Panel Meter vs Multimeter

Submitted: Saturday, Mar 13, 2010 at 13:12
ThreadID: 76817 Views:3134 Replies:10 FollowUps:24
This Thread has been Archived
To all the electronic / electrical experts a little help please ...

I am measuring the output of a solar regulator ..

If I set my multimeter up in series it reads 1.8 Amps fluctuating to about 2.4 Amps

If I set a 0-10 Amp panel meter in exactly the same spot it reads 5.7 Amps !!

Can someone advise on what I could be doing wrong ?? Or more importantly which should be the correct reading ..

I've changed 3 different multi meters and got more or less the same numbers ..

I've also changed the panel meter and got the same 5.7 A ...

This is baffling me as I can't see a logical reason for the difference unless there's a huge resistance in the multi meters ??

Help please

Cheers

Steve

Back Expand Un-Read 0 Moderator

Reply By: Mandrake's Solar Power- Saturday, Mar 13, 2010 at 13:15

Saturday, Mar 13, 2010 at 13:15
Apart from becoming a licensed sparky is there anywhere I can learn HOW to use a Multi meter correctly ?

LOL

Steve
AnswerID: 408648

Follow Up By: dbish - Saturday, Mar 13, 2010 at 15:23

Saturday, Mar 13, 2010 at 15:23
Hi Steve I have used multi meters & analoge meters for years servicing TVs & VCRS & as a mechanic, There is a place for each instrument, any slightly fluctuating voltage or current use analogue you will get a more acurate reading as digital will loose the plot because of is update time. Digital is great for max acuracy on steady voltages or currents. On my solar system I use analogue volt & current meter & i find they are acurate enough for this aplication. Cheers Daryl
0
FollowupID: 678604

Reply By: _gmd_pps - Saturday, Mar 13, 2010 at 13:19

Saturday, Mar 13, 2010 at 13:19
In series to what?
If you are reading the current between the two poles of your solar regulator
without load you read whatever flows depending in the impedance (inner resistance) of your meter. Obviously your panel meters have different impedance than your multimeters, which is quite logical.
good luck
gmd
AnswerID: 408649

Follow Up By: _gmd_pps - Saturday, Mar 13, 2010 at 13:21

Saturday, Mar 13, 2010 at 13:21
This post has been read by the moderation team and has been moderated due to a breach of The Inappropriate Rule .

Forum Moderation Team
0
FollowupID: 678589

Follow Up By: Mandrake's Solar Power- Saturday, Mar 13, 2010 at 13:28

Saturday, Mar 13, 2010 at 13:28
I am trying to ascertain which is correct - perhaps I should add that there is an 85aH battery attached to the reg and it is running a 4 Amp fridge and a 15 amp air pump whilst I am measuring - and I cannot work out why I get such a marked difference in readout - obviously I am doing something wrong - I was hoping that someone might point me in the right direction ...
Obviously you are not that person ..

Thanks

Steve

0
FollowupID: 678591

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 14:42

Saturday, Mar 13, 2010 at 14:42
Steve,

similar thing happended to me in the past.

The solar regulator's PWM circuitry controls the output voltage to the battery within a fairly narrow band. You could assume this voltage is a constant while comparing the currents.

The current now becomes a function of the battery voltage (SOC) and circuit resistance.
I.e. delta V=0.2V, R tot=0.05Ohms, I=4A.

R tot contains the ammeter's shunt resistance and resistances of the test leads.
If you omit the resistance of your test leads (like in case of your panel meter), then the total series resistance would have dropped by say 25 milliOhms, which effectively doubles your measured current (8 amps in this example).

Therefore it's advisable to ensure the lowest contact resistance of your test lead plugs.
Spray them with contact spray and wiggle them in their receptacles.

Or, use a DC current clamp which won't have this problem (these tend to be a bit inaccurate at lower current values though).

Hope this has you un-puzzled?

@ Allan,

the integration times of a standard multimeter are in the order of hundreds of milliSeconds.
The PWM period of a typical solar controller is at least by an order of magnitude lower than this.
Thus, both panel- and multimeters will show a time integrated (time averaged) value of the pulsed charging current.

If the regulator was DC/DC like in MPPT, then the output will be smooth DC, so there won't be any issues at all.

The only way there could be interference is when the regulator is set to 'on/off' control mode with a time period of 250 milliseconds f.e.
But this would result in wild swings on any digital (AND coiled) meter display and would make an accurate reading very hard if not impossible.

Best regards, Peter
0
FollowupID: 678598

Follow Up By: Member - Allan B (QLD) - Saturday, Mar 13, 2010 at 15:00

Saturday, Mar 13, 2010 at 15:00
Hi Peter, Your reference to the multimeter test leads I think may be the key. They almost certainly will be contributing a significant increase in the circuit resistance which will not be present when the observations are taken with the panel meter alone. A convenient check would be to use the digital multimeter whilst the panel meter is in circuit which should then display approximately the same reading on both meters.

However, I still believe that there may be an error in the digital display due to the waveform of the PWM output but probably not as much as Steve has experienced.

Cheers
Allan

Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 678599

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 15:14

Saturday, Mar 13, 2010 at 15:14
Allan,

wiring both of them in series certainly cancels out the test lead variable.

if you expect the same readings from both meters in series, and they probably will, then this would also show that there's no interference with the integration times and the observed effect was exclusively caused by the additional lead/contact resistance.

I'm still waiting on Steve's reply as to whether his panel meter is digital or not.

Best regards, Peter
0
FollowupID: 678600

Follow Up By: Bonz (Vic) - Saturday, Mar 13, 2010 at 15:18

Saturday, Mar 13, 2010 at 15:18
My circuit theory tells me that the lead and contact resistance should be negligible
.
Time is an illusion produced by the passage of history
.

Lifetime Member
My Profile  My Blog  My Position  Send Message
Moderator

0
FollowupID: 678601

Follow Up By: Member - Allan B (QLD) - Saturday, Mar 13, 2010 at 15:23

Saturday, Mar 13, 2010 at 15:23
Yes of course Peter, it did not occur to me that the panel meter may be digital also. I just assumed that it was an analog moving-coil.

But I can't see where you have asked Steve.

Cheers
Allan

Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 678605

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 15:23

Saturday, Mar 13, 2010 at 15:23
Bonz,

only if you're concerned with high series resistances and low currents, like in a voltmeter.
In an ammeter the situation is exactly opposite.

Best regards, Peter
0
FollowupID: 678606

Follow Up By: Member - Allan B (QLD) - Saturday, Mar 13, 2010 at 15:29

Saturday, Mar 13, 2010 at 15:29
Bonz, the standard test leads supplied with the average multimeter have minimal copper in them and in the very low impedance circuit of battery charging the lead resistance could add significantly and change the current flow.

Cheers
Allan

Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 678608

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 15:29

Saturday, Mar 13, 2010 at 15:29
Allan,

you're right I didn't ask Steve that question directly.

I just assumed he'll notice himself that this question could hold the key and that he'll come up with an answer in one way or another.......so to speak - hope that I don't expect too much :P
Meanwhile this thread literally exploded into a number of side tracks, keeping poor Steve occupied.... oh well might take forever to receive an answer.

Best regards, Peter
0
FollowupID: 678609

Follow Up By: Mandrake's Solar Power- Saturday, Mar 13, 2010 at 15:32

Saturday, Mar 13, 2010 at 15:32
Thanks for all this info guys - I was just out hooking up my trailer for a demo tonight at Geelong - ..

Panel meter is a small analogue moving coil ( Jaycar ) - I will try the in -series tomorrow and see if I get different results ...

The whole purpose of the exercise was to find an "easy" test to prove an MPPT controller actually puts out more amps than is coming from the solar panel ..

Not so easy methinks !!

Cheers

Steve
0
FollowupID: 678611

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 15:39

Saturday, Mar 13, 2010 at 15:39
your welcome Steve,

so your regulator is MPPT?

That would put an end to the integration- time- window- influencing the reading on the digital meter.

Either one of your meters is totally out of whack (calibration), or it's due to the additional test lead/contact resistance of your multimeter.
As has been said before, wire them in series and see if the readings are more or less identical.
Wiggle the test leads in their receptacles and watch the display fluctuations....

Best regards, Peter
0
FollowupID: 678612

Follow Up By: Mandrake's Solar Power- Saturday, Mar 13, 2010 at 15:44

Saturday, Mar 13, 2010 at 15:44
Thanks Peter -

But I'm beginning to think my MPPT controller isn't !!

I cannot see any increase in amps from what is or can come from the panel..

Methinks I have been duped

Cheers

Steve

0
FollowupID: 678614

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 15:54

Saturday, Mar 13, 2010 at 15:54
Steve,

this increase can be marginal and barely noticable at times.

If you have a CRO or digital meter with time/frequency domain display, check the output. There should be a ripple of ballpark 150Hz if it's PWM.
And a ripple of 4 Hz if it's a simple on/off controller, and none at all if it's MPPT.
To get a better picture of the ripple, connect a 10 milliOhm resistor in series with the output, and measure the voltage ripple across it.

Best regards, Peter
0
FollowupID: 678615

Reply By: Member - Allan B (QLD) - Saturday, Mar 13, 2010 at 14:12

Saturday, Mar 13, 2010 at 14:12
Hi Steve,

I am not highly experienced with solar panels and regulators but I understand that they are "switching" regulators. In other words, they switch the circuit on and off at a high frequency rather than simply throttling the current. This switching action results in an average current flow and is acceptable to loads such as batteries which are not particularly affected by the switching action.

Panel meters are simple magnetic devices that will respond satisfactorily to the on-off current flow and display the average current although they may exhibit a flickering action if the switching rate is too low.

Digital meters such as your multimeter however work by briefly and repeatedly "sampling" the current then process this value to result in a digital display. When the current is pulsing with the switching action of the regulator, the digital meter sampling rate differs from the regulator switching rate and so "sees" the current at its differing instantaneous values rather than the average value. In other words, it gets all confused.

I would tend to believe the readings of your panel meters rather than the digital meter in this circumstance. Incidentally, the resistances of the two meters will be similar and play no part in this disparity.

Hopefully, someone with practical experience in this area will comment.

Cheers
Allan

Member
My Profile  My Blog  My Position  Send Message

AnswerID: 408656

Follow Up By: Member - Allan B (QLD) - Saturday, Mar 13, 2010 at 14:30

Saturday, Mar 13, 2010 at 14:30
Steve, as a matter interest, remove the regulator temporarily with the solar panel connected directly to the battery (it should not hurt for a short time) and repeat your current measurements in that setup. If I was correct with my above explanation, the meters will read identically.

Cheers
Allan

Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 678597

Follow Up By: Glenndini - Sunday, Mar 14, 2010 at 14:34

Sunday, Mar 14, 2010 at 14:34
As an electronic engineer I agree with Allan. With the uotput from a switching regulator you are not measuring a completely DC current. It's has some AC component. Therefore as the 2 meters measure in a different (over different time scales) way you'll get different readings with the digital reading being pretty much useless.
0
FollowupID: 678724

Reply By: Maîneÿ . . .- Saturday, Mar 13, 2010 at 14:15

Saturday, Mar 13, 2010 at 14:15
Steve,
I would think your using about 100 watt panel and showing roundabout the correct Amperage going TO -> *Regulator* <- FROM panel.

However, because we are not aware where the "spot" is, it's only a guess
I assume the "spot" is between the Regulator and battery?
I assume the battery is fully charged?

therefore; appliances are using ~2 amps which is being replaced by the Solar panel :)

Maîneÿ . . .
AnswerID: 408657

Reply By: Member - joc45 (WA) - Saturday, Mar 13, 2010 at 14:26

Saturday, Mar 13, 2010 at 14:26
What readings do you get when you put the two amp meters in series, reading the same current flow?
If they're still different, then one of them is probably faulty.
Otherwise, a difference in reading when connected between the regulator and the battery may be due to a high internal resistance of one of the meters. The current will be correct, it's just that the meter contributes to the drop in current flow. Many cheap digital multimeters have a 200mV drop across them for full scale current reading, whereas older analogue amp meters will often have much lower voltage drop. This voltage drop will contribute to the different readings in current.
Suggest you take a current reading between the solar panel and the regulator and see if the two meter readings are different. Internal resistance of the amp meters will be less critical on this side.
Gerry
AnswerID: 408660

Reply By: Mandrake's Solar Power- Saturday, Mar 13, 2010 at 14:42

Saturday, Mar 13, 2010 at 14:42
OK - More info required ...
System is a 140 watt Solar panel ( at the time short-circuit Amps 6.22 on the multi meters ) connected to a solar reg the +ve goes to either my multi meter or my panel meter then on to the +ve battery terminal ... -ve is straight connect from reg .. Multi shows 1.8 - 2.4 - then I pull it out of the circuit and replace with panel meter reading is 5.7 A .. Battery is constantly running Waeco on turbo chill mode at -10 C ... ( The Load ) -

Once I get my Clamp Meter things will be better but until then I am baffled ..

Cheers

Steve
AnswerID: 408663

Follow Up By: Battery Value Pty Ltd - Saturday, Mar 13, 2010 at 15:21

Saturday, Mar 13, 2010 at 15:21
Steve,

with your multi connected, wiggle the test lead plugs in their receptacles and watch the display fluctuations.

Best regards, Peter
0
FollowupID: 678603

Follow Up By: dbish - Saturday, Mar 13, 2010 at 15:30

Saturday, Mar 13, 2010 at 15:30
Steve, You will find the analogue will be prety close to being right. Cheers Daryl
0
FollowupID: 678610

Follow Up By: Lex M (Brisbane) - Saturday, Mar 13, 2010 at 17:45

Saturday, Mar 13, 2010 at 17:45
What is the state of charge of your battery.

It would appear your solar panel through a MPPT controller may be capable of more than enough current to run the fridge, depending on what size fridge it is.

In this case the current out of the controller could be being reduced to whatever is necessary to maintain the charge in the battery.

To do what you want to do you need a load that is greater than the solar system is capable of delivering and/or a battery that is discharged enough to be in bulk charge mode.

Under these circumstances the controller should be outputting it's maximum capability. Otherwise it will be controlling the current and putting out less than it's maximum capability. In this case the switching will confuse a digital meter, particularly the cheaper ones.
0
FollowupID: 678628

Reply By: ob - Saturday, Mar 13, 2010 at 16:20

Saturday, Mar 13, 2010 at 16:20
Hi guys, not sure if this plays any significant part in this particular instance but I remember someone telling me that an analogue style of meter shouldn't be used to test electronic components because the current draw required for these meters to operate can cause damage to the equipment being tested. Obviously the amp test being done shouldn't damage anything but would the current requirement for the analogue meter be high enough to cause any inaccuracy????
Also Steve do I understand correctly that you were expecting to see more amperage out of the MPPT than the panel is putting in?????

Thanks Pop
AnswerID: 408675

Follow Up By: dbish - Saturday, Mar 13, 2010 at 16:35

Saturday, Mar 13, 2010 at 16:35
Hi Pop, On 12v solar systems analogue meters are not likely to have any loading afects on any readings. The only time you may run int o trouble if you started taking voltage readings inside a switch mode regulator or power supply, since these days they use MosFets instead of transistors & these dont like static discarge on them, causes instant destruction. Daryl
0
FollowupID: 678620

Follow Up By: Mandrake's Solar Power- Saturday, Mar 13, 2010 at 16:41

Saturday, Mar 13, 2010 at 16:41
Pop ,

MPPT regulators have the ability to convert DC to DC and increase the Amps of input to output by between 10 and 30% ( thats the claim ) -- In every test I have done so far on this unit there has only ever been a slight decrease in Amps between input and output which I guess is expected as the reg runs off the incoming solar power or the battery - Never have I seen the output being higher than the input
. It is possible that this particular unit is a dud and the MPPT circuit is not working and I've only got the PWM circuit running - which is basically what everyone above has mentioned ... that the PWM is somehow interfering with di\gital Multimeters

So as I said before methinks I have been duped ...

Cheers one and all tomorrow will check the 2 metres in series thing ...

Steve

0
FollowupID: 678621

Follow Up By: Member - Allan B (QLD) - Saturday, Mar 13, 2010 at 17:54

Saturday, Mar 13, 2010 at 17:54
Steve, I'm not so sure that you have been duped.

As I have said, I'm not highly skilled with solar panels but is it not the case that an MPPT regulator will only give you a gain in current if two conditions apply. Firstly the load on the regulator must be at least at or above the capacity of the panel in non-mppt mode, and secondly there needs to be sufficient radiation falling on the panel to supply the energy of such a load.

Unless these two conditions apply then the MPPT controller cannot produce a gain in current and will operate simply as a switchmode current limiting regulator.

So, try your assessments with a flat battery and in good sunlight.

Cheers
Allan

Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 678629

Follow Up By: Lex M (Brisbane) - Saturday, Mar 13, 2010 at 18:53

Saturday, Mar 13, 2010 at 18:53
"So, try your assessments with a flat battery and in good sunlight. "

Exactly and use a DC clampmeter to keep it simple.
0
FollowupID: 678636

Reply By: Member - John and Val - Saturday, Mar 13, 2010 at 17:05

Saturday, Mar 13, 2010 at 17:05
Steve,

Everyone else has had their say! Here's my take on it:

Moving coil analogue meters are inherently integrating devices. By their very nature they provide a reading continuously averaged over time. Digital meters also integrate over a short period (typically 0.1 or less of a second), but not continuously. For this reason, a digital meter can give wildly fluctuating readings while monitoring an intermittent voltage or current such as from some switching devices.

It would be worth comparing your meters when they are handling a steady current such as that drawn by a lamp from a battery. If you connect 2 meters in series with each other and monitor a known steady current through them the two readings should be within about 5% of each other. If still wildly different, you have a meter problem. As others have suggested, the resistance of the meters and leads will influence the readings, but should affect the two meters in the same way. (It's not generally realised that when measuring current, a digital multimeter actually measures voltage not current, the voltage developed across a low value precision resistor due to the passage of the current, hence there is always resistance introduced by the meter.)

I think your digital meters are having trouble with the intermittent nature of the current flow from the MPPT controller. I would trust the analogue meter.

One way I've used to get a good average from a digital meter is to fit a low value meter shunt resistor (eg 1/1000 ohm, available from Jaycar and elsewhere) in series with the current to develop a small voltage, feed that voltage though a resistor/capacitor filter to a digital voltmeter. Using the voltage reading, plus Ohms law, will give you a current reading. This way the meter is not exposed to the wild fluctuations present in the original signal.

Suggest try the series connected meters with a steady current to check the meters, then use the analogue one for your present purposes.

HTH

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

Lifetime Member
My Profile  My Blog  Send Message

AnswerID: 408681

Reply By: V8 Troopie - Saturday, Mar 13, 2010 at 21:40

Saturday, Mar 13, 2010 at 21:40
"The whole purpose of the exercise was to find an "easy" test to prove an MPPT controller actually puts out more amps than is coming from the solar panel .. "

And that is where you got astray....

MPPT units put out more *power* from the solar panel.

You get maximum power when the panel voltage is well above the battery charging voltage - a standard solar regulator just cuts this extra power off.

You also get power from the solar panel when its output voltage is below the battery charging voltage (as happens in low light conditions) - a standard solar regulator puts out nothing in this situation.

The battery charging current you measure on the MPPT output has a lot to do with the state of charge of your batteries. The input current to the MPPT reflects that too.

If you want to "prove" your test then get two identical analog meters and put one each in series with the input and output of the MPPT. Do your measurements after the battery has powered a load for a while but disconnect the load for the test.
I found digital meters difficult for that, certainly the digital display in my MPPT jumps about a bit on the Amps reading.

Keep in mind to disconnect the panel and the battery while you mess about with amp meter connections

AnswerID: 408718

Reply By: Atta Boy Luther - Sunday, Mar 14, 2010 at 12:13

Sunday, Mar 14, 2010 at 12:13
Youtube is your friend . Unless you are a sparky never touch AC .
AnswerID: 408764

Sponsored Links

Popular Products (14)