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I'm building a bench to calibrate a device that measures AC currents. As I won't build something accurate, the current will be measured with a current clamp and a multimeter (voltage mode).

For the current load, I first tried a diode bridge with big capacitors and after it, a DC current load (voltage / current converter).

The result is a non sinusoïdal voltage at the clamp output due to the diode voltage which leads to 0 current when the input voltage is less than twice the diode voltage. Like in this screenshot (orange = input voltage, yellow = clamp output (volt)).

enter image description here

The problem is that the multimeter does not integrate correctly the non sinusoïdal voltage of the clamp and outputs a wrong rms value. If I plug the multimeter in-line as an ampere meter, the measure is correct but I need the clamp because I will measure currents that are beyond what the multimeter can do

That's why I want to build a device that operates likes a rheostat. Its resistance should be defined by a microcontroler and the current vaweform should be proportionnal to the voltage waveform. Maximum dissipated power is around 60W (for the purpose of my specific design, I can add a serie resistor to drop the power in the device down to 20W) Input voltage: 6V 50Hz AC sinusoïdal.

How can I do?

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  • \$\begingroup\$ how about resistors? They behave like resistors! \$\endgroup\$ – Neil_UK Mar 6 '18 at 9:29
  • \$\begingroup\$ I would need an infinite number of them. User can choose the value. \$\endgroup\$ – Julien Mar 6 '18 at 9:48
  • \$\begingroup\$ Do you mean a potentiometer or a rheostat i.e. a programmable load? \$\endgroup\$ – Andy aka Mar 6 '18 at 9:51
  • \$\begingroup\$ I suppose what you want to do is actually an AC active load, basically a power resistor with user-settable value, correct? \$\endgroup\$ – peufeu Mar 6 '18 at 10:04
  • \$\begingroup\$ Julien. What is the real problem you are trying to solve? Why can you not use a triac controlled dimmer to vary the output voltage? Does it have to be a sinewave output? Please add the information into your question. \$\endgroup\$ – Transistor Mar 6 '18 at 10:11
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I will interpret this as an AC electronic/active load, basically a "resistor emulator" for AC. I'm thinking about several solutions:

  • Oldskool

Several relays, switching resistors of values in powers of 2 in parallel or in series. Basically a 4-bit number, and each bit controls a resistor. Say you have 4 relays, and resistors of 1,2,4,8 ohms.

If they're in series you can make any value between 1 and 15 ohms.

If they're in parallel, then the binary number controls the inverse of the resistor value, which is basically the current.

  • Simple PWm

In this case we would generate a PWM signal, switching a low value resistor in circuit with an AC switch (like a pair of back to back FETs).

Current is proportional to V/R multiplied by the duty cycle, which makes this an adjustable resistor.

Add a LC for smoothing...

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  • \$\begingroup\$ In fact, the old school version is already my backup version if I can't find anything better. What do you mean by "back to back fet"? I asked google and he tells me about babies and pregnancy... \$\endgroup\$ – Julien Mar 6 '18 at 10:53
  • \$\begingroup\$ check figure 3 on this link -- MOSFETs have body diodes so if you want to switch AC you need 2 FETs to be able to block both polarities. Driving the gates is annoying though, need a floating driver. \$\endgroup\$ – peufeu Mar 6 '18 at 11:01

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