The components are sealed in an opaque package to prevent interference from external light. This provides a barrier to any voltage transients or overvoltage levels that appear at the input from affecting the electrical circuit at the output of the optoisolator. Then, within the same device, a light-sensitive device such as a photodiode, phototransistor, or photodarlington transistor converts the light signal back into an electrical signal. In simplest terms, an optoisolator works by taking an input electrical signal and converting it into a light signal using a light-emitting diode, generally operating in the near-infrared spectrum. Opto-isolators act as a protection mechanism, ensuring that harmful electrical currents cannot flow across the device. Because there is no need to directly pass voltage or current between the inputs and outputs in an optoisolator circuit, these components can be used to provide electrical isolation in two regions in a PCB. Anything up to about 10 kHz should work OK./ For faster PWM a properly designed driver is required.An optoisolator is an electronic device that can be used to pass information between a diode without passing an electrical current. Switching speed : As shown the circuit is suitable for on/off control or slow PWM. A high speed diode here may help slightly but is not essential. An RL204 is just one of a vast range of diodes that would suit. Any 2A or greater current rated diode will do. The BY229 diode shown is OK but is overkill. Without this the system will die almost instantly. gate protection against over voltage transients.ĭ1 MUST be included - this provides protection against the back emf spike which occurs when the motor is turned off. Set R2 = 2k2 gives ~10 mA drive.Īlso, add a 15V zener across R3, cathode to FET gate, Anode o ground, This provides. R2 sets the FET gate capacitor charge current. BUT here I will use 24 Vdc and use R2 + R3 to divided the supply voltage by 2 to limit Vgate to a safe value for the FET. Taking it to +12 Vdc would be good and +5Vdc would be OK if the logic gate FETs mentioned are used. R2 is shown gointo +24 Vdc but this is too high for the FET maximum gate rating. So, make R3 the same value as R2 - see next paragraph. BUT we wil use it here to make a voltage divider to reduce FET gate voltage when on. That's \$ \frac = 360 \Omega\$ - say 330Rīy itself 1K to 10k would be OK - Value affects turn off time but not too important for static drive. Voltage drop will be \$V = I \times R = 6V \times 0.18 \Omega =~ 1.1V\$. That is easily handled in a TO220 package with an adequate heatsink (somewhat better than a flag type preferably) but this much dissipation is totally unnecessary as much lower Rdson FETs are available. Power dissipation = \$ I^2 \times R\$ so at 6A the power loss will be \$(6A)^2 \times 0.18 \Omega =~ 6.5W\$. The latter is not too significant but is unnecessary.Ĭonsider - the data sheet says that the on resistance (Rdson - specified at top right on page 1) = \$0.18 \Omega\$. The main issues are that the FET has a very bad (= high) on resistance, which leads to high power dissipation and a reduced level of drive to the motor. It could be made to work if it was all you had but there are much much much more suitable FETs available, probably at little or no extra cost. Principally, that FET is only very very marginally suited to the task. There is a severe risk that the result will be a smoking ruin :-(. The suggested MOSFET is not well suited to this application.
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