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Feb 22, 2019, 07:43 AM
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DroneSK's Avatar
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Anyone used a Power Supply Unit instead of batteries?


I am talking about something like this:



Of course I am not talking about using it to fly :P I'm talking about using it in your workshop/lab to avoid having to deal with batteries.

I can match the volts and the amps but what makes me curious is about how the ESC on my motor will respond. Upon connection, it beeps to let you know the number of cells in the battery. What would it do when connected to this?

Anyway, do let me know if you have tried something like this or know someone who did. It sucks to realise you forgot to charge your batteries right when you are about to start working on your thing.
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Feb 22, 2019, 09:45 AM
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If it has enough current capability it will work but be very careful. You don't say what you are connecting but never run a quad indoors with props on. But batteries are much cheaper than a good power supply like the one in your picture.
Feb 22, 2019, 10:44 AM
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vimy g eaou's Avatar
Quote:
Originally Posted by DroneSK
I am talking about something like this:



Of course I am not talking about using it to fly :P I'm talking about using it in your workshop/lab to avoid having to deal with batteries.

I can match the volts and the amps but what makes me curious is about how the ESC on my motor will respond. Upon connection, it beeps to let you know the number of cells in the battery. What would it do when connected to this?

Anyway, do let me know if you have tried something like this or know someone who did. It sucks to realise you forgot to charge your batteries right when you are about to start working on your thing.
You would have to build a jig to simulate the what the ESC would expect to see.

The only way I can suggest is to use a voltage divider for the balance connector to plug into.
Feb 22, 2019, 10:59 AM
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DroneSK's Avatar
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Quote:
Originally Posted by Volt_Ampere
If it has enough current capability it will work but be very careful. You don't say what you are connecting but never run a quad indoors with props on. But batteries are much cheaper than a good power supply like the one in your picture.

I planned on using it for both:

1. Powering the RC plane indoors (for ground tests). This will only include the LEDs, motor, ESC, servos and the receiver.
2. Powering the RC plane + Pixhawk 4 ( see here and here for details)

Whenever I will be testing it indoors with the Pixhawk attached to it, the additional sensors (GPS, airspeed sensor,etc) won't be online.

Mine is a plane/fixed-wing. I never run a motor with props on indoors unless it's secured to a test rig (which I don't have right now).

The thing is, I have access to various PSU's. I won't have to buy it. That's why I was thinking about putting them to good use.
Feb 22, 2019, 11:04 AM
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DroneSK's Avatar
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Quote:
Originally Posted by vimy g eaou
You would have to build a jig to simulate the what the ESC would expect to see.

The only way I can suggest is to use a voltage divider for the balance connector to plug into.
Have no prior knowledge/experience of that. Please do feel free to elaborate/explain/share how it's done.

If it's nothing that complicated, I'll give it a go. Meanwhile, I'll check if any of the PSUs I have access to has "voltage divider"s built in.
Feb 22, 2019, 11:09 AM
Registered User
I have done this severall times using a 0-15V. 60A psu for measurement purposes and, providing you have the current capability availability, it works fine.

Interestingly (for me) I plotted rpm against power and it demonstrated that the relationship is a pure cube law; double the rpm and you need 8 times the power.

Wayne
Feb 22, 2019, 11:19 AM
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rampman's Avatar
Quote:
Originally Posted by Wayne Giles
...
Interestingly (for me) I plotted rpm against power and it demonstrated that the relationship is a pure cube law; double the rpm and you need 8 times the power.

Wayne
Humm, I recall reading decades ago that to double your speed in a car you need 8x the power. In this case I "assumed" it was due to wind resistance but I will admit, math is my weakness. Cube law? I have been called a square a time or two but I guess that is different.

Rick
Feb 22, 2019, 11:24 AM
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DroneSK's Avatar
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Quote:
Originally Posted by Wayne Giles
I have done this severall times using a 0-15V. 60A psu for measurement purposes and, providing you have the current capability availability, it works fine.
That's good to know. How did your ESC react?

The ESC I have says in the manual that it automatically detects the number of cells in the LiPo. The first thing it is supposed to do after getting power in a normal start up is to beep out the number of cells in the battery. The number of beeps is equal to the number of cells in the battery.

That's what makes me curious as to what happens when it's connected to a DC PSU. Would it beep just once then?

Quote:
Originally Posted by Wayne Giles
Interestingly (for me) I plotted rpm against power and it demonstrated that the relationship is a pure cube law; double the rpm and you need 8 times the power.

Wayne
Nice! How did you measure the RPM? I have an old tachometer:



Never used it. When I turn it on and set 2 or 3 blades, it shows 20 or 30 (x100 RPM)as the reading. I'd love to either workout, estimate or even come across the power-RPM-Thrust relationships for my motor and prop.
Feb 22, 2019, 11:27 AM
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vimy g eaou's Avatar
Quote:
Originally Posted by DroneSK
Have no prior knowledge/experience of that. Please do feel free to elaborate/explain/share how it's done.

If it's nothing that complicated, I'll give it a go. Meanwhile, I'll check if any of the PSUs I have access to has "voltage divider"s built in.
Unfortunately, I can't draw, even simple stuff.

You use a series of equal value resistors so the voltage drop across each resistor is the same all the way down to 0V. This goes across the output of the power supply.

From there, you can then have leads coming off the connections between the resistors going to the balance connector.

I hope the following helps:

http://denethor.wlu.ca/pc200/divider...iv_lab.shtml.x

https://learn.sparkfun.com/tutorials...e-dividers/all

https://www.electronics-tutorials.ws...tor/res_3.html
Feb 22, 2019, 11:32 AM
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DroneSK's Avatar
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Quote:
Originally Posted by vimy g eaou
Unfortunately, I can't draw, even simple stuff.

You use a series of equal value resistors so the voltage drop across each resistor is the same all the way down to 0V. This goes across the output of the power supply.

From there, you can then have leads coming off the connections between the resistors going to the balance connector.

I hope the following helps:

http://denethor.wlu.ca/pc200/divider...iv_lab.shtml.x

https://learn.sparkfun.com/tutorials...e-dividers/all

https://www.electronics-tutorials.ws...tor/res_3.html
Thanks so much for the links. I got it! Super easy! Barely an inconvenience.
Feb 22, 2019, 11:36 AM
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Quote:
Originally Posted by DroneSK
That's what makes me curious as to what happens when it's connected to a DC PSU. Would it beep just once then?
No, it would beep the number of times according to the number of cells it thinks you have connected.

The ESC doesn't have some magic ability to sense how many battery cells are connected to it. All it knows is that the voltage on its input correlates to what would normally be supplied by a certain number of cells of a LiPo battery. For example, if you plug a fully charged 3S LiPo to it, it will see ~12.6V on its input, and assume that you've hooked up a 3S LiPo.

Hook the ESC to a 12V car battery, and the ESC will assume it's hooked to a 3S LiPo, because 12V is in the normal voltage range for a 3S battery.

If you hook the ESC to a power supply set to 12.6V instead, the ESC still thinks it's hooked to a 3S LiPo battery. Set the power supply to 8V, and the ESC will think it's hooked to a 2S LiPo.

You could hook up a 4S LiPo that's been drained much lower than it should, and each cell is down to 3.1V. This would mean 12.4V on the ESC input, and the ESC would assume you have a 3S battery hooked up.

The balance plug is not used for connecting to the ESC.
Feb 22, 2019, 12:19 PM
Registered User
Spinning prop power will be between the square and cube of RPM.
Feb 22, 2019, 12:49 PM
B for Bruce
BMatthews's Avatar
One point that might bite you. If you flick the throttle up from zero to max the initial current peak on a strong motor that will draw 20 amps out of a 30 amp supply might peak initially at over 30 amps. And if it pushes the supply into current limit the voltage at the output will drop. The ESC might see that as a low battery and cut off the power.

Cure is to smoothly and slowly advance the power so the startup or speed increase load does not push up over what the supply can give you before going into current limit.

On smaller motors that will only draw 10 to 15 amps and with a 30 amp supply it shouldn't be as big of an issue. Might not even notice it in fact. It will only be an issue on motors with more draw that comes up closer to the limit of the supply.
Feb 22, 2019, 05:05 PM
Registered User
Quote:
Originally Posted by DroneSK

Never used it. When I turn it on and set 2 or 3 blades, it shows 20 or 30 (x100 RPM)as the reading. I'd love to either workout, estimate or even come across the power-RPM-Thrust relationships for my motor and prop.
I used a similar tachometer and yours must be accurate as the readings you are seeing are derived from the mains frequency ie 50hz = 50 cycles per second = 3000 cycles per minute. ( unless your mains frequency is 60Hz in which case it is 20% slow )
You are probably looking at it in fluorescent lighting which turns on and off at mains frequency although some tachometers will read in tungsten lighting by just counting the variations in the light level.

When reading prop rpm make sure you are doing so in natural light.

Wayne
Feb 22, 2019, 05:29 PM
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DroneSK's Avatar
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Quote:
Originally Posted by lametec
No, it would beep the number of times according to the number of cells it thinks you have connected.

The ESC doesn't have some magic ability to sense how many battery cells are connected to it. All it knows is that the voltage on its input correlates to what would normally be supplied by a certain number of cells of a LiPo battery. For example, if you plug a fully charged 3S LiPo to it, it will see ~12.6V on its input, and assume that you've hooked up a 3S LiPo.

Hook the ESC to a 12V car battery, and the ESC will assume it's hooked to a 3S LiPo, because 12V is in the normal voltage range for a 3S battery.

If you hook the ESC to a power supply set to 12.6V instead, the ESC still thinks it's hooked to a 3S LiPo battery. Set the power supply to 8V, and the ESC will think it's hooked to a 2S LiPo.

You could hook up a 4S LiPo that's been drained much lower than it should, and each cell is down to 3.1V. This would mean 12.4V on the ESC input, and the ESC would assume you have a 3S battery hooked up.

Well then it's kind of a false marketing thing then. I assumed it'd "detect" because it claims to do that. After reading your post, it seems like all it does is something like dividing the voltage by 3.7, rounding to the nearest integer and guessing the number of cells based on that. Thanks for letting me know that bit.

Hang on a second! Does that mean that using voltage dividers is pointless?

Quote:
Originally Posted by lametec
The balance plug is not used for connecting to the ESC.
Would you mind sharing what you are talking about?
Last edited by DroneSK; Feb 22, 2019 at 05:43 PM.


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