Making a Voltmeter into a Current meter

imix500 · 13th July 2006, 07:42 PM

Hello all, I would like to use one of those cheap single board lcd meters to measure DC current. The apparent problem is that the meter has a 200mV input. The current shunt is 50mV. Is there a way to scale the meter down? Thanks!

anatech · 13th July 2006, 07:55 PM

Hi imix500,
I think 200 mV FS is the basic sensitivity for those chips. You need to build a DC amp. DC drift may be a problem unless you are careful with your integrating cap type and op amp. Leakage paths need attention too.

-Chris

imix500 · 13th July 2006, 09:50 PM

The dc amp seems correct, I wish I could find a 200mV shunt.

Hmm, I found an article that suggests that because the a/d convertor needs no current for measurement, full scale input isn't needed. A sample schematic shows a 50mV shunt resistor feeding a 200mV meter. Here is the page:
http://sound.westhost.com/articles/meters.htm

Does this sound right?

poobah · 13th July 2006, 09:57 PM

What is the maximum level of DC current you wish to measure?

There are several sources for many types of shunts... a shunt is merely a precise resistor with 4 contacts.

For maximum accuracy, you should use the full scale that is available to you. Unless, of course, this gives you some really weird conversion factor for your displayed value.

anatech · 13th July 2006, 09:58 PM

Hi imix500,
All you really need is a resistance that scales your FS current to a convenient range with the meter if you are willing to use a different shunt. You can use less than the full scale keeping in mind that you may possible lose some resolution - or not. If you want 5A FS, your 50 mV shunt is correct the way it is.

A four terminal resistor is the best, make your sense traces come off so you don't share any part of the trace with the load current.

To start, what current do you want to measure?

-Chris

poobah · 13th July 2006, 09:59 PM

Ha ha... beat ya bud...

anatech · 13th July 2006, 10:03 PM

Hey quicks draw!

I was thinking pie as a correction factor, you ?

poobah · 13th July 2006, 10:06 PM

I actually like e better than pi... unless it is warm raspberry with good vanilla ice cream.

anatech · 13th July 2006, 10:08 PM

I was trying to keep it easy. 1 to the power of infinity takes too long to figure out.

imix500 · 13th July 2006, 11:26 PM

You guys crack me up. I was thinking a nice round number like... 50A. it's for a water cooled test rig for power supplies.

13th July 2006, 07:42 PM	#1
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Putnam, NY	Making a Voltmeter into a Current meter Hello all, I would like to use one of those cheap single board lcd meters to measure DC current. The apparent problem is that the meter has a 200mV input. The current shunt is 50mV. Is there a way to scale the meter down? Thanks!

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
how to make this esr meter work with 250uA Panel meter	dytln_02	Everything Else	2	27th January 2007 11:13 PM
Making a milliAmp meter, become a Volt meter	redrabbit	Power Supplies	11	26th May 2006 08:51 PM
Measuring current with an LCD meter	Matttcattt	Parts	10	12th May 2004 01:13 AM
Making LED voltmeter	Dima	Equipment & Tools	5	3rd August 2002 02:11 PM
Making series Vreg to sink current	KasperM	Solid State	1	30th November 2001 07:30 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

13th July 2006, 07:55 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi imix500, I think 200 mV FS is the basic sensitivity for those chips. You need to build a DC amp. DC drift may be a problem unless you are careful with your integrating cap type and op amp. Leakage paths need attention too. -Chris

13th July 2006, 09:50 PM	#3
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Putnam, NY	The dc amp seems correct, I wish I could find a 200mV shunt. Hmm, I found an article that suggests that because the a/d convertor needs no current for measurement, full scale input isn't needed. A sample schematic shows a 50mV shunt resistor feeding a 200mV meter. Here is the page: http://sound.westhost.com/articles/meters.htm Does this sound right?

13th July 2006, 09:57 PM	#4
poobah diyAudio Member Join Date: Nov 2005	What is the maximum level of DC current you wish to measure? There are several sources for many types of shunts... a shunt is merely a precise resistor with 4 contacts. For maximum accuracy, you should use the full scale that is available to you. Unless, of course, this gives you some really weird conversion factor for your displayed value.

13th July 2006, 09:58 PM	#5
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi imix500, All you really need is a resistance that scales your FS current to a convenient range with the meter if you are willing to use a different shunt. You can use less than the full scale keeping in mind that you may possible lose some resolution - or not. If you want 5A FS, your 50 mV shunt is correct the way it is. A four terminal resistor is the best, make your sense traces come off so you don't share any part of the trace with the load current. To start, what current do you want to measure? -Chris

13th July 2006, 09:59 PM	#6
poobah diyAudio Member Join Date: Nov 2005	Ha ha... beat ya bud...

13th July 2006, 10:03 PM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hey quicks draw! I was thinking pie as a correction factor, you ?

13th July 2006, 10:06 PM	#8
poobah diyAudio Member Join Date: Nov 2005	I actually like e better than pi... unless it is warm raspberry with good vanilla ice cream.

13th July 2006, 10:08 PM	#9
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	I was trying to keep it easy. 1 to the power of infinity takes too long to figure out.

13th July 2006, 11:26 PM	#10
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Putnam, NY	You guys crack me up. I was thinking a nice round number like... 50A. it's for a water cooled test rig for power supplies.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)