Re: clamper

On Mon, 04 Nov 2024 16:02:24 -0800, john larkin <jl@glen--canyon.com>
wrote:

I want a small cheap voltage clamper device. I'd like under 2 volts
drop at 1 mA, but minimal current at a few hundred millivolts.
>
I tried two small diodes in series, but that's terrible. Better is a
logic-level mosfet with gate connected to drain. It clamps nicely at
1.5 volts or so but conducts picoamps at a few hundred mV, over 1000:1
better than the diodes.
>
This will go between the force and sense leads of a 4-wire temperature
sensor thing so it automatically works in 2-wire or 4-wire mode.
>
The alternative is to use two SSRs and let the user explicitely
declare 2-w or 4-w mode.
>

Something like this?
Opamps  work  well as  voltage clamps. But your input  voltage must
suit  for the amp.

Version 4
SHEET 1 880 680
WIRE 16 -64 -80 -64
WIRE 208 -64 16 -64
WIRE 208 48 208 -64
WIRE 272 48 208 48
WIRE 272 64 272 48
WIRE 624 64 272 64
WIRE 368 128 96 128
WIRE 528 128 368 128
WIRE 368 144 368 128
WIRE 96 176 96 128
WIRE -288 192 -288 160
WIRE -192 192 -288 192
WIRE -80 192 -80 -64
WIRE -80 192 -112 192
WIRE 16 192 16 -64
WIRE 64 192 16 192
WIRE 208 208 128 208
WIRE 272 208 272 64
WIRE 320 208 272 208
WIRE -288 224 -288 192
WIRE 64 224 32 224
WIRE 32 288 32 224
WIRE 32 288 0 288
WIRE 320 304 320 288
WIRE 320 304 240 304
WIRE 32 320 32 288
WIRE 320 336 320 304
WIRE -288 352 -288 304
WIRE 240 352 240 304
WIRE 240 352 64 352
WIRE 240 368 240 352
WIRE 0 384 0 288
WIRE 624 416 624 64
WIRE 624 416 448 416
WIRE 672 416 624 416
WIRE 528 448 528 128
WIRE 96 464 96 240
WIRE 240 464 240 448
WIRE 240 464 96 464
WIRE 448 464 448 416
WIRE 496 464 448 464
WIRE 592 480 560 480
WIRE 672 480 672 416
WIRE 672 480 656 480
WIRE 736 480 672 480
WIRE 496 496 448 496
WIRE 0 528 0 464
WIRE 64 528 64 352
WIRE 64 528 0 528
WIRE 96 528 96 464
WIRE 240 528 240 464
WIRE 528 528 528 512
WIRE 528 528 240 528
WIRE 448 544 448 496
WIRE 736 624 736 560
WIRE 736 624 448 624
WIRE 448 656 448 624
FLAG 368 224 0
FLAG -288 352 0
FLAG 320 336 0
FLAG 448 656 0
SYMBOL voltage 368 128 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 15
SYMBOL Opamps\LM324-national 96 144 R0
WINDOW 3 -36 131 Left 2
SYMATTR InstName U1
SYMBOL voltage -288 208 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value SINE(0 8 100)
SYMBOL voltage 240 352 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 15
SYMBOL diode 272 192 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName D2
SYMBOL res -96 176 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 10k
SYMBOL voltage 0 368 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value 3.3
SYMBOL Opamps\LM324-national 528 416 R0
WINDOW 3 -36 131 Left 2
SYMATTR InstName U2
SYMBOL diode 592 496 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMBOL voltage 448 640 R180
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V5
SYMATTR Value 0
SYMBOL res 720 464 R0
SYMATTR InstName R2
SYMATTR Value 100K
TEXT -168 504 Left 2 !.tran 100m