All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum


The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
Thread Tools Display Modes
Old 04-27-2011, 12:23 PM
simo_x's Avatar
simo_x simo_x is offline
Senior Member
Join Date: Dec 2010
Location: Nowhere
Posts: 164
Default H Bridge & Voltage Drop calculation.

Hi all, I have a little question about my H bridge circuit with schematics attached as jpg file.

So, I have a circuit which is the same H Bridge as the image where the PNP Darlington are TIP127 and the NPN Darlington are TIP122 but without RX, the comparator and the zener diode.

The circuit I have attached is one that I will make new for a project, where of course, a 12VDC motor will be connected to the H Bridge.
I this new circuit I have to calculate the value for VX, which will be connected to the non inverting input of the comparator, so depending on the value for RX I will have a value for VX and than I'll can determine the value for the zener diode, but I am not totally sure about how to determine this value.

Testing the real H Bridge circuit, I connected it to 12V power supply, in point A e B I measured 10,6V (Voltage drop equals for two Darlington tranistor) and I measured the current consumed by the 12VDC motor connected to it and is about 70mA.

With these values, calculating the value for VX, so, treating the situation as a normal voltage drop formula, if I am not wrong, the formula is:

VX = VAB * RX / (RX + RMotor)

VAB is the total voltage I have in output from the H Bridge and RMotor is the total resisteance of the motor, in my case 10,6V/70mA = 151,4 Ohm.

I am right, or I'm wrong and I am missing some detail?
unfortunately I don't have any potentiometer to determine it physically right now
Thank you for your help.
Attached Images
File Type: jpg HBC.jpg (67.7 KB, 28 views)

Last edited by simo_x; 04-27-2011 at 12:29 PM.
Reply With Quote
Old 04-27-2011, 02:07 PM
ifixit's Avatar
ifixit ifixit is offline
Senior Member
Join Date: Nov 2008
Location: Ontario, Canada
Posts: 595

Hi simo_x,

Start by picking an arbitrary voltage drop you can live with across RX. Usually you will want this to be as low as possible since it would subract from the motor voltage and therefore torque.

Example: if you want to keep the voltage drop less than 1 Volt then try RX=8.2Ω. Therefore, Vrx= motor current X RX = 0.07 X 8.2 = 0.574 Volts.

Use a 4.7V zener and get the detection level (0.574V) with a resistor divider.

Using this approach, you can re-calculate the values to suit your requirements.

MOSFETs would be a better choice for H-Bridge drivers as they would have much less voltage drop across them.

"Learn the rules well, so that you'll know how to break them wisely."
“The quality of an answer is directly proportional to the quality of the question.”
Reply With Quote
Old 04-27-2011, 05:42 PM
simo_x's Avatar
simo_x simo_x is offline
Senior Member
Join Date: Dec 2010
Location: Nowhere
Posts: 164

Hi ifixit, thank you very much. Now is all clear!
Reply With Quote
Reply   Post New Thread

, , ,

Related Site Pages
Section Title
Worksheet Zener diodes
Textbook Zener diodes : Diodes And Rectifiers

Similar Threads
Thread Thread Starter Forum Replies Last Post
Drop Voltage for a Constant Current Supply jpguppy The Projects Forum 11 04-27-2011 04:41 PM
Assessing an unknown transformer someonesdad General Electronics Chat 37 05-27-2010 05:22 AM
Beginners tutorials #2 designing using different transistors hobbyist General Electronics Chat 0 10-09-2009 06:50 AM
Inductor Voltage Drop mach7 Homework Help 7 04-09-2009 05:50 PM
Voltage drop Ady General Electronics Chat 5 02-15-2009 06:47 PM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT. The time now is 12:50 PM.

User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.