Finding voltage drop for motor controllers

Other Hardware Motors, Mechanics, Power and CNC
1

I'm going to build a fleet of small RC robots for my sons birthday. I bought some L239D motor controllers but after hooking them up I noticed they caused a large voltage drop (~1.5v). I'm powering the robots with 3 rechargeable batteries so losing 1.5v really hurt performance. After poking around the web, I discovered this was expected for this motor controller and people suggested using something more modern.

After more poking around the internet I found some other options like DRV8833, A4988, L9110S, L298N, and a few others. My question is how do I figure out how much voltage these motor controllers will consume? For the DRV8833 I found "Low MOSFET ON-Resistance:". Can I can plug that into Ohms law to figure out voltage drop? Most of the data sheets don't have that. For the rest what should I be looking for?

Thanks!

2

Your motor is a stepper?

3

they caused a large voltage drop (~1.5v)

Both the L293 and L298 are ancient and inefficient technology. They are, by modern standards, crappy motor drivers and even worse stepper drivers. They have an inherent voltage drop of 2 to 4V depending on current. The modern drivers are much more efficient so their MOSFET outputs drop little voltage.

Post data sheets for your motors and we can help you find an appropriate driver.

The main specifications that we need are motor supply voltage and stall (starting) current (DC brushed motor). Or for a stepper, type (unipolar, bipolar) and coil current.

4

The DRV8833 datasheet says that the ON resistance is 360 milli-ohms. You need to know the motor current to figure the voltage drop using Ohm's law.

The A4988 is a stepper motor driver - a completely different thing.

...R

5

Thanks for the responses!

The motors I am using are these. The relevant details are:

  • Rated Voltage: 6.0VDC
  • Starting Voltage: 2.0
  • Loaded Current: 250 mA max
  • Stall Current: 500mA max

I'm going to be running them on 3 rechargeable batteries so ~3.6V. They are teeny-tiny so I'm guessing current isn't going to be a problem (almost every motor controller I looked at could handle 500mA). However, I'd like to use almost all of that 3.6V for speed. Slow robots aren't going to be fun to play with :).

6

Get a tiny RC speed controller for RC cars, brushed motors. disable battery saver. That's as simple as it can get.

7

GooperMC:
I'm going to be running them on 3 rechargeable batteries so ~3.6V.

I have built some model train controllers that are powered by a small 1S LiPo (3.7v, 240mAh) and which use the DRV8833 driver.

More recently I have been using a ROHM BD6212 h-bridge chips because they are physically smaller than the DRV8833 module.

If you buy an RC electronic speed control (ESC) make sure it has reverse. Many don't as they are used for model airplanes.

...R

8

GooperMC:
After more poking around the internet I found some other options like DRV8833, A4988, L9110S, L298N, and a few others. My question is how do I figure out how much voltage these motor controllers will consume?

You learn to read datasheets - sorry, but that's what you have to do. The on-resistance or saturation
voltage of the output devices is the key thing to find. MOSFET outputs have an on-resistance, Bipolar
and darlington have saturation voltage.

The A4988 is not a DC motor driver, its a chopper-driver for bipolar stepper motors, no use at all for your
motors.

The L298N is another ancient inefficient darlington driver like the L293D (not L239D BTW - its important
not to miss-spell part numbers, that can really cause confusion - copy and paste them).

Pololu's site has lots of options for modern drivers, good place to find candidates. Chips with
built-in MOSFETs usually have on-resistances down to about 0.25 ohms - for lower you need
separate discrete MOSFETs.

9

Thanks for the responses everyone. I'll try a DRV8833 and look around Pololu's site.

Warmly,