crazy volmeter

[XOKE]

Hi,
I buy this voltmeter from an eBay store.



http://www.ebay.com/itm/Blue-Digital-DC-20V-LCD-Volt-Voltmeter-Meter-Display-/230789974798?pt=LH_DefaultDomain_0&hash=item35bc278b0e

every time that I have a power inverter connected to batteries, the display looks “crazy”, display readings from 11volt to 14volts in a second.
When I disconnect the inverter the reading stays ok
Anyone know why this happens?

Thanks

xoke

[OperaHouse]

You ar picking up line noise.  Put a 470uF or higher capacitor at voltmeter power.  Between battery and neter +power put an inductor.  A small power transformer would work well as an inductor.

[XOKE]

Hi
this is what you are saying?
or different connection?


please help

thanks

xoke

[DamonHD]

If it is a two-wire device, with no separate 'Vin', then yes.

But have the capacitor a lot closer to the meter than the battery along the wiring.

Rgds

Damon

[XOKE]

Ok, thanks guys
I´ll do it soon I get the capacitor.

regards
xoke

[XOKE]

Hi, I check the capacitor today, only now I get some free time for this, and the voltmeter stays ok with modified sine wave, but when connect a pure sine wave the voltmeter stays like a crazy display.
what can I do,?
Buy a good one, or this can be modified'
thanks

Xoke

[tanner0441]

Hi

You could  try a ferrite ring twist both leads from the battery to the meter and wind six to ten turns through the ferrite ring, you could also put a 0.01 capacitor across the electrolytic to handle short high frequency spiked.

You don't mention if the meter is powered separately or just has the two wires coming from it.

Brian.

[XOKE]

Hi,
there´s just 2 wires,
I´ll get the 2 ring and test it

thanks

Xoke

[Flux]

your meter will be high impedance, connect 100 ohms or similar in the positive lead and the capacitor straight across the input terminals. This will filter the noise and have no observable effect on the reading. This will have the same effect as the inductor mentioned by Opera House and may be easier for you to do.

If things are really bad, Tanner's idea of the ferrite ring may help. Try winding the two leads through one ring and keep the resistor or possibly try one ring on each lead, in which case you may not need the resistor but it won't hurt to leave it in.

Flux

[dnix71]

There has to be something else wrong. I have two of those replacing the burned out displays on my Harbor Freight solar kit power taps. The inverter is pulling pulses from your bank. DC chopped up and smooothed to make a/c. If the inverter is a cheap mod sine inverter, it's pulling nasty square pulses. You need to make sure the bank is big enough for the inverter and all the wires are tight.

All that said, mine show unstable volatges when there is a motor load on the bank, like my Engel fridge's compressor running, but the float is only .2v up or down. I have a pure sine Wagan inverter and it causes no trouble. Even the nasty Apple laptop switching power supply doesn't affect the display, but it messes with nearby radios a lot.

[XOKE]

Hi,

I check voltage with my multimeter when the pure sine wave inverter it's on and they stay ok, not like the other one.
Probably it was something wrong with him; I´ll tries the ring and the 100 ohm resistor,

Thanks to all you guys

Xoke

[tanner0441]

Hi Flux

Could it be an RF component from the switching circuit in the inverter, I had a problem on a machine guard a few years ago where I found over 30V at 60Mhz on a 5V track from a 7805 I finished up cutting the track and putting in a R/C pi filter, I can't remember the sums at the moment so I don't want to quote figures.

Brian.

[Flux]

In this case I suspect it is a crude digital meter sampling the ripple ( hence the 100 ohm to form a filter with the 470U capacitor, but there could well be a lot of fast spikes or even oscillation coming from the inverter. If the meter is connected to the inverter rather than the battery it would be worse.

You are right that regulators can oscillate violently unless careful  precautions are taken ( many oscillate anyway and go un noticed).  The other offender is an emitter follower feeding a capacitive load( co-ax cable will do it). Op amps are also prone to this unless the supply pins are decoupled right at the pin.

I was originally going to ask whether the voltmeter wandered with the capacitor across the input and no input wire when the ground was connected to battery, but then I realised that this one derives power from the input ( scuppered that idea).

Between the 100 ohm filter resistor and the ferrite beads I am fairly sure it will tame.

Flux

[XOKE]

HI
when returning home in the next days I will test the 100 ohm resistor and ferrite ring,
then say how it went

thank you

xoke

[tanner0441]

Hi

Apart from the ferrite rings you want whatever components you use for the filter as close to the meter as you can get them, and still be prepared to put the capacitor is as well.

Brian.

[XOKE]

Hi,

thanks for the tip

Regards

xoke

[XOKE]

Hi,
I got this from a microwave Owen



This ring has 2 wires, each one with 13 turns. I think I´ll put it to the meter and test it

regards