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Old 16th May 2012, 02:32 PM   #1
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Default Guys...I need help with simple DC meter

Ultra small Blue LED Volt Meter DC100V ( 0V~99.9V ) | eBay

Monitor range : DC 0V ~ 99.9V
Working power : DC 4.5 ~ 28V
Current consumption : about 5mA
Accuracy : 0.1V
Monitoring frequency : 500ms
Led Size : 0.36″ LED
Meter Size : 29.3mm x 15.2mm x 10mm
Working temperature : -10℃~65℃


Basically, I have 45v DC that I want to monitor. I am building something which will utilize 45v dc, and want to put this led display to monitor the voltage in real time. Couple of questions:



1 - how would I connect this? there are 3 wires.
2 - Should I have a separate dc source for the required 4.5 - 28v needed to power this thing?
3 - It draws 5ma of current...if I have a separate dc power source to power this thing...lets say 9vDC...how long will it take to use up the 9v source?

thanks

Spiro
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Old 16th May 2012, 02:48 PM   #2
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Your asumptions are correct. You need a fixed 4.5 to 28 V to power the meter. You could drop the 45V with a voltage devider and then regulate it with a LM7805 regulator to power the meter. Black is ground and red is working power. Put your 5V to red. Connect the white to the 45V that you want to monitor. Google the +5v regulator because you will need a cap or two described in the data sheet. Should work.
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Old 16th May 2012, 02:52 PM   #3
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Quote:
Originally Posted by firechief View Post
Your asumptions are correct. You need a fixed 4.5 to 28 V to power the meter. You could drop the 45V with a voltage devider and then regulate it with a LM7805 regulator to power the meter. Black is ground and red is working power. Put your 5V to red. Connect the white to the 45V that you want to monitor. Google the +5v regulator because you will need a cap or two described in the data sheet. Should work.
thanks. Only thing is that I want to keep this a simple as possible. I saw a meter that didnt require a voltage source to operate...but it would only read to 30v DC.

Any ideas on how long a 9vDC source will last with a draw of 5ma?
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Old 16th May 2012, 03:19 PM   #4
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How to Connect a Voltage Regulator in a Circuit

I was reading this, and was wondering why would you need the caps to filter AC noise? Where is that coming from if the source is only DC?

Also, I would probably need a heat sink if Im feeding the reg. with a 45vdc input correct?

If this is the case, then its getting too complicated for my needs.
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Old 16th May 2012, 03:46 PM   #5
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Is the 45V fairly constant ?

The components values aren't critical.

You can increase R to 4K7 at 1W with no real detrimental effect.

(By the way the 3K3 only needs to be 1W - I got decimal point in wrong place)

0.6W in both instances will be sufficient but they will get warm.

The cap is only there to decouple the meter, it might not be necessary. 1uF/16V would be more than sufficient. It also decouples the noise from the zener. Depending on how well designed the meter is it probably wont be required.

You cannot use a 7805 as 45V will exceed its maximum input voltage.
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Last edited by KatieandDad; 16th May 2012 at 04:00 PM.
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Old 16th May 2012, 04:06 PM   #6
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What is the capacity of your 9V DC source in mAh at 5mA current draw? If you know that then arithmetic can give you an answer. If you don't know that then nobody can give you an answer.
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Old 16th May 2012, 04:11 PM   #7
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There is no need for a separate supply if the 45V supply can spare 7-10mA.
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Old 16th May 2012, 04:16 PM   #8
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This circuit seems like it ought to work. Sim'd just fine and hard to get simpler. Have I missed something?
If your 9V source is an alkaline PP3 battery, you'd be lucky to get a week out of it at a constant draw.
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Old 16th May 2012, 04:19 PM   #9
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Quote:
Originally Posted by sofaspud View Post
This circuit seems like it ought to work. Sim'd just fine and hard to get simpler. Have I missed something?
If your 9V source is an alkaline PP3 battery, you'd be lucky to get a week out of it at a constant draw.
Why bother with the LM7812 when a simple zener will do here ? In SOFASPUD's circuit, if the 12V is not drawing any current for whatever reason, the 7812 will fail.

You could put a minimum load on it but that is just adding unnecessary complexity.

Last edited by KatieandDad; 16th May 2012 at 04:23 PM.
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Old 16th May 2012, 04:27 PM   #10
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Wouldn't argue with that. I might dispute the need for a 5W resistor, though. 1W should be adequate, shouldn't it?
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