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### Topic: Dropping Amperage (Read 2957 times)previous topic - next topic

#### llhetlet

##### Dec 10, 2011, 04:59 am
I apologize for my ignorance here, but I'm trying to remedy that.

I have a component that needs 3ma.  Currently it's getting 38ma.  I am supplying 9v to the circuit, and it need 5 volts and 3mA.

So applying OHMS law, I subtract the voltage needed (5v) from what's supplied, leaving 4v, and then divide that by the amperage I desire (.003) leaving me with 1333 ohms.

So I threw in a 1K ohm resister and measured everything.  The battery supply is a little low, so I get my 3ma, but only about 2.5v.  If I slip in a 200 ohm resister instead, I get about 5v, but 38ma.

I'm clearly missing something.  Any pointers???

#### cmiyc

#1
##### Dec 10, 2011, 05:05 am
What is the device?
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### llhetlet

#2
##### Dec 10, 2011, 05:22 am
It's a sonar range finder.  http://www.maxbotix.com/documents/MB1010_Datasheet.pdf

I do have other components in the circuit as well, I'm figuring that might be part of my problem???

#### cmiyc

#3
##### Dec 10, 2011, 05:36 am
First, you don't "drop" amperage.  A source can have a very high current capability and the devices connected will only draw from that source what they did.

What is confusing is that you say you have a 9V source and your sonar device is 5V.  How are dropping the voltage from 9V to 5V?  Are you using a regulator?  You can't just use a resistor.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### llhetlet

#4
##### Dec 10, 2011, 05:43 am

First, you don't "drop" amperage.  A source can have a very high current capability and the devices connected will only draw from that source what they did.

What is confusing is that you say you have a 9V source and your sonar device is 5V.  How are dropping the voltage from 9V to 5V?  Are you using a regulator?  You can't just use a resistor.

I apologize for any confusion.  The power source is 9v, the sonar device operates on 5v, so don't I need a resistor to take 4v away from the circuit?

#### cmiyc

#5
##### Dec 10, 2011, 06:07 am
You can't use a resistor to drop voltage for an active device.  You need to use a voltage regulator like a LM7805.  It will regulate 9V down to 5V.

The problem with a resistor is that the voltage drop will depend on the current draw.  Active devices have changing current draws.  A regulator will maintain 5V regardless of current draw.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### llhetlet

#6
##### Dec 10, 2011, 06:42 am

You can't use a resistor to drop voltage for an active device.  You need to use a voltage regulator like a LM7805.  It will regulate 9V down to 5V.

The problem with a resistor is that the voltage drop will depend on the current draw.  Active devices have changing current draws.  A regulator will maintain 5V regardless of current draw.

Okay, that makes sense.  Looks like a trip to Radio Shack tomorrow.

Thanks, James, for helping a total rookie as myself.  Do you have any recommended sites or books I could benefit from?

Lance

#### cmiyc

#7
##### Dec 10, 2011, 07:16 am

Actually, arduino's Playground section has some good information.  Not always the best, but at the least a starting point for most things.

http://www.arduino.cc/playground/
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### cr0sh

#8
##### Dec 10, 2011, 09:05 am
Do you have any recommended sites or books I could benefit from?

You want to be serious about electronics? Pick up a copy of Grob's Basic Electronics (any edition in the last 20 years will do). Read it, understand it, do the math. Then read it some more. Then do some more math. Yes, electronics - if done right - relies on more than a bit of math. Get used to it. Or burn up parts (and money) - your choice...
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

#### llhetlet

#9
##### Dec 10, 2011, 10:28 am

Do you have any recommended sites or books I could benefit from?

You want to be serious about electronics? Pick up a copy of Grob's Basic Electronics (any edition in the last 20 years will do). Read it, understand it, do the math. Then read it some more. Then do some more math. Yes, electronics - if done right - relies on more than a bit of math. Get used to it. Or burn up parts (and money) - your choice...

Thanks cr0sh, I'll look for that book.   Thankfully, math is a strong point of mine, so no fear there.

#### pwillard

#10
##### Dec 10, 2011, 04:19 pm
A copy of "The Art Of Electronics" is also a good way to learn.

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