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| DIY electronic load troubleshooting |
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| bogdan2014:
Greetings, I followed Dave's video and built myself an electronic load. Main difference is the supply voltage, which is 12V, so I can use a standard MOSFET and have a bit more current capability. Problem is that I can only get about 5A through it (I should be able to get about 10A with a 1R resistor). The maximum gate voltage is 10V, which according to the datasheet is enough to drive the transistor so that it sinks a drain current of 11A. I suspect a fake transistor, but it should've blown up by now. I have attached the schematic below. |
| Dave:
The maximum output voltage of the opamp with respect to ground is 10V, but that's not what VGS will be with 5A flowing through a 1R resistor. Why did you use the buffer opamp? ??? |
| bogdan2014:
That's true, and I completely missed that. The actual gate voltage is 5V. So the first obvious solution is to lower the value of the resistor? I used both op-amps because it's a dual op-amp package, and one was unused. |
| duak:
Yes, you will have to reduce the value of the R5. After you reduce the value I think you will find that Q1 is undersized for 10 amps and load voltages much above 5 to 10 V. The IRF640 data sheet shows that it is possible, but it is difficult to achieve. The 1 ohm resistor is probably dissipating much of the power. This circuit will not work well at low load currents. The TL072 is not specified to operate reliably with its inputs within 3 V of Vcc-. You will have to provide a -3 V or greater negative supply voltage or use an opamp that can operate with its inputs at 0 V relative to Vcc-. The first opamp connected as a buffer is redundant. I would consider redesigning the circuit to use two IRF640 with their drain terminals connected together and driven by the two opamp sections with two sense resistors (R5). |
| bogdan2014:
I noticed that it goes down to around 10mA minimum. However, I'm not really interested in low currents, it's designed to test power supplies and their overcurrent capabilities. Any recommendation for a suitable op-amp? Sent from my POCO F1 using Tapatalk |
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