Constant current pulse input circuit

Thread Starter

Paul Speirs

Joined Jun 19, 2016
Hi All,

Looking for some advice on a pulse input circuit as per the image below. I don't fully understand the circuit, it was found as an example to someone else and fit my needs. It works fine if I apply a pulse to it anywhere from about 2V to 24V, however have found that when a standard fluid flow meter is connected such as the FS300A with a 5V supply the output pulse voltage is only around 1.3V, being pulled down by the circuit. Open circuit output voltage is about 4.8V. Current draw on the pulse in line is about 5.5mA


Below is a description of the circuit as supplied.

Using Pulse 1 Input as an example,
When voltage is applied, Q5 is turned on by R37
The current in R39 rises until Q3 is turned on when R39 voltage drops about 0.6V
Q3 now clamps the base of Q5 at a level that maintains its turn on voltage

I(Q5) = V / R = 0.6 / 100
= 6 mA approx

Can be changed to suit optocoupler requirements if needed

At 3V in, V_Q5_b = Vb1_Q3 + Vbe_Q5 ~= 1.2V
so V across R37 ~= 3Vin - 1.2V = 1.8V
Ib_Q5 = VR37 / R37 = 1.8 / 10000 ~= 0.02 mA

To support the required 6 mA in this case Beta_Q5 must be IC_Q5 / Ib_Q5 = 6 mA / 0.02 mA = 300.
R37 can be made smaller to increase I_R37 and so reduce the required beta of Q5

Using a ZXTN25100DFH for Q5, guaranteed beta @ 2V of 300 - 900, typically 450.

At Vin = 39V (D36 Vbr), dissapation in R37 = V^2 / R = 1521 / 10k = 0.15 W (250 mW would be suitable)

Q5 maximum dissapation ~= Iopto * Von = 6 mA * 39V = 234 mW
ZXTN25100DFH package thermal resistance of Rtja = 171 C/w
At 300 mW temperature trise is 171 * 0.234 = 40 deg C.


Joined Aug 7, 2020
It's a standard two-transistor current source, which fixes the current through the optoisolator to 0.6V/100Ω = 6mA
If your input voltage is fixed you can replace the whole lot with a single resistor.


Joined Jan 29, 2010
Hi Paul.
The data for sensor shows less than 10mA at 5V.

I will check the datasheet for the opto coupler.
EG57_ 1684.pngEG57_ 1685.png
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Joined Jan 29, 2010
Hi Paul,
Looking at the above parameters, have you tried driving the opto with a simple series resistor?
Say a 470R or 560R


Joined Jan 23, 2018
Consider that we have no information about the " a standard fluid flow meter is connected such as the FS300A ", other than I am guessing that it is a "Flowscan" company device. The circuit shown is a current regulator, not a constant current supply. The Flowscan sensors are intended to feed a higher resistance voltage input circuit.