4
\$\begingroup\$

I have a 2S Li-Po battery (7.4v) and I want to control 4 small dc motors (3-5V, site says 22mA at 3V) with an arduino nano PWM signal. The nano is also powered by that battery, i do not want another external battery.

So I thought about regulating the 7.4v from the battery to 5v and then there are 2 option that I saw:

  1. use a NPN Transistor, Resistor 330Ω, Diode (1N4004) and use this setup: (But power the motors from the regulated external battery and not the arduino 5V like in the picture) first method

  2. use 10kΩ resistor, RFP30N06LE MOSFET and use this setup: (My motors is 5v, not 60v like in the picture, and again, there are 4) 2

Ok, Questions:

  1. What setup is better?
  2. Is regulating the 7.4V to the 5V max the motor need is a good idea?
  3. Do I need 1 regulator and split the output to each motor, or 4 regulator (one for each motor)?

EDIT: if there is an already made board with all the connections needed, for 4 motors (I saw only 2 motors driver at sparkfun and buying 2 of them costs too much) please write, because I need the setup to be the smallest possible

\$\endgroup\$
  • \$\begingroup\$ What's the stall current of the motor from a 5V supply? \$\endgroup\$ – Andy aka Aug 5 '13 at 10:58
  • \$\begingroup\$ I don't know, please take a look on the site where I bought it from (link in first line of question) \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 11:10
  • \$\begingroup\$ Do you know what stall current is and how it determines the detail of the regulation method? \$\endgroup\$ – Andy aka Aug 5 '13 at 11:25
  • \$\begingroup\$ @Andyaka No, I don't know \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 12:21
  • 1
    \$\begingroup\$ Partial answer which someone else might expand on: there are various Arduino motor driver shields built around the L293 H-bridge ti.com/lit/ds/symlink/l293d.pdf , and if you only need to drive 4 motors in one direction then you can use one of the four "half h bridges" of that device for this purpose. \$\endgroup\$ – pjc50 Aug 5 '13 at 14:04
3
\$\begingroup\$

The eBay link to the motor did not yield any datasheet. So I will assume for the purpose of this answer that the motors are similar to the Precision Microdrives 106-002 motor, which has nearly identical dimensions and electrical specifications.

The datasheet indicates a starting current of 180 mA at 3 Volts, and a no-load current of 17 mA at 3 Volts. Extrapolate that to the 22 mA current rating from the eBay listing, to a 233 mA starting current for the motors in the question.

If the 4 motors are likely to be started or stalled simultaneously, it helps to design for this maximum current: 233 x 4 = 932 mA = ~ 1 A. For normal operation, this value becomes 22 x 4 = 88 mA = ~ 100 mA.

For a 5 Volt supply, we should thus allow for 1.5333 Amperes current, or at least 1.5 Amperes if we need to cut corners.

  1. What setup is better?

The second one: Just use a MOSFET (or 4 of them, one for each if you plan to control them separately) to switch the low-side of the 4 motors.

  1. Is regulating the 7.4V to the 5V max the motor need is a good idea?

Well, you do need to get the supply voltage down to within the motor's specified voltage range somehow. I might even get parsimonious and use 4 diodes such as 1n4007 in series to do the voltage reduction: End result, 4.6 Volts, so the motors will live a bit longer. After that, I'd drive all 4 motors from this rail, with the capacitors and diodes in place of course.

schematic

simulate this circuit – Schematic created using CircuitLab

Do I need 1 regulator and split the output

Option 1: Single regulator for all 4 motors:

schematic

simulate this circuit

  • This would work fine, and a 1.5 A regulator would run pretty well on normal operation load of 22 * 4 = 88 mA = ~ 100 mA. For a linear regulator (e.g. 7805) the normal running dissipation would be around (7.4 - 5) x 0.1 = 0.24 Watts, which isn't much for a TO220 regulator package.
  • Remember to add a capacitor of say 1 uF at each motor, parallel to the reverse biased diode already shown in the question, to bypass some of the commutation noise the motor must generate

Option 2: Separate regulators for each motor:

  • This would work fine too, but at the cost of a significant increase in part count: Each regulator will need a capacitor each, before and after it, besides the 4 regulators themselves. On the plus side, 4 regulators can dissipate heat better than one.
  • Again, capacitors at each motor would help, but smaller values, say 470 pF each, in parallel with each diode would do fine, since the regulators themselves would protect the supply line from the commutation noise.

Recommendation: Note that this is a personal view... I would go with a single regulator and add the diodes + capacitors as close as possible to each motor

\$\endgroup\$
  • \$\begingroup\$ Thank you for your long and informative answer, can you please add a sketch (like the ones in the question) for the Option 1? I understand from that much better. Also, I don't understand your answer on regulating the 7.4v question if you can please clear it up for me a bit. Another thing, in your answer to the first question you wrote "or 4 of them", what did you mean by that? (should I use one, or 4? or you meant one for each motor). Thanks again! \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 12:31
  • \$\begingroup\$ @DanBarzilay See edits. \$\endgroup\$ – Anindo Ghosh Aug 5 '13 at 12:51
  • \$\begingroup\$ Thank you, much more clear to me now, just one last question, is there an already made board with these things to control my motors? connecting all these together will be all loose around and the setup should be durable (uav, it can crash), and making a PCB is not an option. a board will keep it all together. \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 13:28
  • \$\begingroup\$ oops, another question :\ in your second schematic it seems you used a regulator, but before that you suggested using diodes instead, can you please update it with the diodes instead? (I don't really understand whether I should keep those capacitors in there or not so I'd rather take no risks and just ask you :) ) \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 13:39
  • \$\begingroup\$ @DanBarzilay The first schematic shows how the diodes are used to drop 7.4V to 4.6V - each diode drops 0.7 Volts. The motors with their respective capacitors and diodes would be wired up the same as in the second schematic, just that the regulator can be substituted by the diodes in series. Keeping the capacitors C2 and C3 in there will do no harm, leave them there. \$\endgroup\$ – Anindo Ghosh Aug 5 '13 at 13:59
2
\$\begingroup\$

If it's for a UAV (unmanned aerial vehicle) then power hungry linear regulators may prove a bind so I'd consider a switch mode buck converter like this: -

enter image description here

I'd use this for all four motors as well but don't use it for powering other things like the radio receiver or logic circuits. It should be able to handle the stall/start currents of all four motors although this isn't a given because the motors you have linked to are badly specified but given what Anindo has found, it is likely to work.

If you used a linear regulator and the motors drew (say) 2A at full load you'd be wasting about 5W in heat whereas the buck regulator (above) will be ~90% or greater efficiency at 2A burning probably in the order of 1W or less. Battery life to consider of course.

This isn't anything particularly special - all the usual suppliers offer something similar.

I'd consider these FETs for switching the motors because they come in dual packages and weight should be a consideration: -

enter image description here

Don't forget the reverse connected diodes - these need to be rated in excess of the motor stall current so 2A diodes maybe needed.

\$\endgroup\$
  • \$\begingroup\$ Hey, thank you for your answer, can you please make a schematic like the ones in my question? just so i'll know how to connect it all (I'm a newbie). Also, is there an already made board out there that has what you suggesting? because connecting all these together will be all loose around and the setup should be durable (uav, it can crash) \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 14:38
  • \$\begingroup\$ @DanBarzilay I didn't see any schematics in your question. I saw some pretty little pictures of idealized components but I don't draw those type of diagrams. There won't be a ready-made board. \$\endgroup\$ – Andy aka Aug 5 '13 at 15:04
  • 1
    \$\begingroup\$ Yea I meant those "pretty little pictures", anyway, so can you draw a normal schematic? and just out of interest, why won't there be a read-made board if this is considered a good setup? \$\endgroup\$ – Dan Barzilay Aug 5 '13 at 15:05
  • \$\begingroup\$ You'd probably be able to buy an evaluation circuit board for many switching regulators so that a designer can prove its performance in the lab but It's not gonna be eBay prices. You wouldn't expect an exhaust manufacturer to make cars and the same is true of over 99% of electronic components. \$\endgroup\$ – Andy aka Aug 5 '13 at 16:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.