Conflicts from equipment

oracacle

Senior Member
I have been spending a little more time on my long running project in the form of breadboarding. I have found that to get the desired scale I need to run one of the op-amps in the sensors at 7+ volts. I thought before connecting to my 20x2 I would check the output with a measure 7.3v input.

the figure of 7.3v maybe called into question though as while I was testing with my oscilloscope the output was 4.97v, my multi meter (which I measured the supply voltage with) measure at the same place read 6.1v. I will be putting a 5v1 Zener on the ADC pin to hopefully stop any harm to the picaxe in the final project.

But the question, which would you trust, a hanntek 6022BE or a £20 multimeter?
 

inglewoodpete

Senior Member
But the question, which would you trust, a hanntek 6022BE or a £20 multimeter?
It depends a bit on the bloodlines of the multimeter but I'd be cautious about putting too much faith in my Hantek oscilloscope. They're great value for money but how stable and accurate are their calibrations?

A simple solution would be to put some resistance between the output of the op-amp and the PIC's input. Also, add a capacitor from the PICAXE input to ground. The PIC's input protection diodes will then give you a degree of safety.
 

oracacle

Senior Member
the multimeter is a precision gold pc017.
Think I will leave the zenners in there with your suggested resistors and caps.
Glad I checked this before I got too far along with designing and making a board.
 

oracacle

Senior Member
DC only, the op amp is taking a signal from a photo diode. Its setup in a non-inverting fashion with a 4k7 variable from the feedback and 2k2 to ground.
There is a spare op-amp that could be used if needs be
 

RexLan

Senior Member
I would not believe either - far too much variation. The DMM is likely good but it is speculation. I would find a 3rd cross check source before investing time and $$.

You can use a simple LM 7805 and measure the output with a small load (LED). It should, be within ±4% of 5.00Vdc. Or 4.8 to 5.2 Vdc
 

hippy

Technical Support
Staff member
But the question, which would you trust, a hanntek 6022BE or a £20 multimeter? ... precision gold pc017
I've used a Precision Gold PG10B for decades now and I have pretty good trust in it. It reflects what my bench supply says its putting out. Might be worth trying a new battery just in case an issue.

As for the scope I have no idea.

It's a typical "a man with two watches never knows the time" problem. Maybe try and find some reference voltage as RexLan suggests - a USB PSU should also be pretty close to 5V. Or try another meter to see if that sheds some light on the situation.
 

oracacle

Senior Member
I used the MCP1702 that is on the breadboard as a reference, scope showed 4.97, meter showed 5.02.
This maybe a mistake on my part as I see it now, if I switch the probe and software to 10x (completely forgot about this) the scope shows 6.90v - the same as the meter. I don't know if this a limitation of this particular scope, or me fumbling around.
 

techElder

Well-known member
Impedance of the measuring device is important. Higher than the circuit impedance is best because you are creating a voltage divider.
 

premelec

Senior Member
@oracacle note that if you connect BOTH devices _at the same time_ the readings should be close to same - then you can disconnect one or the other device and see if the remaining reading changes - gives good indication of loading problem...
 
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oracacle

Senior Member
That's how I found the difference. it came down to how the probe and software were set for the oscilloscope.
 
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