# step down 10v to 5v with a resistor?

Discussion in 'General Electronics Discussion' started by qweets, Oct 22, 2010.

1. ### qweets

48
0
Sep 26, 2010
hello : ) i wonder if anyone can help?

i have a 15v ac /dc psu powering my circuit.

a 10v regulator running off that which I use to power a motor.

a H bridge that needs 5v to work, so at the moment i am using a 5v
regulator running off the 10v regulator to make that work nicely.

Everything works fine, however i am trying to make my circuit smaller,
so i wonder, instead of a 5 v regulator, can i use a resistor off the 10v regulator to power my 5v H bridge? I have a SN754410 H bridge, but i cannot see how many ma this h - bridge uses on its data sheet
http://www.datasheetcatalog.com/datasheets_pdf/S/N/7/5/SN754410.shtml

So I wonder what i might need to do to calculate the resistor value?

thanks for your help : )

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,849
Jan 21, 2010

The long answer is: Maybe, but it probably won't work.

3. ### bean

4
0
Oct 21, 2010
I know VMP3201 can convert input voltage from 5.0V-24.0V to 1.0V-9.0V(fixed output), current is up to 1.2A. I don't know whether it is what you need.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,849
Jan 21, 2010
He is already using a 5V regulator.

5. ### barathbushan

223
0
Sep 26, 2009
you might be able to do it , you must know the total current drawn by your h-bridge , you must choose a resistor such that , the current drawn multiplied by the chosen resistor , drops the unwanted voltage (in your case if you want 10v from 15v , then 5v must be dropped )

but remember if the current drawn varies , then the IR drop across the chosen resistor varies too , hence you might not be able to get the constant voltage .

i guess the best option is to use a regulator , i dont see any massive difference between the size of a lm713 and a resistor

6. ### ChosunOne

444
137
Jun 20, 2010
What Steve said.

A slightly longer answer is that you might actually get a resistor to work but at the very least you're risking a drastic reduction in the lifespan of your motor. The arrangement could have it operating at currents it wasn't designed for, for small increments of time, but many, many of those small increments.

If you really need a description of what I'm talking about, we'd need more information on your motor. For instance, is the motor designed to run at constant speed? What's the rated RPM? What does it actually do? (e.g. magnetic tape drive? cooling fan?)

If you need to understand why it's a Bad Idea, get clear on the difference between impedance and resistance. Your motor has a very low resistance. At a constant voltage (which no resistor is going to give you), the current through your motor is governed by its impedance, which is varied by the speed of the motor, which is determined by a balance between its torque (force) and the mechanical load the motor is carrying.

With a resistor & motor in series instead of a constant voltage, you have the peculiar and counterintuitive situation of decreased impedance causing a voltage drop to the motor with an increase in current---Not how it was designed to operate.

7. ### davennModerator

13,984
2,015
Sep 5, 2009

So, why dont you use surface mount versions of the 10V and 5V regulators
you will instantly cut the layout size in half

Dave