4 ms·

A current-controlled power supply will often tell you the current-usage. Obviously, it depends on your power supply, but even a $50 one these days has this feat

A current-controlled power supply will often tell you the current-usage. Obviously, it depends on your power supply, but even a $50 one these days has this feature. If you're off of battery power or USB power instead, you'll need to build a current-sense circuit. Analog wise, its very simple: a small 0.01 ohm resistor (called a "current sense resistor") is placed in series, and you measure the voltage drop across it. 0.0001 V across a 0.01 ohm resistor -> V / R -> 0.0001 / 0.01 == .01 Amps == 10 mA being drawn. Now the question is "how do I measure 0.0001V accurately" ?? And that's a trivial op-amp problem. So trivial, that people sell cheap chips that already do this for you, called current-sensing amplifiers. They just multiply the voltage by 200 accurately. (so 0.0001V into the amplifier comes out as 0.02V), and 0.02V can be measured by pretty much anything. https://www.digikey.com/en/products/detail/ohmite/MCS1632R010FER/2003858 https://www.digikey.com/en/products/detail/ohmite/MCS1632R01... https://www.digikey.com/en/products/detail/onsemi/NCS199A3RSQT2G/9169780 https://www.digikey.com/en/products/detail/onsemi/NCS199A3RS... Voltage-multipliers though are a trivially easy OpAmp circuit, and I'd recommend that beginners actually try to build their own current-sense amplifiers from scratch. Its good to learn how to use OpAmps, and learn all the issues (ex: Rail to Rail, positive / negative voltages, etc. etc.). I'd recommend you do a 9V project with the LM358.

amelius 4y ago
I think you might want to wire that opamp as an integrator, in this case.
- dragontamer 4y ago
  An integrator should be in the circuit "somewhere", but there's a lot of creativity available where I left off. Staying in the analog world alone, you can output the voltage into a 555 Timer, and then have it flash an LED every time the capacitor filled up (and the discharge would also dump the capacitor). For example. Or you could just wire it up to a microcontroller and busy-loop ADC-convert the input. So you can perform the "integral / summation" in microcontroller / in C-code rather than circuit magic. The important tidbit is that we've turned 10mA current into a value we can "touch" and use. Hell, maybe the easiest solution is to just hook up a wire somewhere and shove an oscilloscope there to finish things off, depending on how lazy you wanna get. ------- I know one professional chip did Opamp Integral -> Comparator (if integral is full, dump the charge to ground. If +V, do register++. If integral was full of negative voltage, do register--). Then, whenever the register overflowed, it'd generate an interrupt edge on its output (for both overflow or underflow). Alas, I forgot which chip did this. But this way, you could build all your stuff out of 100kOhm resistors, 10nF capacitors, tiny digital logic circuits with micro-amps of power usage that wakes up your microcontroller out of sleep for just a "Current_Reading++" statement on overflow and you go back to sleep. (or current_reading-- on underflow) But that's all advanced stuff that probably doesn't matter for a student making their first practical OpAmp circuit :-)

amelius 4y ago

I think you might want to wire that opamp as an integrator, in this case.

dragontamer 4y ago
An integrator should be in the circuit "somewhere", but there's a lot of creativity available where I left off. Staying in the analog world alone, you can output the voltage into a 555 Timer, and then have it flash an LED every time the capacitor filled up (and the discharge would also dump the capacitor). For example. Or you could just wire it up to a microcontroller and busy-loop ADC-convert the input. So you can perform the "integral / summation" in microcontroller / in C-code rather than circuit magic. The important tidbit is that we've turned 10mA current into a value we can "touch" and use. Hell, maybe the easiest solution is to just hook up a wire somewhere and shove an oscilloscope there to finish things off, depending on how lazy you wanna get. ------- I know one professional chip did Opamp Integral -> Comparator (if integral is full, dump the charge to ground. If +V, do register++. If integral was full of negative voltage, do register--). Then, whenever the register overflowed, it'd generate an interrupt edge on its output (for both overflow or underflow). Alas, I forgot which chip did this. But this way, you could build all your stuff out of 100kOhm resistors, 10nF capacitors, tiny digital logic circuits with micro-amps of power usage that wakes up your microcontroller out of sleep for just a "Current_Reading++" statement on overflow and you go back to sleep. (or current_reading-- on underflow) But that's all advanced stuff that probably doesn't matter for a student making their first practical OpAmp circuit :-)