Current Source mA for calibration
This is easy to rig millivolt source for field
calibration or troubleshooting of 4-20 mA current
loops. Here a Darlington pair is used for current
amplification which reduces the Ib error as gain is
A rotary switch selects, 4-12-20 mA Preset points. A
Bourns multi-turn wirewound Pot can also be used
with a digital dial. Enclose in a dust proof
handheld box. Read more on process
A mA source terminated with a 10 Ohm or 100 Ohm can
turn it into a mV Source. The Resistor should be
above 1W and has good temperature stability. Use MFR
1 % many in parallel to get a Instrument shunt if
you do not get any.
Voltage to Current Source 4-20 mA
to 4-20 mA Converter published earlier is a
current sink, Here is a circuit that is a voltage to
current converter but with a current source.
You can use a LM358 or LM324. The first opamp is a
Voltage to Current with a sink output. That current
creates a varying voltage w.r.t the 12V DC supply,
this varying voltage is mirrored by the second opamp
across the source output resistor. This way a
constant current is obtained with a sourcing output.
The control elements are small signal high gain
transistors. Any suitable equivalent can be used.
Even the opamp can be chosen by the precision and
application you want. In this form of feedback. way
to understand .... "Op-Amp drives the output to
maintain both inputs at the same level" and also the
"Output takes the polarity of the dominant input"
and lastly "dominant means, more positive". +5 is
more Dominant than +3 or 0 or -2. Then -3 is more
dominant than -12. See which is more positive.
Long distance of current loop may need higher
voltage and lower source resistor value. Then the
output transistor needs to change, if you use 24V DC
then that voltage should not reach opamp. Design
needs to foresee all possibilities of I/O troubles,
as these are wired by a customer, mistakes happen.
Hence, Industrial Designs have to be rugged.
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