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### Author Topic: H bridge voltage drop  (Read 1690 times)

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#### javaman

• Beginner
• Posts: 1
##### H bridge voltage drop
« on: March 18, 2013, 04:57:59 PM »
Hi,

I'm working on a motor controller using PWM. I use Arduino Mega and L293d H-Bridge as the motor driver. My Dc motors can be supplied with 6-9 v (they all are equipped with gear box). The problem appears whe I try to stall it or only working it in free rotation, it happens that the L239d H bridge has a voltage drop, for example:

I start suplying it at 6v with High and low signal for make it rotate free but I measure voltage at out pins on L293 and it shows 3.4 v (@ 0.76 A), I think it's a high voltage drop for it.

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

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: H bridge voltage drop
« Reply #1 on: March 18, 2013, 06:35:41 PM »
Is there a Voltage drop in the wires between your battery and the L293? Measure this.
Does the Battery Voltage drop with the motors stalled?

The L293 uses BiPolar transistors which will have some Voltage drop at higher current draw. Look at the Vcesat specs in the L293 data sheet. With 1Amp of current the High side transistor's will drop ~1.35V and the lower side ~1.2V or 2.55V total. So 3.4V (measured at motor) plus 2.55V = 5.95V at the L293 power input. Then there will be some Voltage drop in the wires, connectors, etc so this sounds normal.

To have less Voltage drop use an H-bridge that has power MOSFETs with a low Rds-on spec.
Or use a high battery Voltage to compensate for the H-bridge's Vdrop.

#### felipe80

• Beginner
• Posts: 5
##### Re: H bridge voltage drop
« Reply #2 on: May 16, 2013, 01:50:28 PM »
A separate supply voltage VCC1 is provided for the logic input circuits to minimize device power dissipation. Supply voltage VCC2 is used for the output circuits

#### jkerns

• Robot Overlord
• Posts: 270
##### Re: H bridge voltage drop
« Reply #3 on: May 16, 2013, 08:00:13 PM »
I start suplying it at 6v with High and low signal for make it rotate free but I measure voltage at out pins on L293 and it shows 3.4 v (@ 0.76 A), I think it's a high voltage drop for it.

How are you measuring the voltage? What is the duty cycle when you  make the measurement?

From your description, it appears that you are measuring the voltage while applying a duty cycle to the bridge - if that is true, and you are using a DC voltmeter you might see the average voltage being supplied which, at a 50% duty cycle would be about 1/2 your supply voltage.
I get paid to play with robots - can't beat that with a stick.

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