I built this dummy load

[arun2118]

  hello

I built this dummy load based on Daves and many others ideas.  It features :: temp controlled fan, volt and amp read out using one of these http://www.ebay.com/itm/DC-100V-10A-Voltmeter-Ammeter-Blue-Red-LED-Amp-Dual-Digital-Volt-Meter-Gauge-/171525324264?pt=LH_DefaultDomain_0&hash=item27efb4bde8, and its usb powered. I have attached the diagram R7 is a simulated load.

The issue is when something is connected and I increase load the voltage read out on the meter also goes down, I tested this with a voltmeter and it doesn't show its dropping.

My question is this is definitely a volt meter issue correct? its not the location i'm taking the measurement? can i add a diode or cap to get the reading I'm looking for?

[jeroen79]

In your schematic the voltmeter and ammeter share a common ground, is that the case with the volt/ammeter module as well?

Where is the power supply that is loaded connected?
I assume it is connected to ground and the node in the top-right corner, right?

What kind of power supply is attached (as device under test) to the dummy load?

Why is R7 added?
It puts 1 ohm parallel to the dummy load and the voltmeter and will drain 1A per volt from the DUT.

I would take two multimeters to measure the actual voltage over and current through the load and compare them to what the volt/ammeter module is indicating at various settings of the dummyload.
You can then make a table in excel and use that to make a graph which may show how the various parameters relate to eachother.
Then I'd make a table

[arun2118]

Yes both are sharing a common ground.

R7 in the diagram is the load its not actually in the circuit  but i was trying to simulate 

A wall wart that's 12v and 4 amps has been tested also 5v 3amps both increasing the load decreases the voltage.

the volt/ammeter module is indicating correct current but not voltage.

[sleemanj]

So you are saying that the voltmeter is NOT agreeing with your multimeter when you measure across EXACTLY the same places as the voltmeter is? 

Remember, every conductor has resistance (short of superconductors), and if something has resistance and you pass current over it, the voltage by necessity drops according to Ohm's law, do not for one second think "that's just a bit of wire/pcb trace, I can forget about that resistance", no, no you can't, it has resistance, it's carrying current, it drops voltage. 

So think carefully about where you are measuring, and what sources of voltage drop comes before and after that that might explain what you are seeing.

[arun2118]

So you are saying that the volt/ammeter module is reading voltage incorrectly because its connected directly on the board right next to the mosfet and my external voltmeter is reading different because its further away? yes you are right i just took it apart and probed next to the fet and see the same volt drop  so how can i prevent this from happening?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

[CodiJack]

Are you sure that common ground is proper connection for such meter?
Some of them have common Vcc, see attached sketch.

[arun2118]

No not sure take a look

[jeroen79]

You haven't given us a value for that voltage drop yet.
How much is dropped? Volts, millivolts?
And at what current?
How does this voltagedrop progress as you increase the current drawn from the supply?

Take measurements of current at voltage over the whole 0% to 100% load range
at 10% increments.

What about the wires between the power supply and the dummy load?
How long and thin are they?
Are they getting warm when you put a high current through them?

[CodiJack]

They are the same So from this point it's OK.

[arun2118]

same thing happens with any source
https://youtu.be/sVxI1dRU4Hk

the wires between the power supply and dummy load get as low as 18/22g

and the main cord from the brick is 3ft.

their probally getting warm ...as i crank the current up i can smell some plastic melting 

[jeroen79]

If you can feel and smell that the wires are getting warm then they may have a significant voltage drop.

As far as I can see in your video the multimeter leads are attached to the breadboard's powerrails, as are the wires going to the dummy load. Is that correct?

That means that the multimeter will be measuring the voltage across the dummy load's  wires and the dummy load's poles while the internal meter will only be measuring the voltage across the poles. (or wherever it is attached)

The multimeter has a high impedance so it will not draw much current and there will be no significant voltage drop along it's cables.

Connect the multimeter directly to the dummy load's poles instead of to the breadboard and then see if the meters agree.

Ie:
PSUpos ===== DummyLoadpos ----- DMMpos
PSUneg ===== DummyLoadneg ----- DMMneg
Instead of:
DummyLoadpos ===== PSUpos ----- DMMpos
DummyLoadneg ===== PSUneg ----- DMMneg
The double lines carry high current and will have a significant voltage drop.

What about the wiring inside the box?
Neat and tidy with short leads or a big knot of wires?
Where is the meter connected?

[arun2118]

Yes the multimeter and wires going to the dummy load are on the power rail.

inside the box i used 12/14g wires anywhere high current is present all connections are soldered.
the volt/ammeter module is reading voltage at the fet drain.

with the multimeter connected directly to the poles i get almost the same voltage drop on the meter as the volt/ammeter module.

I see when i eliminated the breadboard and directly connect the 12v supply(wall wart) to the poles theirs very little drop in voltage. 

Thanks guys