# LM3914 - question

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by caslor, Mar 22, 2010.

1. ### alex69New Member

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Ok, I'm sure you have enought knowledge about electronics scheme but I'm not the same.
I've read datasheet and that's my output:
My questions:
1 - I can't define R2 (don't know Vout or VR2)
2 - is right the R1 value?
3 - is right the R3 value?
4 - how determinate R on pin5 (need it or not?)
5 - how can I determinate Vled=3V?
Many thanks
Alex

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2. ### ericgibbsWell-Known MemberMost Helpful Member

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hi alex,

I was going to post a link to my Forum Blog for the LM3914 calculator,,, but for some unknown reason its says I have deleted it, which is incorrect.!!!

R2 is calculated from Vmax, you state its 5V, but you only have a LM3914 supply voltage of 5V, you MUST have at least 1.5V to 2V higher for the supply above your Vmax in.
R1 is chosen to give the required LED current.
R3 sets the Vmin for LED#1 to light, which is also dependant upon I3 which a function of Vmax voltage.
The R on pin#5 is not required.
The Vled is determined by the colour/type of the LED's that you are using, not the LM3914
Different colours have different Vfwd.

For a Vmax of 5V use a 9V battery and set the LED current to about 15mA.

3. ### alex69New Member

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Many thanks mr erigibbs.
This afternoon I'll try next time to make exact calculate.
Alex

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5. ### alex69New Member

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I've found this scheme.
I've just realize this circuit and it works well (for my usage) but I want to modify it :
1 - increase led voltage to 3,0V
2 - increase led current to 30mA
because I must use it like a speleo-torch.
I use 4 battery 1,2v 2800mA in series and I want all led on with full charge.
When the battery power decrease, 1 by 1 the led must turn off in auto to save power.
I study this weekend to make it work.
Alex

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6. ### ChippieNew Member

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Things to note.....

The Vled is determined by the colour/type of the LED's that you are using, not the LM3914 ( Eric stated this in his previous reply.......)

2. Increasing the led current to 30 ma will probably destroy them unless they are designed to draw that much current.....

Last edited: Oct 8, 2010
7. ### alex69New Member

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So, if I use high bright led 3-3,4V with 50mA there aren't problem to burn them? (I set Iled to 30mA)

8. ### audioguruWell-Known MemberMost Helpful Member

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Most ordinary 5mm LEDs have an absolute max current of 30mA if they are cooled somehow. If they are not cooled then they fail in a few minutes, tomorrow or next year.
Philips Luxeon SuperFlux LEDs have an absolute max current of 70mA if they are cooled somehow. They have a case with 4 pins that carry away some of the heat.
If the LM3914 is set to the BAR mode with all LEDs lighted then at 30mA the IC will get very hot and might fail unless you add a power resistor to share the heat.

9. ### alex69New Member

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Thank you Audioguru.
I'll continue experiment.
Alex

10. ### alex69New Member

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....or I think so.
I've just make the circuit and for 10 hours all ok :
1 - Iled=30mA (high bright 3,3v - 50mA)
2 - Vbatt=4,0-5,1V
3 - Vmin=3,6V
4 - Vmax=4,8V
I've no problem of heating of LM3914 neither led.
For now.... ALL OK.
Alex

PS : in the attachment there is a little scheme and how determinate R value.

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11. ### alex69New Member

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I've made some correction on resistance-calculator for LM3914.

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