Hey, I am trying to power a small RC car DC motor using a motor driver with my Arduino (Hobbytronics). However, it seems the motor driver is consistently providing less power than the battery can supply (current that is). The driver is supposedly rated at 2A per channel, but after testing it with a multimeter...it seems it doesn't want to pull more than 0.9A. I am using an external 9V battery and I have also tried 4 x 1.5V batteries and a 5V 2200mah external charger. All of these only supply +- 0.9A through the motor driver, but if I connect them directly...they spike at about 1.7A.
At 0.9A the motors stall when on the floor and without it pulling more current...I can't get them to start turning. I am pretty new to electronics and I am sure I am missing something obvious. The reason this is strange to me is that this motor use to be powered by a 4.8V battery in the stock RC setup and it had more than enough power to get around quite quick. It doesn't seem right that a 9V battery cannot supply enough current to the motor now. Any suggestions? Let me know if I need to provide more information.
You're probably dropping a couple of volts across the bridge, and I'd be surprised if a 9V battery could supply anywhere near 2A.
What is the nominal voltage for the motor?
Add a couple of volts and use that for a supply.
Since it's rated at about 3v, I am concerned that pushing more than 9 volts at it might cause problems. 9v already feels like a stretch, since it was initially powered by a 4.8v battery.
I have created a quick video to illustrate the difference in current and behavior when powered through the driver and straight from the battery. I did it quickly so hopefully you can see something, the first clip illustrates the car powered through the motor and the external 9v battery. It mostly stalls and pulls around 0.9A. The second clip shows the motor powered directly off the battery and shows current at about 1.2 - 1.3A. The third clip illustrates the motors behavior when powered straight off the battery, it's motion is quick and it doesn't stall like the first part.
I am concerned that pushing more than 9 volts at it might cause problems. 9v already feels like a stretch, since it was initially powered by a 4.8v battery.
Where did I say more than 9volts?
If anything, I'd ditch the 9volt battery and go for something like a 7.2 or 6 volt pack of Nimh rechargeables.
9 volt batteries are OK to power smoke alarms, but not a lot else.
L298 isn't designed for low-voltage motors, its loss of 2.5V or more is poor for a 6V motor,
to say the least.
Also its rating of 2A means "when water cooled" in practice - manufacturers assume you will
provide as much heat sinking as is necessary. The L293 and L298 aren't up to much above
1A in practice.
A good MOSFET H-bridge for low voltage is far better - alas they are not as cheap. The LMD18200T
I've used before, 0.3ohm devices so it wastes 0.6V at 1A, 1.2V at 2A, rated for 3A (with loads of cooling).
@MarkT I am having a similiar problem. I am using the ardumoto shield (check the link below) which has the L298 H bridge driver on it.
When I measure the voltage out of channel A with no load, this is where the motor will be connected, I am getting about the input voltage, minor loss because of voltage drop in the chip.
When I connect a load of 30 ohms are less there is significant voltage drop. The current is initial about 1amp but drops quickly and settles at about 200mA.
The input voltage is coming from an adjustable power supply that can output up to 20W. I have been doing these tests with varying voltages between 5-15V and have been seeing the same results.
The L298 chip gets really hot. If I add a heat sink will this solve the voltage drop and decreeasing current problem?
Thanks
The chip has internal thermal protection. When it gets too hot, it reduces output to save itself from destruction. A heatsink will certainly help provided there is a resonable place to attach it.
I am using the Ardumoto and the L298 chip to drive a transfomer, so i am basically telling the h bridge driver to switch from foward to reverse repeatedly to get a bipolar square wave (sorry cant post code).
I reduced the frequency from 200kHz to 100kHz and that solved my problem. The voltage drop through the arduino and the ardumoto shield is about 1.5 V and the output current is now constant. I am not positve if thie frequency constrant is a cause from the arduino or the other components that I am using in my circuit but hopefully this is helpful to someone.
mbr6486: @MarkT I am having a similiar problem. I am using the ardumoto shield (check the link below) which has the L298 H bridge driver on it. https://www.sparkfun.com/products/9815
When I measure the voltage out of channel A with no load, this is where the motor will be connected, I am getting about the input voltage, minor loss because of voltage drop in the chip.
When I connect a load of 30 ohms are less there is significant voltage drop. The current is initial about 1amp but drops quickly and settles at about 200mA.
The input voltage is coming from an adjustable power supply that can output up to 20W. I have been doing these tests with varying voltages between 5-15V and have been seeing the same results.
The L298 chip gets really hot. If I add a heat sink will this solve the voltage drop and decreeasing current problem?
Thanks
The heat sink will make it cooler - the voltage drop is fundamental to any darlington-based H-bridge, use
MOSFET H-bridge for better performance?
mbr6486:
I am using the Ardumoto and the L298 chip to drive a transfomer, so i am basically telling the h bridge driver to switch from foward to reverse repeatedly to get a bipolar square wave (sorry cant post code).
I reduced the frequency from 200kHz to 100kHz and that solved my problem. The voltage drop through the arduino and the ardumoto shield is about 1.5 V and the output current is now constant. I am not positve if thie frequency constrant is a cause from the arduino or the other components that I am using in my circuit but hopefully this is helpful to someone.
I'm no expert but I don't think they are supposed to be used like that, when you apply power to an inductor and the power is turned off rapidly you will get a "back EMF" voltage spike, caused by the field collapsing in the inductor, I suspect at 200kHz you will be getting a very hot LM298 if it does not have some decent protection diodes, this maybe why reducing the frequency made things better for you.
charlie_smith87:
I'm no expert but I don't think they are supposed to be used like that, when you apply power to an inductor and the power is turned off rapidly you will get a "back EMF" voltage spike, caused by the field collapsing in the inductor, I suspect at 200kHz you will be getting a very hot LM298 if it does not have some decent protection diodes, this maybe why reducing the frequency made things better for you.
True! An expert would understand the difference between an inductor and a transformer - the sum of amp-turns
for all windings is the quantity that has to change rapidly to cause voltage spikes, so you cannot treat the
primary like an inductor unless all the secondaries are open circuit.
Also that motor driver board does have flyback diodes...