# [EVDL] Help with RC filter circuit for voltage input on microprocessor



## EVDL List (Jul 27, 2007)

In previous thread I mentioned that I am working on a battery meter and
balancer based on a PLC. I have a simple 0-5V input monitoring pack voltage
with a voltage divider. Lee suggested I add an RC filter and a zenor at my
voltage input to minimize noise. I already have the R with the voltage
divider, so I put a capacitor across the input thinking that would be
sufficient. Given that I only want DC, I assumed, maybe incorrectly, that
any value capacitor would do but bigger is better so I got the biggest one
Radio Shack had. The voltage sensor works fine until I press the
accelerator. The reading is abnormally high voltage (+2-3 volts) when I
press the accelerator and it appears that my input card faults when I let
off the accelerate (the reading ramps down to zero as soon as I let off the
accelerator, sometimes before I let off). I believe the root of the issue
is on the input voltage itself rather than the power supply to the PLC since
I have a DC-DC and a 120V inverter between the pack and the PLC supply
voltage.

Can I tune my filter circuit better to eliminate the spikes? Is there a
better circuit for isolating this noise?

My existing circuit:
http://s227.photobucket.com/albums/dd106/skskarda/?action=view&current=VoltageSensor.png
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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> > In previous thread I mentioned that I am working on a battery meter and
> > balancer based on a PLC. I have a simple 0-5V input monitoring pack voltage
> > with a voltage divider. Lee suggested I add an RC filter and a zenor at my
> ...


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