Electronics > Beginners
555 Motor Speed Controller
mike_mike:
Can you please explain why on the oscilloscope, the Vpp is about 35V, while the supply voltage of the circuit is about 12V (please have a look at the latest screenshots) ?
Bulb:
--- Quote from: mike_mike on April 22, 2019, 08:44:54 pm ---Can you please explain why on the oscilloscope, the Vpp is about 35V, while the supply voltage of the circuit is about 12V (please have a look at the latest screenshots) ?
--- End quote ---
With 10 ohm resistor in gate path, switching speed of MoSFET is very high (under 1 microsecond). What that means? That means that mosfet will shut down under 1uS and current through load should stop. With RL load that isnt possible because L. This why is parallel connected diode with RL load. When mosfet dont conduct, energy stored in the inductor is transfered via diode back to load. Beacuse diode cant start conduct imidiatly there is voltage spike on load. Decreasing switching speed by increasing value of mosfet gate resistor, you will reduce it. To sum, spike, or Vpp is ove 12V (power supply voltage) because high switching speed, inductive load and nonideal diode.
This why (beacuase L) there is undershoot (negative spike). So oscilloscope make difference between max and min peak voltgae value and display it.
mike_mike:
I used the attached schematic and I tried to reduce the frequency by increasing the 100pF capacitor to 470pF.
I made the tests with R7 and without R7. I used as load a 27R/5W resistor.
The results are:
1. With R7=1k and C=470pF
a. pin 3 of 555: 0688.jpg
b. E of the PNP and NPN transistors: 0689.jpg
c. G of mosfet: 0690.jpg
d. on Rload: 0691.jpg
a,b,c were measured with the crocodile clip on GND.
2. Without R7 and C=470pF
a. pin 3 of 555: 0692.jpg
b. E of the PNP and NPN transistors: 0693.jpg
c. G of mosfet: 0694.jpg
d. on Rload: 0695.jpg
a,b,c were measured with the crocodile clip on GND.
1. Please have a look at the screenshots and tell me what you think.
2. Is R7 really necessary ? Does 555 have pull down resistor ?
Bulb:
Waveforms looks good. I would recommend to put a snubber in parallel with load and use of UF4007 diode instead of 1N4007. Reason for it is voltage spikes on load and MOSFET. This voltage spikes are high (double of power supply voltage) and can hurm MOSFET if higher currents and voltages are used. With snubber you can reduce voltage spikes.
What is snubber and how to dimension it is discraibed in this pdf https://www.illinoiscapacitor.com/pdf/Papers/RC_snubber.pdf
Snubber is RC circuit connected in paralled with inductive load and purpose of it is to reduce voltage spikes caused by fast switching of mosfet (or any other switch).
No, R7 is not needed. Output of NE555 is also totem pole configuration (there are two transistors, one switch to positive power supply input and other to negative power supply side (gnd)) but it can deliver max +/- 0.25A. With external totem pole you can achive higher currents (like +/-0.5A) that are useful for fast switching of MOSFET.
Nice work :-+
mike_mike:
The snubber is needed even if the max load current is 5A and max supply voltage is around 12V ? Or the snubber is needed when the voltages are bigger than 12V ? I'm sorry for asking again but I did not understood clearly from the initial message.
In the last screenshots, the frequency was reduced to about 50KHz, from about 160KHz.
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