Re: ssr's as analog switches
- From: Tony Williams <tonyw@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 02 Nov 2005 09:32:45 +0000 (GMT)
In article <dk53gq02jnk@xxxxxxxxxxxxxxx>,
Winfield Hill <Winfield_member@xxxxxxxxxxx> wrote:
> With what oscillation amplitude?
Just scratching around to find some gate charging
mechanism...... but this discussion has just given
me one of those 'oh sh*t' moments. See below.
> I'm curious, why did you need to use such a low Ron
> massive-die-area MOSFET switch?
To switch low value precision resistors onto the low
sides of 3x half-bridges.
10Vref---/\/\---+----+----+----+----+/+---+
1k | | | | Sw5a |
\ \ \ | |
All 0.1% or 1k/ 680/ 330/ | |
better \ \ \ | |
| | | | |
+ + + + |
Sw1/ Sw2/ Sw3/ Sw4/ |
+ + + + |
| | | | |
0Vref----------+----+----+----+ |
|
|
10Vref---/\/\---+----+----+----+----+/+---+
2nd identical switched h-bridge Sw5b |
0Vref----------+----+----+----+ |
|
|
10Vref---/\/\---+----+----+----+----+/+---+
3rd identical switched h-bridge Sw5c |
0Vref----------+----+----+----+ |
|
0v |
______|_______________________ |
| | |
|precision current-sink, 0-7mA |---<----+
| clamped at 0v. |
|______ _______________________|
|
-15v
The Sw1/2/3/4 sets are wired as dc switches
for lowest ON resistance. The Sw5a/b/c are
wired for ac operation because the polarity
across them could be either way. Switch
resistance doesn't matter when in series
with a constant current source anyway.
Now for that 'oh sh*t' moment.........
I'm now wondering whether those Sw5's did
actually get laid out for bipolar operation.
That would be a very plausible reason for why
the OFF Sw5's had to be biassed upwards.
Have to try and recover the pcb photographs
and have a look at them.
Thanks for that..... chatting over a prob
certainly forces your mind to focus on
possibilities.
K.I.S.S (on all four cheeks). :)
--
Tony Williams.
.
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