Another L298 post regarding voltage drop

Hey

Sorry if this has been done to death already.

Firstly, Hi, i'm Shane, Electronics newbie.

So i have a prebuilt L298 DC motor controller which i purchased off ebay to have a play around with. I've connected it up to a 5v then a 12v supply written some code and had a play with it. All seems to be working almost as expected.

With the 5v Supply the voltage drop was about what i expected from the datasheet, 5v in about 3.2v out. When i swapped up to a 12v Motor and supply the output voltage seems drop down to about 2.8v i.e. the motor barely moves.

I've checked my supply voltage, 12.5v input while the motor is turning, 2.8v stable on the output. If i disconnect the motor it reads 4.1v across the outputs.

Where to i start trying to track down where all the voltage is going :slight_smile:

Or is this as expected and i should throw the L298 away and find something better?

Thanks in advance

Shane

Is it hot? If so, can you cool it at all?

Doesnt seem to be hot.

Its has a heat sink on it

Its one of these.

Cheers
Shane

Well, that's really strange. So your battery maintains its 12V even when powering the motor, its the motor side that has low voltage, right? 12.5 - 2.8 = 9.7V. It's hard to imagine that much voltage "disappearing" with motors turning and nothing getting hot. Did you measure the current flowing thru the circuit?

Glad its not just me who thinks its weird.

Its no being run off a battery, its a 12v line on a computer PSU (its the most convenient 12v source i have) does that make a difference?

Trigger pin LOW, i get 12.5v across the 12v input, 0v over the motor out
Trigger pin HIGH, i get 12.5V across the 12v supply and ~2.8v across the motor output. If i disconnect the motor and measure across the open terminals its reading 4.1V

Just thinking, if i'm not pushing the trigger pin with a full 5V would that cause this? I havent check the voltage there.

Orginally i thought it might be the 12v motor being silly, so i tried it with a small 4.5v motor basically same result. i end up wiht about 3.7v across the outputs./

Where should i be measuring current draw? the 12v supply line? or the motor outputs?

Thanks for taking time to help me wih this

So i just measured the voltage on the trigger pin to ground and its only registering 2.8v i’m thinking this should be 5v?

I’m setting the pin with digitalWrite(forPin,HIGH); Where forPin is the trigger pin

what am i missing :smiley:

Schematic? Pictures? If the output pin of the Arduino board is at 2.8V, then it is either extremely heavily loaded or the whole board is under a low voltage condition. Check your board Vin. Measure current from PS into the circuit.

Yeah. i'm running the Hbridge off the arduino (for 5v) maybe i should be running dedicated 5v from the computer PSU?

This is how its currently wired (its not clear but the grounds are wired to the same place)

Should i be doing this?

As i test i ran 5v direct to the trigger input on the H-bridge, made a slight difference on the motor output (about 3.8v vs 2.8v)

Do you think the Hbridge is drawing too much juice from the Arduino?

Cheers
shane

So that would be it

I repowered the 5v supply to the Hbridge from the ATX PSU and its now putting out 8.8v at the motor inputs :slight_smile:

Thanks for everyones help i appreciate it :slight_smile:

I assume its because the Arduino wasnt supplying enough to saturate the hbridge fully?

The board shouldn't be drawing very much from the 5V supply, I think something else was the
actual problem (missing ground connection?).

Ground was definitely there, and didnt change between tests

Theres a jumper the on the Hbridge which (i’m assuming) when in place steps down the motor supply to 5v for the H bridge. Or alternatively you can remove the jumper and supply external +5v to the Hbridge (which i was doing with the arduino)

The only changes i made were to add the jumper and removed the +5v wire from the Arduino to the H bridge, everything else stayed the same.

Perhaps i’ll remove the jumper again and pipe +5v from the ATX PSU and measure the current draw. 20mA is the recommended from the arduino?

An Arduino on USB ought to be able to provide several hundred mA, I don't believe the motor board takes that at 5V!
There might be a bad/dry connection on the motor board, a faulty hook-up wire or something like that - the way
to check is multimeter on every power rail, then measure current taken by every power rail.

MarkT:
An Arduino on USB ought to be able to provide several hundred mA, I don't believe the motor board takes that at 5V!
There might be a bad/dry connection on the motor board, a faulty hook-up wire or something like that - the way
to check is multimeter on every power rail, then measure current taken by every power rail.

On my laptop, Device Manager shows that my Uno R3 has asked for 100mA. My laptop would be well within its "rights" to shut the Uno off if it tried to draw more than 100mA from the port. Maybe the OP's computer current limited the port when he tried powering the bridge from the Arduino board.

MarkT:
An Arduino on USB ought to be able to provide several hundred mA, I don't believe the motor board takes that at 5V!
There might be a bad/dry connection on the motor board, a faulty hook-up wire or something like that - the way
to check is multimeter on every power rail, then measure current taken by every power rail.

Yeah i'll check that out.

I've tried it with two identical motor boards with the same results. So i'll check my current draw and see where we go from there. Its fixed now, but i'm still interested it working out why it didnt work in the first place :slight_smile: