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tong
Joined: 07 Aug 2004
Posts: 29
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 05 Mar 2008 10:03 Many questions about LM3914 |
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My English is not native. I read the datasheet but don't know the exact values.
http://www.national.com/ds/LM/LM3914.pdf
1. What is the minimum mV per step ?
2. How much current required at pin 5 ?
3. Current for one LED is equal to current from REF OUT x 10 ?
In the example, set REF OUT to 1mA. So current flow 10mA per LED.
4. REF OUT is regulate and limit at < ( V+ - 1.5 ) < 12 volt ?
I want to design a generic circuit for all purposes.
- Use voltage divider for SIG IN.
- No need to adjust REF HI. Use REF OUT as REF HI.
- Can set offset (REF LOW) in full range (0 to REF OUT).
Last edited by tong on 05 Mar 2008 22:14; edited 15 times in total |
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tong
Joined: 07 Aug 2004
Posts: 29
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| 05 Mar 2008 10:05 Re: Many questions about LM3914 |
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If mV between each step is more than 50mV, you can use these circuit.
SIG IN can be 0V to 35V.
V+ should be more than 1.25V + 1.5V = 2.75V.
1. Apply voltage that you want to light on LED#10.
2. Adjust cermet 100K until LED#10 turn on.
3. Apply voltage that you want to light on LED#1.
4. Adjust cermet 2K until LED#1 turn on.
5. Apply voltage that you want to light on LED#5.
6. Check out LED#5 should be turn on.
Last edited by tong on 05 Mar 2008 22:43; edited 3 times in total |
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tong
Joined: 07 Aug 2004
Posts: 29
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| 05 Mar 2008 10:06 Re: Many questions about LM3914 |
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If mV between each step is less than 50mV in above circuit, you can use these circuit to expand the mV per step.
These circuit increase VREF. So V+ should be more than 2.5V + 1.5V = 4V.
Last edited by tong on 05 Mar 2008 22:55; edited 6 times in total |
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tong
Joined: 07 Aug 2004
Posts: 29
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| 05 Mar 2008 10:08 Re: Many questions about LM3914 |
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Someone adjust REF LOW like these.
I don’t know the formula to calculate offset voltage.
Last edited by tong on 05 Mar 2008 22:52; edited 3 times in total |
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tong
Joined: 07 Aug 2004
Posts: 29
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| 05 Mar 2008 10:25 Many questions about LM3914 |
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Cermet 100K is suitable for 0V to 35V ?
How much PIN#5 need current ? I guess 100nA.
Last edited by tong on 05 Mar 2008 23:01; edited 4 times in total |
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Sinisa
Joined: 03 Mar 2003
Posts: 210
Helped: 38
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| 05 Mar 2008 20:25 Re: Many questions about LM3914 |
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| Quote: |
1. What is the minimum mV per step ? |
Depends on Vrhi and Vrlo. Step size is (Vrhi-Vrlo)/10
| Quote: |
| 2. How much current required at pin 5 ? |
You should really read data sheet. 100nA max.
3. Current for one LED is equal to current from REF OUT x 10 ?
In the example, set REF OUT to 1mA. So current flow 10mA per LED.
Again, clearly stated in data sheet. Iled=12.5/R1 (page 2)
| Quote: |
4. REF OUT is regulate and limit at (V[/b][/size]+ - 1.5 ) < 12 volt ?
I want to design a generic circuit for all perposes. |
Yes.
| Quote: |
- Use voltage divider for SIG IN.
- No need to adjust REF HI. Use REF OUT as REF HI.
- Can set offset (REF LOW) in full range (0 to REF OUT). |
Main problem with low step voltage is offset error or comparators. Typical 3mV seems high for just 20-30mV step. Note that maximum offset might be higher.
Use appropriate amplifier for input scaling.
And for last question, yes, 100kΩ pot can be used, but you still have problem with offset.
For small signal you should use precision opamp like OP177, OP07 or simillar to scale it up and avoid problems with offset error and drift LM3914 would cause.[/
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tong
Joined: 07 Aug 2004
Posts: 29
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| 05 Mar 2008 21:49 Re: Many questions about LM3914 |
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Ok Thanks..
I draw an important rules for reference. May useful for the other peoples.
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