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Topic: Wildly varying voltage (Read 1 time) previous topic - next topic

mibix

Hello.

I have a simple circuit that has 8 potentiometers wired to a 4051 multiplexer.

If I connect 5v and GND to the Arduino pins, I get a steady value shown from the pots.

However, this is part of a larger project, and I have created a custom power board, where a 12V line from mains connects to a 5V voltage regulator. I have a cap on the regulator's output as per the spec sheet. If I use this to power the 4051 and the pots, the values jump all over the place.

I have a multimeter, but don't want to risk getting a shock. If I connect the multimeter to the 5v and GND outputs of the regulator, can I see if this is actually changing without risking a shock?

Does anyone have any idea why the pot values are jumping when using the voltage regulator?

Cheers

pito

The 5V voltage regulator may oscillate when its input and output is not decoupled properly. Show us your schematics.

retrolefty

All the ground connections wired together?

dc42

#3
Apr 21, 2013, 08:18 pm Last Edit: Apr 21, 2013, 08:20 pm by dc42 Reason: 1
Do you have the recommended cap on the regulator input as well? See reply #1.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Grumpy_Mike

Quote
I have a multimeter, but don't want to risk getting a shock.

Have you ever seen a kids electric train set. That has lots of exposed 12V in the rails and no one gets a shock from that.

mibix

@ Pito, I've included a schematic - Gnd in Black (Except 12V input), 5V in Red

@ Retorlefty - yes all GND through the project is connected to the 12V input GND.

@ Grumpy_Mike - good point. I was also worried about shorting the whole thing connecting 5V directly to ground via the multimeter but I suppose they are made for that!

I was told an input cap wasn't necessary but that may be the problem. I'm not going to be able to get the right one until tomorrow to test that, unless someone sees a problem with the schematic.

Sorry for a scruff schematic. I don't have access to my software at the moment.

Cheers

lax123

if that schematic is complete and you dont use a dmm how u know its varieng?


Quote
I was also worried about shorting the whole thing connecting 5V directly to ground via the multimeter but I suppose they are made for that


the thing you have to worry about is how to measure voltage and amps with it and switching to the right mode, so maybe take 5min to read about a dmm

Runaway Pancake


Sorry for a scruff schematic. I don't have access to my software at the moment.


Draw it out on paper
Take a picture of that much
and
Post/Attach it here
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

mibix

Because the reading of the pots shot up and down randomly with the regulator, but with Arduino power it stayed as it should. That was the only part of the chain I changed.

I've checked with a multimeter now and the voltage hovers somewhere between 4.97V and 5V.

I checked the Arduino for reference and it seems to stay pretty much constant around 4.93V & 4.94V.

What's the best thing to try now? Could it simply be the lack of an input cap?

Cheers

Runaway Pancake

"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

mibix

Yes its a 7805 with a ceramic 0.1uf cap on the output.

Runaway Pancake

Good to know, no previous mention of it (surprising.)  They're low maintenance. 
This 12V source is some sort of wall-pack?  They usually have a smoothing cap built in.

How does the 12V fare when you make your adjustments?

And you have no other capacitors you could tack on (for grins & giggles)?
Should be rated > "12V".

[N.B. Looking at the label, with the leads pointing down, it's input-ground-output]
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

mibix

Hi Runaway,

No I have no other caps. I just have a bag of 0.1uf... Remember just needing 1 for another project, and it was just as cheap to order 100 from eBay as it was to pop to Maplin.

The 12V is a wall pack, but again it's a cheapy ebay one. The 12V in is reading ~12.24V on the DMM.

I haven't tried doing any readings when I'm adjusting things. All the reading were done when idle. I'll have a go later at taking reading as I'm moving things. Need an extra set of hands and wait for my housemate to come home.

Cheers.

Runaway Pancake

Next time you're round to Maplin, look into a set of test leads - banana plugs at one end (they plug in to the meter) and alligator clips or mini-grabbers on the other that you clip onto your points of interest, leaving your hands free.

They have this -- http://www.maplin.co.uk/test-lead-kit-5669
Better, they have all sorts of related parts so that, if you're handy, you could make your own.
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

cjdelphi


Quote
I have a multimeter, but don't want to risk getting a shock.

Have you ever seen a kids electric train set. That has lots of exposed 12V in the rails and no one gets a shock from that.



maybe he's scared he might stick the 12v terminals in his mouth?


you say that but i once stuck my hand (small cut on my finger barely noticed i had it) into a jar of water i had two electrodes in at 14.5v creating hydrogen ... i sure as hell got my hand out fast! the cut was deep enough to feel the current flow! weird sensation so curiosity took over d/ced the power placed my hand in... nothin! 


but yeah blame tv/movies where you see people torturing people with 12v car batteries!

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