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PWM voltage drop in a motor controller (AT89S5 and L293 H-Bridge)

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caringin

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Hi,

My recent project is to design motor controller using PWM. I use AT89S5 and L293 H-Bridge as the motor driver. The DC motor is 12 V (DC motor with gear box). If I connect to DC motor, the voltage with 100% PWM dropped to 9.5 V. If I don't connect it to DC motor, the voltage is still 12 V.

Is it normal? Anybody can explain why? Do you have any suggestion on how to fix it?
 

Re: PWM voltage dropped

duty cycle maybe ??
or your cicruit is wrong or has a week link acting as a resistor
if the motor draws more than 1 amp thin wires will drop 1 volt or more per foot

lots of reason post a schematic
 

    C

    caringin

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PWM voltage dropped

I think this voltage loss is due to transistors (bridge) drop out, in L293.
For prove you can see L293 data sheet.

for this reason you can use 13.5-14 volt insted of 12V to get 12 volt
maximum at output (motor terminals).

Regards,
Davood.
 

    C

    caringin

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Re: PWM voltage dropped

Hi,

Do you know how to calculate the voltage of DC motor with PWM duty cycle 50%, 75%, 100%? For example, duty cycle 50% of VIN=12V is 6V, is this correct?

I think it is due to impedance matching. I measure the resistance between OUT1 and OUT2 without connecting it to DC motor and the resistance is about 45 Ohm. I also measure the resistance of DC motor (not connected to anything), the resistance is about 30 Ohm. When I connect DC motor to OUT1 and OUT2 (just like in the picture below), the resistance is about 27 Ohm. That's my opinion, what do you think?

Here is the schematic:
 

Re: PWM voltage dropped

Dear caringin

Output average voltage is
VDC = (1/T) ∫ V(t) dt. from 0 - T
And it is ( for PWM) equal to Vin * [Ton/(Ton+Toff)]
It means for 25% duty cycle you get 25%*12V = 3V
And for 50% duty cycle you get 50%*12V = 6V and so on.

you can not measure output impedance (or resistance) by multimeter.
because it is active circuit which include transistor and other active component.
In the case of motor anything you measure is armature and/or stator resistance,
DC motor behavior (current consumption) is subject to another parameter, for example revolution number.

Regards
Davood

Added after 14 minutes:

you can download L293 data from **broken link removed**

and as you can see in datasheet, output voltage high and down.
we have typically 1.4V and 1.2V Vsat (saturation) for high and low outputs.
for 600mA output current.
If output current higher, voltage dropout is much and vice versa.
 

    C

    caringin

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Re: PWM voltage dropped

Hi,

Changing 12V up to 15V is my last alternative. But, the reason why the voltage dropped to 9,5-10V when the DC motor is connected is a big question for me. The wire is fine, none is acting as resistor I guess.

A friend told me that calculating VDC is the average of VIN. For example, for VIN = 12V, duty cycle 50%, the VDC is= 12×√½=8.49V, is it true?
 

PWM voltage dropped

No, 50% for PWM is 0.5*Vmax(eg.12) = (6)
as you can see in data sheet :
when you connect motor to output terminal, current will produce dropout.
it will be 12-(1.2+1.4)= 9.4V for 100% (at 600mA)
and 9.4 *50%= 4.7V for 50% duty cycle
 

    C

    caringin

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Re: PWM voltage dropped

Hi,

I guess you're right, but with or without DC motor isn't it the same voltage between OUT1 and OUT2 if using the calculation you gave? The actual is different, without DC motor, PWM 100% is 12V. With DC motor is 9.5V. How can you explain this? Do you have any schematic of H-bridge (real circuit inside the IC)? I am so confused of the block diagram on the first page of L293 Datasheet.
 

PWM voltage dropped

caringin,

if you read the third line of Davood Amerion's last post, you will see the answer (you have some voltage drop across the output transisters when you draw current = 600 mA).

jonathan
 

    C

    caringin

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Re: PWM voltage dropped

Hi,

I am sorry, I didn't notice that. The datasheet said

600mA OUTPUT CURRENT CAPABILITY
PER CHANNEL

Does it mean that 600mA is fixed? I measure the current on OUT1 and the current is only 70mA.
 

PWM voltage dropped

It means maximum current not should be grater than 600mA.
Any value below 600mA (eg. 70mA) is alowed.
 

    C

    caringin

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Re: PWM voltage dropped

600mA OUTPUT CURRENT CAPABILITY = the IC can drive a 600mA current continuously.

The maxium current always go with a peak time.
 

    C

    caringin

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Re: PWM voltage dropped

There will be some voltage drop in the bridge transistors, but 2.5V seems rather high.
Can you check that the "12V" actually stays at 12V? My guess is that your supply voltage drops because of the load.
 

    C

    caringin

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Re: PWM voltage dropped

Hi,

What is the different if I use PWM 1kHz 75% and 100 Hz 75%? Is it true that the voltage is different? Or maybe the energy supplied to motor DC is different? Any comment?
 

PWM voltage dropped

Average voltage are the same in both case.
if you increase switching frequency of PWM, switching loss of bridge increased.
and if you decrease switching frequency, ripple current in motor increasd and it may produce power loss.
Switching frequency depend on motor inductance (in case of you dont use of seperate LC network).
I think 100Hz & 1 KHz (both of them) are good for your project.
 

    C

    caringin

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Re: PWM voltage dropped

Hi Davood,

Why if I calculate PWM voltage, I use V average? Why not using V rms? Do you have any reason about it?
 

PWM voltage dropped

because output DC voltage is important.
and another reason is output waveform produce DC current.
For DC values rms=average [if voltage is filterd and fixed].
for better discussion, consider use LC filter in output.
 

    C

    caringin

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