Re: Hockey puck SCRs

On Wed, 12 Dec 2007 03:15:03 +0000, KW <n8023299@xxxxxxxxxxxxxxxxx> wrote:

I recently bought a hockey-puck SCR, without realising it needs to be
clamped to a heatsink. I'm trying to decide whether to return it or
whether to build a mount.

The SCR in question is an Internation Rectifier 250PA140, specs pasted at
the end of this topic.

First of all, I haven't managed to find any information about how much
force you need to clamp these things with. I'm assuming 2 metal plates
with 3x6mm bolts should be ample.

Secondly, do I need to apply force or displacement? For example, if I need
to apply exactly 1mm of displacement to the plates of the puck, then the
puck contracts by 0.1mm this might be a disaster, unless there is some
kind of springy contact inside that will take up the slack. Perhaps I
should use some kind of springy bar to apply a constant pressure despite
changes in temperature etc...

Thirdly, I presume that too much force will break the die, and too little
will mean the contact has insufficient area for the device to function
properly? How can I know when I've tightened the clamp the right amount?

Finally, I understand there is meant to be a snubber resistor between the
gate and the cathode. But I get very low resistances of < 0.1 ohm and I
wonder whether this implies it is broken?

If I(GT) Max. is 150m and the parallel resistance is < 0.1 ohms then as I
see it the voltage can't rise high enough to cause the device to trigger -
I'm assuming it needs at least a diode drop.

Thanks in advance for any help.


V(DRM) Max.(V)Rep.Pk.Off Volt.=1.4k
I(T) Max.(A) On-state Current=250?
@Temp. (?C) (Test Condition)=70
I(TSM) Max. (A)=4.2k
I(GT) Max. (A)=150m
V(GT) Max.(V)=2.5
I(H) Max.(A) Holding Current=500m
I(D) Max. (A) Leakage Current=11m
@Temp. (?C) (Test Condition)=125
V(T) Max. (V)=2.3
@I(T) (A) (Test Condition)=250
dv/dt Min. (V/us)=200
t(q) Typ. (s)=60u

My old GE manual specifies 700-900lbs force for their
press pak mounted SCRs. I mounted some where I didn't have
to be concerned about dissipating heat (single pulse operation)
and found an online calculator to determine clamping force resulting
from a given torque on a bolt then used a small torque wrench to
tighten it up. I don't remember the numbers, but it took surprisingly
little torque to get 800lbs clamping force. You might try googling
"presspack" to see what you come up with.

When truth is absent politics will fill the gap.