stepping down voltage from 12 to 8 volts. A motor turns on from the regulated stepped down voltage, and when it does I get a voltage sag that drops the voltage down 1.5 volts (so from 8 to 6.5). it lasts for about 2 milliseconds which is unacceptable for my application.
How can I deal with this voltage sag? I have no passive components from the regulated 8volts to the motor driver.
attached is an oscilloscope image of the sag when stepping it down to a lower voltage, but still I get the same sag.
All motor require current (amperes) in order to turn. Your voltage step down device have absolutely no current rating based on your link, so I am surprised they have not burst into flames. All the associated questions in your link never ask the most obvious question which is how much current they will supply.
In short, those boards are totally unsuitable for motors and there is nothing you can add or change to make them move a motor. Sorry.
Im not sure how you came to the conclusion that these boards are not suitable for motors or that they would burst into flames. They are rated for 3 amps as is shown in their datasheet. my motor does not draw more than 1 amp.
what other buck converter would you suggest that would suit this better without giving any voltage sag?
I couldn't find any data sheet for the motor but from a seller selling a similar motor they say the stall current is 2.2A . I ran the motor at much lower voltages but still I get that same 1.5v sag for 2 ms.
I can try another buck converter if you have a recommendation for one that you believe should not have that voltage sag on start up.
To estimate the stall current for your motor, measure the winding resistance. Take several readings, rotating the motor a bit between readings. Note the lowest resistance reading. The motor supply voltage divided by the lowest measured resistance is the stall current.
I dont think current is the issue. I found the specs for my exact motor from the seller site. the specs are the same. 2.2A as the stall current. After the voltage sag the voltage is regulated properly. I dont have any other buck converters to test this with but am willing to get different ones if the problem was with the lm2956. Are voltage sags an expected thing with any buck converter on a motor start up? are passive components needed to rectify this?
aether5:
Im not sure how you came to the conclusion that these boards are not suitable for motors or that they would burst into flames. They are rated for 3 amps as is shown in their datasheet. my motor does not draw more than 1 amp.
what other buck converter would you suggest that would suit this better without giving any voltage sag?
That conclusion was based on the advertisement link you gave in the first post.
jremington:
Actually, the current draw is the ONLY issue. The circuit cannot supply the starting current of the motor.
Period.
So how would this get resolved if it is a current issue? If a different buck converter is needed with a higher amp rating, what would you recommend? the source power comes a clean 12v power supply able to provide up to 10 amps.
Paul_KD7HB:
That conclusion was based on the advertisement link you gave in the first post.
Paul
Sorry for the confusion but its definitely not an ad. in fact, I'm probably going to drop a bad review later if I find that it was that device that was the issue. Im seeing from other places that Im not the first to have problems with it as others are stating there is bad ripple with the device.
jremington:
Use a motor power supply that can easily provide the stall current, which you MUST determine.
What are you really trying to do, and why do you think you need a buck regulator?
I am trying to power everything (motor, sensors, microcontrollers )from the same power supply.
So my setup is
a clean 12v power supply (up tp 10amps) lm2956-> 8 volts regulated to a sensors and motor driver driving a motor that (from the specs of the exact motor) never draws more than 2.2A
So I feel I need the buck converter to regulate the noisy voltage the motor creates to the microcontrollers and sensors. I even tried this setup as well
a clean 12v power supply (up tp 10amps) lm2956-> 8 volts regulated to a motor driver driving a motor lm2956-> 5 volts regulated to sensors and microcontroller.
You could power the motor directly from the 12v if you used a PWM output from your processor ( via a suitable drive circuit ) and limited the output value to around 170 , (8/12)*256; which will give you an average of 8volts .
It’s not a good idea anyway to run a motor from the same supply ( the 8v) as the processor as motors tend to create a lot of electrical noise which can cause problems.
Something you can try is placing a capacitor across your 8 volt side of your step down. Something along the lines of 1,000 uF rated for 25 VDC. Then see how your 1.5 volt 2 mSec drop looks. The cap is upstream from however you are switching your motor.
Again, no clue as to the quality of your buck converter or exactly what your motor is doing. Yes, as mentioned a motor on start can draw as much as 10X normal rated current. This is merely a suggestion.
I am running a platform using 2, 6 Volt motors and a 12 Volt battery. I am doing just as was suggested and using PWM to keep the duty cycle at about 50% and I have had no issues. I am also running my Nano off of a separate 6 Volt (4 AA batteries) supply with common grounds. Nothing on the motor side effects the controller side that way and no voltage regulator is needed on the motor side.
aether5:
A motor turns on from the regulated stepped down voltage, and when it does I get a voltage sag that drops the voltage down 1.5 volts (so from 8 to 6.5). it lasts for about 2 milliseconds which is unacceptable for my application.
Why is it unacceptable, what do you have that is sensitive to that drop?
Have you tried to place a diode and capacitor in the 8V wire to the sensitive device?
Will it tolerate 7.3V?
TomGeorge:
Why is it unacceptable, what do you have that is sensitive to that drop?
Have you tried to place a diode and capacitor in the 8V wire to the sensitive device?
Will it tolerate 7.3V?
Tom...
PS, You could use a second DC-DC to power the sensitive device, giving it its own power supply may be an advantage.
the sensitive device is a hall sensor that drops by 2 volts on low.
Thanks for the diagram. I tried out what was there. unfortunately I only had a 220 uf electrolytic capacitor though. The sag was still there though. I did try to use a second buck where both were at the 12 volt supply but one went to the motor load and the other to the sensitive device and still was there. However it just occurred (and I dont know why i did not think of this sooner) to me that the I was sending the two regulated voltages to the same bread board but on different rails. when i seperated the two regulated voltages to two different board entirely, the sag was reduced to only a .1V sag . So I guess breadboards have a quirk where voltages will interfere even if not on the same rail? I still dont seem to be finished with the problem entirely because the two regulated voltages are still pretty noisy even without the motor starting.
JoeWillson:
I am running a platform using 2, 6 Volt motors and a 12 Volt battery. I am doing just as was suggested and using PWM to keep the duty cycle at about 50% and I have had no issues. I am also running my Nano off of a separate 6 Volt (4 AA batteries) supply with common grounds. Nothing on the motor side effects the controller side that way and no voltage regulator is needed on the motor side.
Joe
Ive had a few comments about using a PWM instead and two different power sources. I am aware that this would resolve the issue but for my application I would really like to stick to one power source for all components and am trying to learn how to do so by using converters appropriately. I will be trying PWM later as I will have to change some things from a separate library I wrote (not arduino related so will take a bit more time than changing to analogwrite() )
I will be getting different converters to see if there is any difference with that. I appreciate any other help about the noise issue and insight to why the slight sag of voltage with motors starting.
Hi,
Connect the scope so one channel is on the input to the DC-DC converter and the other channel on the output of the DC-DC converter.
See what both traces do when the drop occurs.
The amount of current that a HallEffect should draw would be relatively small, even 220uF should be enough.
What diode did you use and what was the scope trace of the voltage across the capacitor?
Can you post a picture of your project and a schematic, even a picture of a hand drawn circuit, including power source?
What else do you have connected to the 8V supply, and do you have anything connected to the power in side of the DC-DC converter?
