Problems with a potential divider

Discussion in 'The Projects Forum' started by MMH, Mar 7, 2013.

Feb 8, 2013
143
4
Hello there everybody!!

I have some tiny problems with a potential divider. I needed 1.8v and 120mA out of 5v 500mA supply for my project. I constructed one using a 1k resistor on one side and 1.75k resistor on another side. I got 1.85v but alas! The current dropped down to 2.8mA!!

Can you tell me any other ways to get this 1.8v 70-120mA from 5v 500mA??

2. tindel Active Member

Sep 16, 2012
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Hmm - I'm a bit confused about where you want your current and how you arrived at 2.8mA. 5V/2.75k=1.8mA Maybe a schematic would help

If you want 100mA out of your 5V supply then you need a total load of 50 ohms off of your 5V. I.E. Both of your resistors should sum to 50 ohms. Try 18ohm and 33ohm. That should give you ~1.8V and ~100mA out of your 5V supply. BTW - You'll need at least 1/2W resistors for this.

3. tubeguy Well-Known Member

Nov 3, 2012
1,157
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You could use an LM317 regulator.

MMH likes this.

Feb 8, 2013
143
4
Thanks you very much!!

What about using a 6.8ohm(1/4W) and 10 ohm(1/2W) resistors. i cant find 1/2W 6.8ohm resistor anywhere in the home.

5. tubeguy Well-Known Member

Nov 3, 2012
1,157
197
You can get a certain voltage drop through a resisitor only if the current stays exactly the same, all the time.

To calculate it use Ohms Law.
5v - 1.8v = 3.2v through the resistor.
3.2/120ma = 26.7 ohms ~ 27 ohms
Then wattage = 3.2v x 120ma = 0.38 watts.

The trouble with using a resistor is that as soon as the current changes the voltage out changes.
The same thing happens with a resistor voltage divider.

A voltage regulator like the LM317 keeps the voltage constant when the current changes.

6. DickCappels Moderator

Aug 21, 2008
2,647
632
You can only get away with using a resistive divider at that current if your load is well known and absolutely stable. Otherwise the LM317 is probably a better solution for your application.

You did not mention the kind of load you expect and how much variation in load you expect, and to what tolerance you want to keep the 1.8 volts. Knowing so can help determine whether merely using a resistive divider is a good solution.

MMH likes this.

Feb 8, 2013
143
4
Thank you very much for answering!!

I will now use the LM317 for my projest