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.
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.
A red LED. I’ve measured some which leaked less than 50 fA at 20 C, from
-5 to +0.5 V.
I used them as switches for a pyroelectric array, with bias current
supplied by illuminating them faintly under processor control.
They were cheap CML parts in early transfer- molded board (TMB) SMT
packages, and are now discontinued.
Cheers
Phil Hobbs
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.
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.
On 5/11/2024 11:02 am, john larkin 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.
Use two diodes in series, with the junction between the diodes actively >driven by an op-amp follower to be the same voltage as the low-leakage
node. The op-amp can be a low bias current type, like LMC662 etc.
On Wed, 6 Nov 2024 00:32:48 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:
On 5/11/2024 11:02 am, john larkin 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.
Use two diodes in series, with the junction between the diodes actively
driven by an op-amp follower to be the same voltage as the low-leakage
node. The op-amp can be a low bias current type, like LMC662 etc.
https://www.dropbox.com/scl/fi/pj7fd43f787urqav1dewc/B450_Front_1.jpg?rlkey=69oh858vsod4efnoes1ya5uql&dl=0
Seems to me that poking any current into the junction of the upper
diode pair must make a current source error.
Maybe I'll just use two isolated switches and let the user select 2w
or 4w measurement mode. One ACPL227 should work, and I can use that
somewhere else too.
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.
On 6/11/2024 1:23 am, john larkin wrote:
On Wed, 6 Nov 2024 00:32:48 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:
On 5/11/2024 11:02 am, john larkin 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.
Use two diodes in series, with the junction between the diodes actively
driven by an op-amp follower to be the same voltage as the low-leakage
node. The op-amp can be a low bias current type, like LMC662 etc.
https://www.dropbox.com/scl/fi/pj7fd43f787urqav1dewc/B450_Front_1.jpg?rlkey=69oh858vsod4efnoes1ya5uql&dl=0
Seems to me that poking any current into the junction of the upper
diode pair must make a current source error.
Maybe I'll just use two isolated switches and let the user select 2w
or 4w measurement mode. One ACPL227 should work, and I can use that
somewhere else too.
You didn't tell me that *both* ends of your clamp needed to have low
leakage.
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