Electronics > Projects, Designs, and Technical Stuff
I need a method to control an AC load
capt bullshot:
Virtually short the output of the sensors (or load them with a rather small resistor) and measure their short circuit current / voltage across the rather small resistor. This should rule out the internal circuitry, which I believe is just a higher value shunt resistor, diode bridge rectifier and smoothing cap that gives you a poor "peak detect" current measurement. If that peak detect circuitry can be ruled out this way, you can apply your own more appropriate filtering to deal with your current waveforms.
Zero999:
--- Quote from: jackbob on October 22, 2019, 08:14:43 am ---Thanks for the suggestion, however, I cant put a variac before the mains because each channel must act independently. The mains transformer has four 12V secondaries and each one needs to serve these variable loads but also needs the 12V available for other functions on the project.
To give a little more background so that it makes sense. The project is simulating a small electircal distribution grid and will also simulate protective relay schemes to respond to faults. Each 12V circuit will have the load resistors monitored at different points but will also have another fixed load resistor bank of very low impedance which needs the full 12V to create the necessary current magnitudes. The purpose of this resistor bank is to simulate faults and the current sensors will detect this spike in current and reconfigure the circuits to redistribute the load to isolate the fault.
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Presumably it's not possible to use four smaller transformers?
--- Quote from: jackbob on October 24, 2019, 01:18:06 am ---Now for my quick hand drawn schematic below, I built what I think is a MOSFET being used as a constant resistor. This circuit DOES work successfully and it appears to operate as a resistor. The amplitude of the AC current sinusoid can be adjusted without affecting the quality of the sinusoid (with the exception of some zero cross distortion due to the bridge). However, doing some research, I do have some concerns using the FET's as resistors. Turns out most FET's are designed to operate as switches (no surprise) but with this optimization, most cannot operate in the linear region for more than a few 10's of ms at most. It did work fine with an IRFP250N MOSFET when I tested it but that doesn't mean it will continue to work that way. The fets tend to form hotspots in the die and suffer from thermal runaway. There are FET's designed to work for this application, I'm looking at IXYS L2 line of FETs. I havent bought them yet as they are $10 each transistor.
So I have a few questions regarding this application of FET's.
Will using them in their linear region be any different with pulsed (full wave rectified) DC? They are still acting as a resistor but will be subject to a pulsed rectified sinusoid.
Doing some quick math using maximum power transfer theorem, the FET will dissipate the most power when it is equal to the load resistance (1.5 ohms) in this case the RMS current will be 4 amps so I expect to dissipate a maximum of 24W per FET worst case.
Realistically, could the IRFP250N's handle this reliably? How does Rigol use these transistors in their electronic loads for constant resistance mode if they are not designed to do so?
The last thing I want is a FET to blow, especially while demonstrating the project haha! So if I have to I will go for the IXYS L2's made for the application, but I do have a handful of IRFP250N's and IRFP150N's and wouldn't mind using them if I felt they were up to the job.
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Oh, I thought you were talking about some kind of voltage controlled resistor. Yes, that will work, but it relies on a potentiometer for controlling the resistance. Since you need a potentiometer anyway, why not simply replace your MOSFET circuit with a high powered variable resistor known as a rheostat? It will make it much simpler.
Yes, MOSFETs are designed to be used as switches, but can work quite well in the linear region. The parameter you're looking for is known as the safe operating area. It's true that pulsing vs DC makes a difference, but at these low voltages, I believe the average power dissipation is the limiting factor and will depend on the temperature and thermal coupling between the MOSFET's semiconductor junction and the ambient, i.e. the total thermal resistance of the die, package, any thermal insulator and heat sink.
JacobPilsen:
Another thread: Where can i find an adjustable AC (AC output!) power supply?
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