Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

A very stupid question

Not open for further replies.


New Member
I have a problem with one of my ICs. I think that is what is the problem:

When on the picture (taken straight from the IC's datasheet) it says connect the lead to the ground I usually connected the negative terminal to that spot, am I wrong?

What I have is an H-bridge: L398N. I am building a robot where I need a forward and reverse for the driving dc motor. With the picture from the L398 datasheet (connecting the 'ground' pin to the negative) I get the voltage output from one of the 'out' pins (as measured against the negative terminal) but the other 'out' seams not to be connected to the negative to make the motor turn. I tried using an independent transistor instead of the other 'out' but it does not work. (it does work with a bulb :roll: :!: :evil: )

I redid the circuit about 30 times now, I have five of those H bridges this thing still does not work. My woltage suply is 7.2 volts (it turns the motor if pluged straight in). For the logic voltage is 5 v from a voltage regulator.

So if all my mumble is very confusing, my question is how do I connect the ground pin? However do help me if you know about those H-stupid-bridges :(
Are you using L298N or L398N. I couldn't get any reference to L398N that means you have done a typo there.

Are you using same circuit as given on page 6 of L298N datasheet?
It showd pin 6, 8 & 11 should be grounded. There is a resistor between pin 1 and ground and pin 15 and ground for current sensing.
Sorry you are right its L298N, my mistake.
**broken link removed** This is where I got it from and I am using this datasheet for L298 (maybe that is the trouble? is L298N any different? The picture is labeled explicitly "1/2 L298N") **broken link removed** and this is the datasheet the HWVTech directs me to.
Yes, I am using the picture from page six, but only the pin 8 is grounded = connected to teh negative terminal of the batery. (Pins 11 and 6 are for enable of the bridges)
This thing works as its supposed to if I plug one motor pin to the output and the other to the negative (but this means that i cannot use the reverse feature :( )
:arrow: :arrow: Is the negative pin the ground? :!: :?:
I tried using a TIP31 as a switch for the negative terminal. It works with a 250 mA bulb but not with a motor (same principle, isn't it? you need voltage across the ends but also more current. The battery is a ni-cd 7.2 V battery, its definitelly not dead and it can supply way over 2.5 amps needed for the motor: it evaporated the 22 gauge wires rated for 8 when I connected the positive and negative together by mistake on my breadboard :p hee hee :lol: good that I had nothing connected from the ICs to it)
!!!HELP!!! :( :cry:
Simply put, Connect your motor to the out pins( 2 and 3........)
Make sure you got the RL in circuit too......

Make pin pin 6 Hi,( connect it to Vss in need be) logic Vss must be lower than 7.0v( make it 5v from a 7805 or similar)......then applying logic to the input pins will make the motor: stop free wheel rotate cw or ccw.

It couldnt be any easier.... :eek:
I did that connection about 30 times now, I tested it with a voltmeter and everything. I connected the Vs to positive of 7.2, ground to the negative of 7.2 batery. I connected the current sensing to the negative though a resistor ( :arrow: it also says a control circuit, what is that?) I send the 7.2 through a 5 v voltage regulator (I have an OOPic microprocessor, thats where I take the 5 v and I also tested it with a volt meter: 4.98 v) I connect these 5 v to enable and one of the inputs. If measured against negative terminal of the battery, one of the outputs outputs my 7.2 v from the batery, (so if I connect the motor from this output to the negative it turns) however if I measure the voltage across the two outputs, there is nothing)

:arrow: What is the RL circuit?
Something else: what is the diference between the positive, negative and ground terminals?
The RL circuit is just an external current limiting facility that will shut the ic down if the motor stalls, the voltage developed across RL is used to sense motor current and is fed back to some external protection logic....You need the resistor to provide a return dc path for the lower elements of the H bridge.......


I'd suggest connecting every thing up as per fig start with, leave out the motor...

Now connect pin10 to a 5v supply via say a 1k resistor( for safety...for the ic)conect pin 12 to ground. Ensure the enable pin has the correct volt level on it....Now measure between pin 13 and ground, pin 13 and Vss.
Either way you should read a voltage, now repeat with pin 14 and gnd/Vss

If you dont read a voltage at pin 13/14 wrt gnd/Vss then the chip is suspect!!.

Connet up using the other half of the chip and repeat.......

HTH Chip
I have been doing exactly that, not only with 1/2 of the chip, but I have been repeating that on three same chips. I get the same story: one out is showing voltage with respect to the negative, but nothing with the other output (the other output is no voltage, whatever you measure)

When you said ground, is it the negative terminal of the batery?
You said when you measure output with respect to other output terminal, you get nothing. That means you haven't connected any load between the terminals. Try connecting 1k resistor at the output terminals and measure voltage across the resistor. See if the polarity changes when you change the input logic.

Also, the other reason may be (it sounds stupid but may be true) that your motor only rotates in one direction, i.e. It is a type of Cassete Player Motor which only rotates in one direction as it is internaly protected by diode and voltage regulator. Check this out if it rotates in both directions by connecting it directly to the batteries.
Tried that. I am really getting a voltage diference now across the terminals, and the voltage is reversed if you change the input. (The 'ground' out pin is showing some voltage ~0.5 v) If I connect this to the motor it will not turn :? It looks like there is not enough current.
I thought that maybe its the current protection (my motor has a starting current of about 5.2 A and a free running current of 1.3 A while the L298 is rated for max of 2A constant) so I decided to try with a bulb, and still the same story :cry: :cry: :cry:
Like I don't think I am that stupid that I cannot make a chip work following a black-on-white-half-a-page diagram but it looks that way. I am getting really frustrated. Why did I like electronics? :cry: :(
By the way: I tried the motor if its one way, and yes its two way!!! at least something works
oh that current sensing on pin 1, maybe that is the problem? I have no idea what is it for and if it looks like a not-enough-current to the ground problem, maybe its that pin (it provides the pass to the ground, right?)
Do I need that RL (???) circuit?
I was going to send the picture of my circuit on the bredboard but have no idea how to atach the image
I am going to buy more components next monday, so till that time I will be still testing my IC. I did not think the current as a trouble: I have 5 of those H bridges. I have to use them for the four pumps in my robot. Sinse it does not matter which way they rotate, I could mount them all on one chip, instead I am going to use up two (sinse turning one way is not the trouble)
I was gonna wire all three ICs together giving me 6 H bridges, 2 Amps for each you get a continuous of 12 Amps. So that should not be the problem, however, maybe the switching will require more current (another transistor can always solve that)
Do you think I still should buy the descreet components?
What I do no get is why it does not work with a 250 mA bulb if current is the problem :?:
Are high school projects always this frustrating?!
There may be several reasons why your IC is not working as desired. Some of them are:
1) Even after 30 times of rebuilding your circuit you are still messing up some where in the connections.
2) The whole lot of ICs that you have bought may be faulty.
3) By mistake you have blown all the ICs while testing.
vopap said:
Are high school projects always this frustrating?!

Nop! not at all :) I always enjoyed doing my high school projects. It is lack of interest/attention/patience that makes it frustrating. Problems always arise while doing project, but instead of losing patience, you got the find the solution and keep trying until it works.
kinjalgp said:
There may be several reasons why your IC is not working as desired. Some of them are:
1) Even after 30 times of rebuilding your circuit you are still messing up some where in the connections.
2) The whole lot of ICs that you have bought may be faulty.
3) By mistake you have blown all the ICs while testing.

Thats a very happy solution!!! :lol: :cry: :?:
hey!!!found it: not enough current for the motor. Now I got a way smaller motor and it works :D :D
Have another question: I am wiring OOPic to the PC through a serial port by the way of SN75188 and SN75189. The datasheet call for a +/-12 volts. Can I use somehow serial port as power supply? I am having trouble finding the proper voltage regulator. (in the shops)

Thanks a lot for you help. This website rocks!!!
NOP! You can't use serial port as power supply. It can drive a few ICs/Transistors/LED's but not Motors. The output current is limited to few mA which is not at all sufficient for motors. Also if tried to drain too much current you'll end up blowing your port.

I can't figure out why you have trouble finding regulator IC's? 78xx is the most popular series regulator ICs. If you want 12V regulated output use 7812 regulator. It can provide constant 12@1A. If you need more current, use a series pass transistor. The scheme is given in the datsheet of 78xx.
I did not mean to use the serial port as my power supply for the motor:!: That is indeed VERY stupid thing to do. What I need to wire up is a 75188 and 75189 quadruple line driver to go from a TTL level of the OOPic microprocessor to the serial port. I already have the line drivers but I need +/-12 volts power supply to power those chips at low current

I just did not want to complicate the wiring any more with two more voltage levels. So do I use 7812 and then a voltage inverter of some kind?

Just a comment: I know some electronics.
Instead of Line Driver Chips that you have, i suggest you to use better RS232 driver i.e. MAX232 or HIN232. These operate at +5V supply. It uses four capacitors as charge pumps to boost the voltage levels at the output side. MAX232 is the most widely used RS232 driver and you'll get it very cheap. A much cheaper version of MAX232 will be HIN232 or any other 232 variant.

See Schematic of RS232 based level converter. It has two converters which means you can have 2 serial ports using one chip. THe main advantage is that it does not requite +/-12V Supply.

For quad drivers use MAX238 or two MAX232 (cheaper solution).


  • MAX232.gif
    3.3 KB · Views: 879
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips