Help understanding Darlington CB leakage current and its ram

[John Larkin]

On 22 Jul 2005 05:21:14 -0700, gerbermultit00l@yahoo.com wrote:

Even so, I would support John Larkin's suggestion that you use a >FET. No drain on the micro and none of these other problems. It is >a match made in heaven.

The original design used a FET. That's why the resistor values are the
way they are. The darlington was supposed to be a drop in replacement.
FETs for this purpose cost 5x as much as the Darlington in my circuit
now. Their are only a hand full of options for a N-Channel MOSFET with
a 100V breakdown rating in a SOT-23 package. The limited number of
competitive products on the market dictate higher prices. When you
manufacture 2 million+ products a year this sort of thing adds up.

Thanks

Ge0rge

What's the actual load voltage?

John

 

What's the actual load voltage?

Normal operation range 9VDC to 18VDC. Circuit must survive 100V
transient for 400mS. Also 24VDC for 30 minutes.

Ge0rge

 

[John Larkin]

On 22 Jul 2005 11:15:39 -0700, gerbermultit00l@yahoo.com wrote:

What's the actual load voltage?

Normal operation range 9VDC to 18VDC. Circuit must survive 100V
transient for 400mS. Also 24VDC for 30 minutes.

Ge0rge

Car?

John

 

[Rich Grise]

On Fri, 22 Jul 2005 11:15:39 -0700, gerbermultit00l wrote:

What's the actual load voltage?

Normal operation range 9VDC to 18VDC. Circuit must survive 100V
transient for 400mS. Also 24VDC for 30 minutes.

http://www.vishay.com/diodes/protection-tvs-esd/trans-zorb/

Pick about a 30V or 33V one of those and put it across your supply
leads, maybe with a hash choke or some ferrite beads.

Good Luck!
Rich

 

[John - KD5YI]

Rich Grise wrote:

On Fri, 22 Jul 2005 11:15:39 -0700, gerbermultit00l wrote:


What's the actual load voltage?

Normal operation range 9VDC to 18VDC. Circuit must survive 100V
transient for 400mS. Also 24VDC for 30 minutes.



http://www.vishay.com/diodes/protection-tvs-esd/trans-zorb/

Pick about a 30V or 33V one of those and put it across your supply
leads, maybe with a hash choke or some ferrite beads.

Good Luck!
Rich

Hmmmm. He must have some way to protect his low-level (micro) from
transients because he is only worried about the relay's switch. So, why not
put a 40V TVS from the FET drain (or darlington collectors) to ground?
Nothing would happen for the 24V case and 60V of the 100V, 400ms transient
would be dropped by the relay. The relay provides an impedance for the TVS
to work with. The relay might pull in temporarily. This way, he could use a
60V FET and a really low power TVS.

Just an wild thought.

John

 

[GerberMultit00l]

UInfortunately a trans-zorb will not work for me. My assembly requires
a direct battery feed (low resistance path to module). The device I
need would have to be large to absorb transient spikes on an
automobiles charging system without some means to limit current.

The relay coils add some current limiting but the transient suppressor
would still be too big. I'm working in very tight confines. I have a
hard enough time finding room for a 1210 package resistor. Don't want
to go to a through hole part. We haven't had a lot of luck with that.
The assembly cannot be wave soldered due to its physical layout. Hand
soldering is not that reliable.

Thanks,

George

 

[Ge0rge Marutz]

"Hmmmm. He must have some way to protect his low-level (micro) from
transients because he is only worried about the relay's switch."

Correct.

" So, why not
put a 40V TVS from the FET drain (or darlington collectors) to ground?
Nothing would happen for the 24V case and 60V of the 100V, 400ms
transient
would be dropped by the relay. The relay provides an impedance for the
TVS
to work with. The relay might pull in temporarily. This way, he could
use a
60V FET and a really low power TVS."

A few reasons I don't do this.

1.) I am switching high current loads (60A). I cannot allow the relay
contacts to close during such a high transient. It could tack the
relay contacts shut. I would no longer have control over the load.
This would not be good.

2.) The cost of two components and machine use to place them on the
board exceeds the small incremental cost to place a single higher
priced 100V part. On top of that, I have very little board space
available. I would be pressed to find room for two small SMA package
TVS's, more or less the SMC size that would be needed. IMHO Sometimes
its better to try and immunize yourself against transients by letting
them crash against a barrier vs. trying to absorb them.

Ge0rge