4-20 mA loops and hall current sensors
Archiver
Posts: 46,084
Hi Guys:
I have a fair amount of experience in both of these items so I will throw in
my 2 cents worth:
a. 4-20 mA loops: anytime something is connected or added into the loop, it
must be a series connection so the same current is passed through each
device on the loop from the transmitting element. Therefore, be very careful
about using voltage drop resistors (more on this later). Basically I size
the total loop resistance based on the 20 mA level and the supply voltage
used to power the loop (typically 24VDC in most industrial systems but with
everyone trying to reduce the amount of components in their circuits, the
supply voltages are actually getting lower and in some cases the maximum
resistance is only about 250 ohms!), with a little bit of buffer thrown in
to compensate for IR drops in any of the instrumentation wiring. In terms of
using voltage dropping resistors, ensure that they are not tied to ground
because of the series connection! This now implies that the voltage drop
resistor should be coupled into an ADC via an op amp set for unity gain (or
whatever is required) that is configured with a differential input. I have
these circuits available if anyone needs them and they are fairly simple and
have been used in robust industrial instrumentation now for over 25 years.
b. For a hall effect sensor, LEM makes some nice ones that will measure the
current and return a linear DC or AC signal that can be directly fed into an
ADC. All these sensors need is a power supply (usually 5 or 12 VDC and
depending on the module, these supplies may have to be +/- as well) and a
voltage dropping resistor so the increased cost is compensated by reduced
circuit components you would have to supply. LEM makes a wide range of these
units that range from PCB mounted units that will measure in the amp range
to units that are field mountable and capable of reading up to 5000A or
higher.
If anyone has any further questions, please feel free to e-mail me at your
convenience.
Regards,
Gerry Shand
gwshand@c...
(780) 471-0470
res@c...
(780) 922-2099
I have a fair amount of experience in both of these items so I will throw in
my 2 cents worth:
a. 4-20 mA loops: anytime something is connected or added into the loop, it
must be a series connection so the same current is passed through each
device on the loop from the transmitting element. Therefore, be very careful
about using voltage drop resistors (more on this later). Basically I size
the total loop resistance based on the 20 mA level and the supply voltage
used to power the loop (typically 24VDC in most industrial systems but with
everyone trying to reduce the amount of components in their circuits, the
supply voltages are actually getting lower and in some cases the maximum
resistance is only about 250 ohms!), with a little bit of buffer thrown in
to compensate for IR drops in any of the instrumentation wiring. In terms of
using voltage dropping resistors, ensure that they are not tied to ground
because of the series connection! This now implies that the voltage drop
resistor should be coupled into an ADC via an op amp set for unity gain (or
whatever is required) that is configured with a differential input. I have
these circuits available if anyone needs them and they are fairly simple and
have been used in robust industrial instrumentation now for over 25 years.
b. For a hall effect sensor, LEM makes some nice ones that will measure the
current and return a linear DC or AC signal that can be directly fed into an
ADC. All these sensors need is a power supply (usually 5 or 12 VDC and
depending on the module, these supplies may have to be +/- as well) and a
voltage dropping resistor so the increased cost is compensated by reduced
circuit components you would have to supply. LEM makes a wide range of these
units that range from PCB mounted units that will measure in the amp range
to units that are field mountable and capable of reading up to 5000A or
higher.
If anyone has any further questions, please feel free to e-mail me at your
convenience.
Regards,
Gerry Shand
gwshand@c...
(780) 471-0470
res@c...
(780) 922-2099
Comments
Nice post, I've been interested in current loop stuff since it seems to be
the way to go with sensors as far as noise reduction, false signals,etc.
Trouble is I don't know how to get sensor mV output translated into the
current loop and then get it back out again.
I would be interested your circuits, can you post them?
Thanks,
Mark
Original Message
From: Shand, Gerry W., Celanese/CA <gwshand@c...>
To: 'basicstamps@yahoogroups.com' <basicstamps@yahoogroups.com>
Date: Friday, February 16, 2001 6:26 AM
Subject: [noparse][[/noparse]basicstamps] RE: 4-20 mA loops and hall current sensors
:Hi Guys:
:
:I have a fair amount of experience in both of these items so I will throw
in
:my 2 cents worth:
:
:a. 4-20 mA loops: anytime something is connected or added into the loop, it
:must be a series connection so the same current is passed through each
:device on the loop from the transmitting element. Therefore, be very
careful
:about using voltage drop resistors (more on this later). Basically I size
:the total loop resistance based on the 20 mA level and the supply voltage
:used to power the loop (typically 24VDC in most industrial systems but with
:everyone trying to reduce the amount of components in their circuits, the
:supply voltages are actually getting lower and in some cases the maximum
:resistance is only about 250 ohms!), with a little bit of buffer thrown in
:to compensate for IR drops in any of the instrumentation wiring. In terms
of
:using voltage dropping resistors, ensure that they are not tied to ground
:because of the series connection! This now implies that the voltage drop
:resistor should be coupled into an ADC via an op amp set for unity gain (or
:whatever is required) that is configured with a differential input. I have
:these circuits available if anyone needs them and they are fairly simple
and
:have been used in robust industrial instrumentation now for over 25 years.
:
:b. For a hall effect sensor, LEM makes some nice ones that will measure the
:current and return a linear DC or AC signal that can be directly fed into
an
:ADC. All these sensors need is a power supply (usually 5 or 12 VDC and
:depending on the module, these supplies may have to be +/- as well) and a
:voltage dropping resistor so the increased cost is compensated by reduced
:circuit components you would have to supply. LEM makes a wide range of
these
:units that range from PCB mounted units that will measure in the amp range
:to units that are field mountable and capable of reading up to 5000A or
:higher.
:
:If anyone has any further questions, please feel free to e-mail me at your
:convenience.
:
:Regards,
:
:Gerry Shand
:gwshand@c...
[noparse]:([/noparse]780) 471-0470
:
:res@c...
[noparse]:([/noparse]780) 922-2099
:
:
:
:
: