![]() R1, R2 and C1 form a compensation network to ensure stability. The feedback loop of the Op Amp ensures that the voltage drop across the current sensing resistor is the same as the reference voltage coming from IC1A. The buffered voltage reference is then passed into the current sink formed by the second Op Amp and the MOSFET. But for now, the current is controlled by a 10K multi-turn trimpot. ![]() A benefit of using 5V is that this circuit can be easily controlled from an MCU. So I chose 5V as the voltage reference with a 0.5 Ohm current sensing resistor. In my case, the desired maximum current of this load is 10A. The reference voltage range along with the current-sensing resistor value (R3) determine the current sink capability. This reference voltage is then buffered by IC1A to provide the voltage reference to the Op Amp that drives the by-passing MOSFET. A simple Zener diode is used to generate a voltage reference at around 5V. ![]() The Op Amp I used is a general purpose LM358N. The circuit I am using is pretty standard, although it has been modified a little bit from what you saw in Dave’s video and I will explain a little bit here. So I thought why not use it to build a dummy load myself? The die-cast aluminum case is relatively thick, making it an excellent heat sink. I happened to have an old aluminum hard disk cooler case collecting dust. Reproduction in whole or in part in any form or medium without the.Basic constant current dummy load using an Op Amp and a by-passing MOSFET is very easy to build and had been made quite popular following Dave’s video on EEVBlog. Were doing this to protect your privacy and ensure you successfully receive your e-mail alerts. #IC1A OP AMP UPDATE#You may wish to update your subscriptions or try using another email. When the probes pin is pulsing, both LEDs alternately turn on and off at the pulse frequency. IC 1A s output is low, and IC 1B s output is high because its inverting input voltage is higher than 0.7V at its noninverting input voltage. IC 1B s inverting input voltage, which is the probe voltage, exceeds that of its 0.7V noninverting input voltage, and IC 1B s output is low. In CMOS mode, when measuring logic high, IC 1A s inverting input voltage, which is 90 percent of the supply voltage, is greater than the voltage at its noninverting input. When measuring logic low, IC 1A s 2.7V inverting input voltage is greater than the voltage at its noninverting input, which is the probe voltage. IC 1B s inverting input voltage, which is the probe voltage, is greater than its 0.7V noninverting input voltage. In TTL mode, when measuring logic high, IC 1A s 2.7V inverting input voltage is less than its noninverting input voltage, which is the probe voltage. ![]() IC 1B s 1.3V inverting input voltage is greater than its 0.7V noninverting input voltage. When the probe is in its high-impedance state in either CMOS or the TTL mode, IC 1A s inverting input voltage is greater than its 1.3V noninverting input voltage. In CMOS mode, the voltage at the inverting input of IC 1A is nearly 90 percent of the input voltage through voltage divider R 6 R 7. In TTL mode, the voltage at the inverting input of IC 1A is 2V. Voltage divider R 3 R 4 divides voltage from 2.7V zener diode D 1, providing 1.35V at IC 1A s noninverting input and IC 1B s inverting input. It uses 0.7V as logic low for both TTL and CMOS because their logic-low levels approach 0.7V. ![]() The circuit uses 90 percent of the power-supply voltage as CMOS-logic high and 2.7V as TTL high. ![]()
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