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Hi
I made a small car using 4 DC motors ( 12 V, 300 rpm ). The motors on the left ar connected in parallel and same for the right. Im using differential steering.

For the supply :- Im using an 18 V, 3 Amp transformer. DC regulation is done by LM 317. I vary the voltage for speed control.

Since the single LM 317 was specified for 1.5 Amps only... I connected 3 LM317's in parallel with input given to all 3, their adjust common, and then their output combined together again. My thought is that the current will get divided among them. ( but i feel something is wrong, even thought it works ).

Now i have learnt that controlling DC motors by changing voltage is highly inefficient and gives problems with back emf. I want to improve on this.

1. The supply ( want it to be as small as possible )
2. Protection of motors from back emf using simple components like diodes & capacitors. ( Also could someone explain back emf in simple terms )
3. I would like to control the car through computer programming.
4. How can i make simple RF control ckt for it.

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Hi,

Try using only one LM317 and a “by pass” power transistor instead of paralleling if you are going to control the motors by voltage.
However changing the voltage is not the best way to control motors, PWM is a better way. ;)

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Which transistor should i select ? I know there is a ckt given but what if the transistor mentioned is not available.

What damage is done by using 3 LM317's??

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Hi Siddarth,
There are many transistors with the same or better spec's than the old one used in the datasheet. If overheating isn't a problem then you could use the LM350 for 3A or the LM338 for 5A without a booster transistor.

Voltage regulator ICs each have a slightly different output voltage. If you parallel them then the one with the highest voltage will give the load as much current as it can, and the other regulators will do nothing or give the remainder of the current required by the load. So one regulator gets very hot.

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The LM350 or the LM338 are alternatives but they are very expensive compared to one LM317 and one power transistor if money is an object!? Not any of the regulators are needed if PWM is used though. 8)

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2. Protection of motors from back emf using simple components like diodes & capacitors. ( Also could someone explain back emf in simple terms )




dcseries.jpg

Ok I did this pic in Matlab.  It's a simulation of a Series DC Motor.  Now the red line which I hand drew (imagine it as a smooth curve) would represent the Torque over speed curve. 

To start off with this is how I understand it and how it was represented to me so take it with a grain of salt.  I'm in my last year of a BSEE curriculum.  Anyways the original pic is as labeled armature current over time.  This is from starting position to synchronous speed ( I did this awhile ago for a class).  Pay close attention to the armature current at the very beginning.  See that spike well that's when there is no back emf.  The armature current can't be that high for long or it would create heat and probably short motor windings.  When the armature of the motor starts to turn it creates a voltage that has an opposite polarity (back emf) with respect to the power supply.  As speed increases the back emf increases which reduces the potential seen by the motors windings (power supply minus back emf).  Which also reduces the current going through the windings as with the torque.  Torque is proportional to current as speed is proportional to voltage (red curve).  Anyways back emf is good.  When the load on the motor increases the speed drops, back emf drops, torque goes up and motor speeds back up (with in reason).  Any ways one concern is that controlling the motor by voltage will increase the current (red curve).  Remember decrease the voltage/speed increase the torque/current and current creates heat.  Anyways I know this is all over the place, but I hope it is understandable.  I got most of this from memory I didn't refer to my books so there may be an error here for anyone wanting to pick it apart.  ;D  Also if it's confusing let me know.

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Audioguru, so if i use one LM317 with a heatsink can it provide the needed current of 3A? ( Where could i get samples of power transistor or LM338. Texas doesnt have any regulator as sample which can supply about 3A )


wvengineer, thanx got the basic idea. But u havent mentioned how to avoid back emf using simple components.

Now can the bellow ckt work.

Earlier i used switchs that would change the polarity and thus steer the car. But in this case while steering one motor used to remain OFF. Hence the turning radius was quite small ( The centre would be the rear wheel which was OFF )

In the ckt bellow i do not completely switch OFF the motor on one side but make it run a bit slower than the one on the other side. By using a variable resistor i can
do so ( RESISTOR R4 ).

post-1256-14279142644214_thumb.jpg

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An LM317 is rated at 1.0A to 1.5A in a TO-220 case that can be bolted to a heatsink. Since the samples you ordered from TI are rated at only 500mA then they are probably in a tiny surface-mount case that can't be fastened to a heatsink.
If your morors draw enough current that the LM317s need heatsinks and current-boosting power transistors then you need them in a TO-220 case.

You need to measure the motors' current to calculate how much heatsinking the regulators and booster transistors need. A motor draws it max current when starting and when working hard.

Worry about back EMF only if the car is coasting downhill and the motors become generators. A high current diode between the regulator and the motor will prevent back EMF from damaging the regulator.

I corrected errors on your schematic.

post-1706-14279142644325_thumb.jpg

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The value of resistors that i selected are :-

R1 = 746 Ohms( Adjusted by preset )
R2 = 10k Ohms Potentiometer
R3 = 1.6k Ohms( Adjusted by preset )
R4 = 42.88k Ohms Potentiometer ( The resistance was measured using multimeter ).
R5 = 1.6k Ohms( Adjusted by preset )

( Output voltage is set to be Max. 18 Volts )
Now i took half of R4 ie 21.44k ohms and calculated R3 for that. And R3 must equal R4. Therefore when the R4's resistance is equally divided on both sides the voltage at the remaining 2 regulators will be the same.

Now the regulator (1) was getting heated up a lot, but when i took the output only from the regulator(1) and not from remaing regulators. This output was used in my earlier switching ckt ( there are two rocker switchs which i switch ON/OFF to run the motors ) and now it dosnt heat up.

Whts going on ?



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Hi siddharth, seems u are a bit confused with the lm317 u need. u require an LM317T (the T prefix specifies that the device has a current rating of 1.5A). a normal LM317 has a rating of 1A. Since these devices are in the TO-220 package they can be easily heat sinked. but still u cannot get a cuurent output exceding 1.5A. use a bypass transistor  as suggested for quite high cuurents u can use the old 2N3055 which comes in the TO-2 package ( like earlier power transistors) with a maximum current of 15A but a TO-220 package is also available but it handles lower currents. or u can use a TIP42 from texas which can handle currents upto 4A and is similar to the TO-220  Package. i suggest u use a PWM control as it is more efficient. use a Power Mosfet for PWM control (like IRF540). as for the turning problem, to make your turn to a side just stop the rear motor of that side and the car will swing on the rear wheel of that side using the wheel as a pivot, like if u need to turn left just stop the left rear wheel and the car will swing around using the rear left wheel as the pivot point.

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