Help on Ampere Meter using LM3915

[aq_mishu]

lm3915

Can some body help me in designing an ampere meter capable of measuring 5A max (shunt will be a 5w, 0.1ohm resistor) ampere meter using the LM3915/LM3914?? please....

 

[horzcat]

using lm3915

Use current divider circuit to limit the current to a few hundred milli-amps. Then use a current to voltage converter circuit using any op-amp. For reference check coughlin's book on op amps. Use the voltage output to drive a sereis of window detectors. These windows will represent the voltage levels and thus your original current levels.

 

[Audioguru]

lm3915 volt meter

The LM3915 has logarithmic steps. You should use an LM3914 because it has linear steps.

With a 0.1 ohm shunt, 5a produces 0.5VDC. The lowest LED for the LM3914 will light at an input ov only 0.05V which will not be accurate due to the input offset voltage error of 10mV max.

Use an input amplifier using an opamp with a very low input offset voltage if accuacy is important. Them the LM3914 can have a max input of 5V or 10v and its input offset error will be much less.

 

[aq_mishu]

amplifier with lm3915

so if a .1ohm produces 0.5v drop, i can make it to a 10V and then i can use the LM3914 for the bar graph right?? and the voltage amplify should be made of an opamp like 741/1458?? if so, then i'll also have to manage a negative supply for the same... right?? Also let me know what will be the drop in case of a .01ohm 10A ??
Also if i use 3915 (i have many of them surplus) it will only give me log output.. will it be a great problem?? or acceptable?? I am trying to make a Batt changer for LEAD Achid batt and also a PSU of 10A for VHF Tcvr. and i need to know the current. But not exactly the exact value... just for an idea.. a metering..

 

[Siniša]

lm3915 lm358

For amplification you could use LM358 that is fairly common. In offset of 3mV(typ to 7mV(max) might be acceptable for your purpose. Mind that in your area there are probably amplifiers available that are better performers in terms of offset for same or less money. Look for "single supply" amplifiers. All you care that input voltage range goes down to 0 and offset is acceptable. 741 and 1458 are not good choices if only single power supply is available.

 

[Audioguru]

pin connection lm3915

The first LED lights when the input to an LM3915 is at 1/23rd of the input that lights the 10th LED.
Its steps are logarithmic so when the input voltage is at half of the voltage that lights the 10th LED then the 8th LED lights.

The first LED lights when the input to an LM3914 is at 1/10th of the input that lights the 10th LED.
When the input voltage if at half of the voltage that lights the 10th LED on an LM3014 then the 5th LED lights.

 

[aq_mishu]

price of lm3915

seems that 3914 is better in that case... Well, so i should use 358 instead of a 741 due to single supply right?? Fine... 0.5V should be 5V after amplification right?? the gain is 10. (.5A for each LED). But in 3914, the signal input is pin 5 and pin2 where pin2 is grounded. But i can't measure against ground. It is the voltage across the shunt. Then what will be wiring?? In amplifier and in 3914 both... Guys, i'm getting confused.... plz help..

What will be the voltage drop across shunt of 0.01ohm incase of 10A??

Also advice on using another meter for 10A application, or even 5A application, should i build a DVM for range of 2V full scale using the 7107???

I also have surplus LM324. Can I use that for the single supply application? because I'll need total 2 op-amp. one for gain 10 (inverting config) and another for unity gain in inverting config. Price of LM324 is same as 1458 in my country.

 

[Audioguru]

lm3915 19 led

I recommended using an opamp with a low input offset voltage. Since you are not using one that has a low input offset voltage then the opamp is not required.

If the max input voltage to the LM3914 is 0.5V then set the voltage reference of the LM3914 to 0.5V and live with whatever amount of input offset voltage it has.

 

[aq_mishu]

10 led volt meter lm3915

well... itn't it becoming an instrumentation amp? also a difference amp config across the shunt right?? I think for accuracy, it should be amplified to 5V from 0.5V and then it should be fed into the 3914. Now, for this, i simply utilize a difference amp config with the gain I require and I think i can use the lm324 in this case for single supply. If i am right, let me know.. if wrong, also let me know plz...
mean while, for digital display, what should be the volt meter for 10A?? 0-2V fullscale?

 

[Audioguru]

lm3914 application circuit

Ordinary opamps and the LM3914 have an input offset error of up to 10mV.
There are some opamps that have their inputs laser-trimmed so that they have a very low input offset voltage.

 

[aq_mishu]

lm3915 problem

guys, i was in india and was kinda busy. anyways... starting to work on it... i'll let you know...