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Topic starter
2024-08-16 8:43 pm
How can I determine the current requirement of a servo? Can I extract or derive it from the servo datasheet, or do I need to make measurement?
I have a pair of 9g servos that I want to connect to an ESP. I power the ESP with the 3v rail of a breadboard PS, and power the servos from the 5v rail. I also have 2 LEDs, and 2 MOM button switches that are connected to the 5v.
I didn't think about the power usage until, after about a week, the circuit failed to drive the servos. AFAIK, the breadboard PS can deliver ~700ma. I thought that would be sufficient for the circuit but apparently not. I suspect the 2 servos may be drawing more amps than the PS can deliver.
How can I determine if the servos are the cause?
2024-08-16 8:57 pm
It should be able to drive them under no load, but a single 9g can draw up to about 700ma at max pull.
In a perfect work the datasheet would say but often they don't.
You could hook up just one servo inline with a multimeter and get it to hold a position, then press on the horn and see how high the current reads as you increase the force.
I would have expected it would drive those servos though, do they not respond at all?
Topic starter
2024-08-16 9:25 pm
o they not respond at all?
The symptoms are they act like there is a mismatched ground, i.e., they jitter or don't move at all. I've assumed the breadboard PS uses a common ground for both the 3 and 5v rails.
What's peculiar is I have 2 ESPs with this set up. One runs fine; the other doesn't. I did some ad hoc component swapping and it looks like it may be the ESP but decided to be a bit more rigorous. I finished a continuity test of the breadboard and starting to rebuild the circuit. It's just got me stumped right now.
Edit: forgot to mention, these are essentially under no load.
2024-08-16 9:38 pm
It certainly sounds like a grounding issue given the symptoms. But if I understand correctly you can swap out the ESP and it works fine?
You could test to see if both ESPs are sending PWM signals correctly. Or even check if the ground pin on the ESP is actually a ground pin. I've had issues where I build a circuit board assuming the pinout diagram for my ESP was correct and it turned out the Ground pin wasn't grounded.
2024-08-16 9:47 pm
Probably worth watching online examples of how to drive multiple servo motors from an esp32.
One issue I mentioned was voltages spikes so using say a 100 µF capacitor between the ground and the power supply to the servo might be useful.
Topic starter
2024-08-17 2:57 am
It certainly sounds like a grounding issue given the symptoms. But if I understand correctly you can swap out the ESP and it works fine?
The Good News is the I found the faults in the circuit. One was a connection error with the LEDs and other was I used different power rails from a single power supply on both ESPs. So I powered the ESPs from the 3v rail of the power supply but then powered the servos from the 5v rail of the same power supply. That's a no-no. If I want to use the same power supply then everybody would need to use the 5v. My intentions were good, but I came up a little short on execution. So, I learned something.
Still gonna search for a servo current requirement though. Probably a measurement for no-load and just before stall load.
2024-08-17 1:30 pm
@tfmccarthy The datasheet will have the servo current values.
First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.
Topic starter
2024-08-18 2:28 pm
sigh. This feels like a "it depends on where you get the datasheet"
Previously I searched for the SG90 datasheet and wasn't able to see a current rating. But based on your advice, I searched again and found this on ProtoSupplies and sure enough, there's a current rating:
Current (idle) 10mA (typical)
Current (typical during movement) 100-250mA
Current (stall) 360mA (measured)
So, I should expect to find a current rating on a datasheet
And these values make me feel better about the load I'm using.
Thanks Ron.
2024-08-18 7:11 pm
@tfmccarthy There is a bit of 'black magic' to finding 'good' datasheets. I just use common sense and 50+ yrs experience to augment my gut feel.
First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.
2024-08-19 7:41 pm
Do you have the datasheet/ model number of the particular 9g servo that you're using?
Topic starter
2024-08-19 11:20 pm
Do you have the datasheet/ model number of the particular 9g servo that you're using?
I did a Google search and looked at 2 datasheets that looked like this one.
Then a wizened wizard, walking his Wookie, came along and threw pixie dust at me and said, "Those are not the datasheets you're looking for. Try again."
When I did, I found this.
2024-08-20 3:13 am
@aliarifat Please use the Reply link at the bottom right of the post you are replying to otherwise that person does not get notified.
First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.
2024-08-20 5:30 pm
@tfmccarthy If you find the correct datasheet, you can find the current ratings in the specs. It might help more if you post a picture or model number.
Topic starter
2024-08-20 10:07 pm
It might help more if you post a picture or model number.
I going to assume the link went bad on my post and you were unable to follow it to the data sheet.
Let me repost it in the clear here: http://www.ee.ic.ac.uk/pcheung/teaching/DE1_EE/stores/sg90_datasheet.pdf
If you did follow it, you would see the model number right at the top. And it has a picture and dimensions. And the specs. No current.
The second link I posted, which also seems broken now, was: https://protosupplies.com/product/servo-motor-micro-sg90/
And that one also shows the model number and code and a picture and the specs.
I don't want to get my blood pressure up over this. At the risk of being accused of boasting, my suggestion to you, @aliarifat, is to look at my bio and ask yourself if I might know a thing or two about technical communication.
If you were unable to follow the link, then mention that rather than instruct.
At the same time, my post was meant to be lighthearted and it clearly missed the mark.
Noted.