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Old 4th February 2008, 09:46 PM   #1
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Default Need help with LED panel voltmeter wiring

Hello all.

I have bought this module from Velleman

http://www.velleman.be/ot/en/product/view/?id=19215

to use as a car voltmeter.

I am using a 7808 voltage regulator IC to keep the main voltage in the manufacturer specs.

My problem is: if I directly connect the module inputs to 12V (assuming VCC is 12V), the module will simply go bonkers, not displaying anything except some lit up segments.
If I connect the inputs to another voltage source, it will display correctly the voltage.

I've attached a schematic of the circuit. Basically, what should I place in the [?] box to cure this problem, as the voltage to be measured in the car will be the same as the supply voltage.

Thank you very much for your help
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File Type: gif velleman.gif (2.9 KB, 73 views)
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Old 4th February 2008, 10:35 PM   #2
hermanv is offline hermanv  United States
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1. The 7808 will like an output filter cap to prevent oscillations, anywhere from 4.7uF to 100uF electrolytic.

2. There is a lot of noise on a car 12V line, try 100 Ohms in series with the module sense lead and a .1uF to local ground. Verify that the input resistance of the 100 Ohms doesn't make the meter read low with a second DVM.

3. The 7808 might not last, I've seen spikes from the starter motor of over 50V when the start relay de-energizes I think the 7808 is limited to 35V input.
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Old 4th February 2008, 10:44 PM   #3
Cobra2 is offline Cobra2  Norway
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Try powering it with a separate 9V battery, some of these modules cannot be powered from same source as they measure.

Arne K
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Old 4th February 2008, 11:07 PM   #4
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Do you have any more information on this meter? The webpage you linked to is really lacking in details. It says the input sens. is 200mV FS, which I would take to mean it measures from 0-200mV, but then it says it's 3.5 digit and doesn't tell you how the measurement is scaled for display. I guess that 200mV input reads as 1999 on the display. It mentions that you can place a decimal point wherever you want. So in other words, each count represents ~100uV.

You will need a resistor divider network in your '?' box to scale the input voltage to the 0-200mV input requirement. For example, if you're trying to read the operating voltage of the car's electrical system, a range from 0-19.99V is reasonable. You would need to divide the input by a factor of 100. Use resistors like 1K and 100K in your divider. Then place the jumper so the decimal is in the second position, and you're set.
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Old 5th February 2008, 08:04 AM   #5
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The module itself already has precision resistors that form a divider network so it can read 0-20V.

Seems like a 9V battery is the only way to go

Thank you for your advices, I really appreciate them
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Old 5th February 2008, 08:38 AM   #6
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Quote:
Originally posted by Simpleton
The module itself already has precision resistors that form a divider network so it can read 0-20V.

Seems like a 9V battery is the only way to go

Thank you for your advices, I really appreciate them
Are you saying that powering the meter in your car from a 7808 causes the device to go bonkers? It shouldn't be too difficult to get any electronics working in a car...it's a nasty environment but it's not an unsolvable problem. There are tons of sensitive electronic devices in a modern car, and they all work fine.

Some things you might try:
- more supply decoupling. power in a car is dirty, more decoupling will help smooth out any sags. before the regulator is probably more effective.
- a low-pass on the measurement lead. add a capacitor across the meter reading pins to smooth fast transients on the measured voltage
- an inductor (choke, coil) in series with the power supply will help smooth any spikes. place it before the regulator.
- a zener diode, reverse biased from the input to ground, maybe a 16-20V unit will help conduct any high voltage spikes to ground and protect the circuit from voltages that aren't in its SOA, but I don't think they'll solve this problem. you could use one on both the power supply and measurement leads.
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