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sco77ie
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Join Date: Nov 2007
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LM3914 IC runnning from variable current souce

11/26/2007 5:06 AM

hey, ive created an LED bar graph using an LM3914 driver, it measures from 0 - 2.4V and when i use a potentiometer to simulate this it works fine. the problem is i need to run it off an output that would normally control an hydraulic solenoid, we can control it to put out 200mA - 700mA, when i run that current through a 3.3 ohm resistor and measure the voltage dropped it is within the range i want, but upon hooking the signal wires across the resistor the system wont work, can anyone give me some idea's as why it wouldn't work when it runs fine off the potentiometer ????
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JohnDG
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#1

Re: LM3914 IC runnning from variable current souce

11/26/2007 5:34 AM

It depends how you're powering the meter circuit and how you're connecting to your measuring resistor.

If the LM3914 is 'floating' (e.g. supplied by a battery), you should connect the 0V side of the supply to the lower voltage side of the resistor, and the input of the LM3914 to the higher voltage side.

If the supply isn't floating, you may need to buffer the signal. Come back with more detail if it still doesn't work.

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sco77ie
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#2

Re: LM3914 IC runnning from variable current souce

11/26/2007 6:34 AM

i am supplying it with Vref from the system which is a 5V supply, i then have the signal wire connected to the positive side of the resistor and common ground going to the negative side. here is a link to the datasheet for the LM3914, i have built and identical circuit to the 20 segement one with mode selector for bar or dot but i have left out the switch and wired it up for bar mode.
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JohnDG
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#3
In reply to #2

Re: LM3914 IC runnning from variable current souce

11/26/2007 7:14 AM

1) Were you supplying it from the system Vref when testing with the pot?

2) Can you put a meter between the LM3914 input & 0V (with the system signal as I/P) and measure the right sort of voltage swing?

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Andy Germany
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#4

Re: LM3914 IC runnning from variable current souce

11/27/2007 3:45 AM

It sounds as though at the voltage you have got from the supply, there is not enough current available to maintain that voltage as a reasonable constant value, therefore the chip gets unhappy with having its voltage jerked about....

Measure your constant current voltage and see what you have over a period of time....add a diode and a reservoir capacitor for the 3914. That might work....

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sco77ie
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#5
In reply to #4

Re: LM3914 IC runnning from variable current souce

11/27/2007 5:31 AM

thanks for the advice, would a 47 microfarad electrolytic capacitor be alright u think? if not what would u recommend or how would i be able to work it out myself?
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Andy Germany
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#6
In reply to #5

Re: LM3914 IC runnning from variable current souce

11/27/2007 6:18 AM

Start there, but as I have no information as to the impedance of your current source, or the actual voltages, its a case of "Suck it and see!" - that was the answer from the parent when a child asked how a particular sweet tasted!

If the voltages are very low (highly probable!) then you may need some sort of DC to DC converter to get a usable voltage.......complicated as I personally have never had occasion to use one.....you may need some sort of Zener/resistor circuit to make sure that the input to the DC2DC device does not get a "too high" voltage......

On the other hand, if this 3914 is only used occasionally, then the simple answer is to use a push button and a battery!

Experiment!!

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