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My controller outputs square waveforms in 5 volts PWM SIGNAL. I want to change the waveforms into current waveforms with some circuits.

nazmul0087

Mar 29, 2024
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My controller outputs square waveforms in 5 volts PWM SIGNAL.
I want to change the waveforms into current waveforms with some circuits.
The final output could be 10 milliampere square current waveforms, and the amplitudes will not be changed with different loads. Can any one suggest me a circuit for this?
 

Alec_t

Jul 7, 2015
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Welcome to maker.pro!
Constant-current circuits are well-known.
What are your different loads?
What is the PWM frequency?
Is your "10 milliampere square current waveform" one which goes from 10mA to 0mA, or from +10mA to -10mA?
 

nazmul0087

Mar 29, 2024
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Welcome to maker.pro!
Constant-current circuits are well-known.
What are your different loads?
What is the PWM frequency?
Is your "10 milliampere square current waveform" one which goes from 10mA to 0mA, or from +10mA to -10mA?
PWM Frequency: 0.487KHz, Current goes from 10mA to 0 mA, Would be grateful if you can suggest some solution.
 

bertus

Moderator
Nov 8, 2019
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Hello,

A constant current source will give different voltages with different loads.

Bertus
 

danadak

Feb 19, 2021
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A simple method is to use a low pass filter on PWM output producing a V proportional
to PWM duty cycle, and feed that to a V to I converter. There is a latency associated
with this approach, how much time from a step change from 0 to 10 mA can you live
with ?



Output of filter then goes to V to I converter :

1711761247297.png

Note the above circuit needs a well regulated accurate Vcc so only variation in current is due to
sense R and Vref (Vinoput). Use a better opamp than a 741, a rail to rail type would work fine.\

There are other architectures, sinks, transistor-less versions (Howland).....search the web.


Regards, Dana.
 

nazmul0087

Mar 29, 2024
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For load 500 the current I am getting is 10mA. If i change the load, the output should be 10mA as well. Any solution for that please?
 

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danadak

Feb 19, 2021
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What is the OpAmp supply V value ? Desired range of input V to OpAmp ?

Keep in mind Iload = Vinopamp / RS, so 10 mA thru .25 Ohms says Vin to
opamp has to be 2.5 mV which the OpAmp has 2 mV typical of offset.

If you want 0 - 5V in = 0 - 10 mA load then RS must be ~500 ohms.

1711839535631.png


The above sims (Vin 5V ) 10 mA for R1 1 to 500 ohms. Note supply has to be >= 12 V
to insure Q1 does not go into saturation and out of current regulation.

Regards, Dana.
 
Last edited:

danadak

Feb 19, 2021
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Here is the load R swept from 1 ohm to 500, with Vin @ 5V :

1711885984689.png

Current only changed by 35 uA.


Regards, Dana.
 
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