L293N Motor running slow or not at all

[Ubergeek63]

ditch the idiotic L293 in favor of a FET IC!

 

[Lyncos]

ditch the idiotic L293 in favor of a FET IC!

You have one in particular to recommend for dual 600mA motors ?

 

[audioguru]

An L293E is fine if you understand the unwritten details.
Maybe the Italian datasheet from ST Micro has all the details.

Most other manufacturers write their datasheets in clear detailed English.
Even their graphs are clear.

 

[Ubergeek63]

You have one in particular to recommend for dual 600mA motors ?

**broken link removed** will handle a 2A motor

Dan

 

[audioguru]

Each bridge in an L298 drives up to 2A continuously.

 

[Ubergeek63]

Each bridge in an L298 drives up to 2A continuously.

for a total dissipation of 12W that you have to somehow get rid of and REQUIRING freewheeling schottkys.

While there are better ones on that site, that particular one yields 6W dissipation to get rid of and incorporates synchronous rectification eliminating the need for the schottkys.

Dan

 

[audioguru]

for a total dissipation of 12W that you have to somehow get rid of .

That is why it has a case that can attach to a heatsink.

 

[Lyncos]

Thanks for your help guys my 2 motors are now running with a 1Ohm sense resistor... Now I will try to read the voltage drop and get the motor stop when the current is too high.... but I have no Idea how to read it... I get +/- 0.50 volts voltage drop to my resistor... Any idea how to read it from my mcu ?

Maybe I'm better to start a new thread on this ?

Thanks a lot for the help

 

[Sceadwian]

If you just want an upper current limit you're better off using a comparator from a fixed voltage refrence, it will respond way faster than a micro controller can. If you need to be able to read the current from the MCU anyways you need to amplify it a bit and send it into the ADC port of your micro controller. You could read the .5 volts (positive at least) directly but you're not going to get much resolution. What is your current limit?

 

[Lyncos]

If you just want an upper current limit you're better off using a comparator from a fixed voltage refrence, it will respond way faster than a micro controller can. If you need to be able to read the current from the MCU anyways you need to amplify it a bit and send it into the ADC port of your micro controller. You could read the .5 volts (positive at least) directly but you're not going to get much resolution. What is your current limit?

My Max voltage will be 24 Volts and i should not go over 1.2 A or .600mA .. i'm not sure for now.. is it possible to have something adjustable ?

With a 1 ohm resistor I get Voltage drop of about .50 volts at about 630mA

I'm pretty noob with these comparator... I already have an LM239N chip i could use...

 

[Sceadwian]

If the resistor is 1ohm you should get a voltage drop of .6 volts at 600ma. Basically one tenth of a volt per 100ma. .5 volts at 630ma is either your meter, or more than likley the exact resistance of the resistor. If your readings are accurate your 1 ohm resistor is actually .78 ohms. 20% tolerance, typical for a generic resistor.
You would simply feed the comparator a refrence voltage for your cutoff and have it trigger the circuit to shut off.

 

[Lyncos]

If the resistor is 1ohm you should get a voltage drop of .6 volts at 600ma. Basically one tenth of a volt per 100ma. .5 volts at 630ma is either your meter, or more than likley the exact resistance of the resistor. If your readings are accurate your 1 ohm resistor is actually .78 ohms. 20% tolerance, typical for a generic resistor.
You would simply feed the comparator a refrence voltage for your cutoff and have it trigger the circuit to shut off.

Should I use the Enable Pin of the L293 to shut it off ? and if it is the case how I would put that pin to ground with the when the current is too hight with LM239 ?

As I said i'm really noob with these kind of circuits a example schematic would be welcome ..

Thanks

 

[Lyncos]

I suppose I would need a logic inverter ?

 

[Lyncos]

If the resistor is 1ohm you should get a voltage drop of .6 volts at 600ma. Basically one tenth of a volt per 100ma. .5 volts at 630ma is either your meter, or more than likley the exact resistance of the resistor. If your readings are accurate your 1 ohm resistor is actually .78 ohms. 20% tolerance, typical for a generic resistor.
You would simply feed the comparator a refrence voltage for your cutoff and have it trigger the circuit to shut off.

My Resistor is .95 Ohm .. when I read It i get 1.18 and when i test the probes of my meter it say I have 0.23 ohm so I suppose my meter is offset about 0.23 ohms.

 

[shokjok]

Most of the voltage loss is in the diodes. The bridge could have a shorted diode, robbing the motor of a power source. The diode bridge allows a fractional voltage to the motor. Eliminating this diode bridge should improve performance.

 

[audioguru]

The diode bridge is not in series with the motor, it is parallel and is normally reverse-biased and does nothing. It conducts when the flyback voltage spike from the motor exceeds the supply voltage. Without the diodes then the very high flyback voltage destroys the bridge.

Hold both terminals of a motor in one hand and apply a battery to power the motor. When the battery is disconnected then your hand will feel the flyback high voltage spike.

 

[Lyncos]

Any 1 can help me to have an LM339 stopping the motor when I get 1.2 amps ? I read the LM339 datasheet but i'm a little lost.. I would like to drive Logic with it but would like to input 12volts from another source and have no Idea how to do it.

 

[Sceadwian]

Lyncos, the meter doesn't test the resistor under load. If your voltage is accurately measured at .5 volts and the current is also accurately measured at 630ma then the resistance under those test conditions is .795 ohms, you can't violate Ohm's law =) But you have to factor in the accuracy of your meter on current and voltage measurement, and you may be changing something you're not aware of.

Since you're using an h-bridge the current will reverse itself so you need an opamp setup to measure absolute voltage. I've seen a few opamp cookbooks with examples but I've never tried to do it myself so I acn't suggest a circuit directly, just what to look for. Since you have to use an opamp anyways you can amplifier the signal to make it a little easier fro the MCU or a comparator to read.

 

[Lyncos]

Lyncos, the meter doesn't test the resistor under load. If your voltage is accurately measured at .5 volts and the current is also accurately measured at 630ma then the resistance under those test conditions is .795 ohms, you can't violate Ohm's law =) But you have to factor in the accuracy of your meter on current and voltage measurement, and you may be changing something you're not aware of.

Since you're using an h-bridge the current will reverse itself so you need an opamp setup to measure absolute voltage. I've seen a few opamp cookbooks with examples but I've never tried to do it myself so I acn't suggest a circuit directly, just what to look for. Since you have to use an opamp anyways you can amplifier the signal to make it a little easier fro the MCU or a comparator to read.

My reading is done on the Sense pin wich is connected to ground thru a 1ohm resistor.. now what i would need is a voltage divider to get 1.2 Volts into my lm239 comparator.....

but i cannot find a resistor pair to acheive this...