mode MOSFET conducts current without a gate bias. In an
n-channel device, a thin n-type region exists under the oxide in the absence
of an applied bias. It connects the source and drain allowing current flow.
In fact, a negative voltage is required to drive the electrons out of
(deplete) the region to increase channel resistance and reduce current
flow."

so basically its normally on though I found they actually have a on
resistance of 100ohms going by the ones ive found so far. All sounds good
until you read that it needs a negative to switch it, this reminds me of
thermal valves also, maybe if I fake the ground level by a volt or 2 then in
effect the gate voltage will be 2 volts under the source voltage... even so
the actual supply rail goes high so not sure how I can get negative switch
from a possitive rail.. oh the joy.....

Chris

 

[CWatters]

so basically its normally on though I found they actually have a on
resistance of 100ohms going by the ones ive found so far. All sounds good
until you read that it needs a negative to switch it,

I'm a bit rusty but I think you are referring to an N-Type device. Perhaps
look at at P-Type instead.

 

[CWatters]

I'm a bit rusty but I think you are referring to an N-Type device. Perhaps
look at at P-Type instead.

See also P-Type JFETs

 

[exxos]

I'm a bit rusty but I think you are referring to an N-Type device. Perhaps
look at at P-Type instead.

will have a look for some p devices,

cheers,
chris