PDA

View Full Version : replace L293



gyaanguru
07-12-2006, 12:20 PM
hey guys... i m building a wireless circuit using market available modules.....
but i need more curent.
L293 gives only 600 mA....
can anyone tell me a replacement for this Ic...
i need to give approx 2A..
plz help

lhx
07-12-2006, 03:14 PM
Use L298. It gives 2A per channel and has provision for attaching a heatsink too.

a7400
07-12-2006, 09:59 PM
dont forget to add external freewheelin diodes in l298... l293d has built in diodes... normal 1n4001 will do ...

docel
07-13-2006, 11:58 AM
....and some 'good' sense resistors...

Jabir
07-13-2006, 11:16 PM
or LMD18245 ; current= 3A

gyaanguru
07-15-2006, 02:10 PM
wat do u mean by good sense resistors??

subot_robot
07-15-2006, 07:32 PM
What will happen if we don't connect any resistor at all and connect those current sense pins to ground directly. I understand that sense resistors are used to limit current. And also how r we going to compute the value of resistance for particular current limit.

Vivek
07-15-2006, 08:36 PM
Sense resistors are not required if you dont intend to sense the current in your motors.
Connecting the sense pins directly to GND would be better in this case, as it will give your motor more voltage, and will reduce power wasted in the sense resistors.

Current sense resistor itself does-not "limit" the current of the motor, but is used to measure/sense the current in the motor.

Simply follow Ohms law to calculate the sense resistor value.
But to use the Sense resistor you will need a simple op-amp circuit with a Gain. (say G), and amplify the voltage on the sense resistor so that your CPU ADC can read it.


Example -
------------
For example consider that you have a Motor which can take peak current of 1A.

So if you use a sense resistor of Rs=1ohm,
the voltage drop across sense resistor during peak current is 1V.
This is bad, because now your motor will get 1V less.
Moreover the power wasted in sense resistor during peak is 1W. !

So a good solution is to have a very small sense resistor (and PRECISE).

Consider now that the sense resistor is 0.2ohms
The peak voltage on this at 1A, would be 0.2V.
Seems better than first value.
(also calculate the power dissipation of this, and dont use resistor with lower power rating)


Now you see that, the voltage on the sense resistor during peak current is 0.2V. This value is too small for accurate reading in our ADC range which is usually 0 to 5V.
So if you have an op-amp circuit which will amplify this Sense-Voltage (0.2V max) to say 4.5V (max) you need a Gain of 22.5.

So if you have a simple Op-Amp circuit, and if you use Resistors of 22K and 1K, you would be able to sense the current of the motor by the ADC in your Microcontroller.

When the Microcontroller sees that current has exceeded a certain limit (set by you) it could switch OFF the motor.

There are details I have skipped - for example you will need a simple RC filter to avoid the motor shutting off due to spikes of start-up etc. These things are to be compensated in the MCU software too.

You would learn and enjoy this better while experimenting yourself.

cheers!

PS: by 'good sense resistors', i think Docel meant, low value and High precesion. (Atleast 1% or better, whichever you can get). If you can't get very small values (such as 0.2ohms), you know how to make one!

goodluck!



--------------------------------------------------------------------------------
Win XVI Board: H8/3664, BHARAT
http://www.ne.jp/asahi/dream/robots/xviboard/

gyaanguru
07-16-2006, 07:51 PM
hey guys...since this is wireless project i need to build it as light as poosible & have onboard power supply(batteries) that make it heavy..... so i cant afford to waste any amount of voltage...
so can i just skip these sense resisitors.... to reduce power wastage

Vivek
07-17-2006, 06:01 AM
Yes you should skip, if you are not planning to have a feedback of Motor-Current-measurement to the CPU.

Since you did ask this question again, aftre reading my previous post, it seems that you can skip these resistors.

regards

Vivek
07-17-2006, 06:16 AM
Gyaanguru
By the way, since you are so concerned about power loss and voltage drop - I hope you have already seen the datasheet of L298 in detail.

There would be voltage drop across the high-side and low-slide transistors in any H-bridge.
If, for example, your motor current is 1A, typ, the datasheet of L298 says,
total drop across these transistors could be 2.5V approx (typ) and 3.2V(max) at 1A. (please see datasheet for details of these values)
I hope you have already tried your motor with intended battery and the L298, and verified that you get required torque/speed in final setup.