CLD/CRD requirements of 24 ma or greater.

[Jamie]

We have some commission devices that use a current loop in the input
and it appears that basic over current protection wasn't thought of in
the event the sensor decided to short or the lead wires got shorted.

The event hasn't taken place yet but I see a potential of it
happening, so i've been looking around for CLD/CRD's and it appears
the best I can find is 20 ma.

Even though the window is 4.20ma , I would like to have a little head
room above this. I know I can do this by other means how ever, I need
something that can be installed in line for the time being..

All I can find for current regulating diodes is ~ 20..23ma? does
any one have a part# that would be like around 30..100 ma ? that can
handle 24 volts or more.

Jamie

 

[Joerg]

We have some commission devices that use a current loop in the input
and it appears that basic over current protection wasn't thought of in
the event the sensor decided to short or the lead wires got shorted.

The event hasn't taken place yet but I see a potential of it
happening, so i've been looking around for CLD/CRD's and it appears
the best I can find is 20 ma.

Even though the window is 4.20ma , I would like to have a little head
room above this. I know I can do this by other means how ever, I need
something that can be installed in line for the time being..

All I can find for current regulating diodes is ~ 20..23ma? does
any one have a part# that would be like around 30..100 ma ? that can
handle 24 volts or more.

How about using a current source chip like this

http://www.national.com/ds/LM/LM134.pdf

or an adjustable voltage regulator wires as current limiter? It also
depends on your voltage compliance range and the max expected
dissipation. And mind stability issues.

Another option could be paralleling CLDs but those things are rare
beasts, not many sources.

 

[Jamie]

How about using a current source chip like this

http://www.national.com/ds/LM/LM134.pdf

or an adjustable voltage regulator wires as current limiter? It also
depends on your voltage compliance range and the max expected
dissipation. And mind stability issues.

Another option could be paralleling CLDs but those things are rare
beasts, not many sources.

You know, I never thought about pairing them. I think I actually have
enough room on the edge of the board to drop a couple of smt packages
there..

One of the other guys has a temporary fix that actually seems to be
comical in some ways. He places a 40 ma 24 volt panel mount incandescent
lamp in there. It acts as a short indicator and protection all at once..

But I don't want any more units to be distributed to our other
locations of operation before I fix this. I already had a bunch of
boards done so it's not like a simple fix with a couple of drag and
drops on the screen. But I can patch in a couple of smt CLDs until
those boards are used up.

We'll look into that.

Thanks

 

[Jamie]

Polyfuse?

John

THat was my first idea how ever, I think the response time maybe a
little to slow.

I have a 2.49 ohm load R so that I can a 10..50 mv translation and
that R isn't vary large. 1/8 W

The supply used for the sensor is a 15 watt unit and must remain that
size for other things so I need to come up with a basic limiting source
for the sensor input.

I was thinking about doing a JFET current limit that I could pot into
a discrete looking thing how ever, it looks a little rough when looking
around for JFETS that I can get my hands on that has Idss well above 20ma

This does not have to be a precision current limit but it does need
to have a small foot print to squeeze it in there.

Jamie

 

[John S]

We have some commission devices that use a current loop in the input and
it appears that basic over current protection wasn't thought of in the
event the sensor decided to short or the lead wires got shorted.

The event hasn't taken place yet but I see a potential of it happening,
so i've been looking around for CLD/CRD's and it appears
the best I can find is 20 ma.

Even though the window is 4.20ma , I would like to have a little head
room above this. I know I can do this by other means how ever, I need
something that can be installed in line for the time being..

All I can find for current regulating diodes is ~ 20..23ma? does
any one have a part# that would be like around 30..100 ma ? that can
handle 24 volts or more.

Jamie

Take a look at the J310 available from Mouser.

John S

 

[Jamie]

These are cool:

http://www.supertex.com/pdf/misc/d_mode_mosfets_SG_device.pdf

These are depletion-mode (normally on) mosfets. Pick one with Idss
higher than you need, and add a source resistor to bring it down.

A 3t voltage regulator (LM317, LM1117) and one resistor makes a nice
little current limiter, too.

John

The DN2535 looks to be the winner!

It looks like I can use that one "Idss of 150 ma" directly as is with
out an R.

And Thanks to Fred Bloggs on boosting the CLD ratings PDF.


Jamie