Timeline for Weird DC motor behaviour at low PWM duty cycle
Current License: CC BY-SA 4.0
23 events
| when toggle format | what | by | license | comment | |
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| Oct 20, 2023 at 11:50 | vote | accept | Solmyr999 | ||
| Oct 20, 2023 at 10:16 | answer | added | Solmyr999 | timeline score: 0 | |
| Oct 20, 2023 at 8:43 | comment | added | Solmyr999 | @wbeaty - Isn't there a risk of voltage spike from drain to source if I remove the 100 µF capacitor across the 24V battery ? Anyways, I'll give this a few more tests before dismantling and building the circuit again from scratch. My physical layout is bad and needs to be redone properly. The very long wire from 555 output pin to mosfet gate, among other things, is something I need to correct. I'll make one final post on this thread with all the things I plan to change in my new layout so that you guys can tell me if they are good ideas or not. | |
| Oct 19, 2023 at 20:22 | comment | added | wbeaty | @Solmyr999 mosfets can unexpectedly oscillate at MHz, and fry themselves. To stop this, the gate resistor must be placed at the mosfet gate, and not at the 555 pin. 100ohm or 1K is typical, giving RC of roughly a microsecond. Also, circuit noise usually comes from capacitors. If it's from your 100uF, try removing the RF snubber, or greatly reducing its R*C value. | |
| Oct 19, 2023 at 18:39 | answer | added | Antonio51 | timeline score: 1 | |
| Oct 19, 2023 at 17:56 | comment | added | Antonio51 | @Solmyr999 If you want to use a pulldown, use a 1 kOhm. 10 kOhm seems too "high". | |
| Oct 19, 2023 at 17:00 | comment | added | Solmyr999 | @Antonio51 - Do you think a 100 Ohm gate resistor with a 10 kOhm pulldown resistor would do the trick ? | |
| Oct 19, 2023 at 16:37 | comment | added | Antonio51 | 10 Ohm is too low. I should test a 100 Ohm. | |
| Oct 19, 2023 at 16:37 | comment | added | Solmyr999 | @coal - It comes from the 555 output (mosfet gate), that's what worries me. I have regular pwm signal when I disconnect the motor. Whenever the motor is connected and spinning, I get this weird signal I measured. Do you have any idea what's going on ? | |
| Oct 19, 2023 at 16:05 | comment | added | coal | The signal you measured with the oscilloscope comes from the motor output, or from the 555 output (at the mosfet gate) ? Does the 555 output gives you a normal pwm signal ? | |
| Oct 18, 2023 at 16:37 | history | edited | JYelton | CC BY-SA 4.0 |
No space before punctuation in English; you needn't put "help with..." in question title as it is redundant
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| Oct 18, 2023 at 13:49 | comment | added | Solmyr999 | Yes I was refering to the sum of rise and fall time, sorry for using the incorrect terms. I've just edited my post, with an additional video showing oscillo reading of the gate voltage. It doesn't seem right to me but I need some help interpreting it. | |
| Oct 18, 2023 at 13:41 | comment | added | Vladimir Cravero | Yes, probe with everything connected. And you keep calling this "dead time", but dead time applies only in a push-pull configuration, dead time is when both the FETs are off. You want some small positive dead time to avoid shoot through current. In this case the correct term is rise time, and fall time - not sure if there is a good term for both the positive & negative transitions. | |
| Oct 18, 2023 at 13:37 | history | edited | Solmyr999 | CC BY-SA 4.0 |
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| Oct 18, 2023 at 12:49 | comment | added | Solmyr999 | @Antonio51 - I've put the 10 ohm resistor because I've read that it prevents ringing of the mosfet gate. Maybe it's completely unnecessary. The 555 output (gate voltage) is perfectly fine even at low duty cycle when the motor and 24V battery are not connected. Somehow the motor and/or the 24V battery act on the mosfet gate in a way I don't understand. I will probe the gate voltage with everything connected to see if I can get what happens. | |
| Oct 18, 2023 at 12:44 | comment | added | Solmyr999 | @danmcb - I've just read the following thread with a very good answer on pulldown resistors : electronics.stackexchange.com/questions/60427/… Maybe my MOSFET turns on at low PWM frequency because the gate is oscillating from capacitive coupling ? A pulldown may help in that case | |
| Oct 18, 2023 at 12:43 | comment | added | Solmyr999 | @Vladimir Cravero - At which amount of dead time do you think a stronger driver is required ? I've read that dead time below 1% of total period is best, but I guess this also depends on the amount of current you're moving through the MOSFET | |
| Oct 18, 2023 at 12:32 | comment | added | Antonio51 | The 555 driving a 10 Ohm resistor is quite unusual ... Scope the output of 555. | |
| Oct 18, 2023 at 12:22 | comment | added | danmcb | very nice and complete problem description. yes, a pull down is a good idea, keeps the FET off if the 555 out is high impedance for some reason. The FET will in theory only consume power when switching, so at a higher frequency you expect more heat. Shorter rise times reduce this but can give other problems such as RFI. It's a balance of concerns. | |
| Oct 18, 2023 at 12:22 | comment | added | Vladimir Cravero | Regarding your edit - you can probe the transistor gate at various PWM settings, and assess whether you need a gate driver or not. Actually you just need an experiment at one (mid-range) PWM setting, to assess rise and fall time of the gate voltage. | |
| Oct 18, 2023 at 12:11 | history | edited | Solmyr999 | CC BY-SA 4.0 |
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| Oct 18, 2023 at 11:49 | history | edited | Solmyr999 | CC BY-SA 4.0 |
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| Oct 18, 2023 at 11:44 | history | asked | Solmyr999 | CC BY-SA 4.0 |