Jaycar panel meter, QP5585

The catalogue describes it as "not just a digital display, but a fully functional high impedance digital panel meter that only requires power to operate. It will measure voltages from 200mV to 500VDC (jumper selectable) and has a maximum count of 1999".

Based on the above description it would be reasonable to expect that the meter would have 5 decadic ranges, namely 200mV, 2V, 20V, 200V, and 500V (2000V), as do all (?) other panel meters and multimeters. But, inexplicably, this meter does not have a 2V range. There doesn't seem to be any logical reason for this because the ranges are selected by a single potential divider - an additional range would have required only an extra resistor and an extra couple of PCB pads.

You can guess which voltage range my application requires. :-(

- Franc Zabkar

-------------------------------

Surely you can use the 200mV range and provide your own multiplier resistor, or possibly swap one of the ones already there?

MrT.

On Tue, 14 Nov 2006 20:15:26 +1100, "Mr.T" put finger to keyboard and composed:

resistor,

I had resigned myself to modifying the 20V range (the 200mV range is a direct input to the A/D chip), but at the cost of precision. The existing resistors have special values.

In any case my post was intended as a complaint about the product as well as a warning to others.

- Franc Zabkar

--=20 Please remove one 'i' from my address when replying by email.

Excuse my ignorance (it's been a while since I did much analogue electronics), but can't you just use a 1K/10K divider on the 20V setting? This doesn't need any special values, just low tolerance resistors.

- Gadget

. . .

Depending on which way you connect them you would have 220V or 22V giving a reading of 20V.

Andy Wood snipped-for-privacy@trap.ozemail.com.au

Firstly he wants to measure 2V, not 200V. Secondly measuring anything with an 11k ohm input impedance is asking for loading errors, even assuming the DVM had an infinite input impedance itself (and it doesn't).

MrT.

Ah, the brain to keyboard police are out in force :)

My point is that is the bottom range bypasses all the 'special' resistor values then providing your own divider with the correct ratio will do exactly the same thing, avoiding rewiring the existing ranges. If the existing 2V one uses 100M and 1M then replace it with either 10M and 1M, a

100M pot, or 2x33M/33M.

As Franc 'needs' a 2V solution there's a good chance he knows exactly what the impedance can be and can avoid the loading issues.

Gadget

On Wed, 15 Nov 2006 22:17:19 +1100, Jason Sobell put finger to keyboard and composed:

setting?

for

itself

a

what

This is the existing divider network:

Vin o----|----o o---| 200mV Ra 9M9 | |----o o---| 20V Rb 90K |---------> to A/D chip ( >100Mohm ) |----o o---| 200V Rc 9K | |----o o---| 500V (2kV) Rd 1K _|_ =3D

Clearly the input impedance is 10Mohm, yet the panel meter's specs state ">100Mohm".

If the meter had been designed "properly", then the divider would have looked like this:

Vin o----|----o o---| 200mV Ra 9M | |----o o---| 2V Rb 900K | |----o o---| 20V Rc 90K |---------> to A/D chip ( >100Mohm ) |----o o---| 200V Rd 9K | |----o o---| 500V (2kV) Re 1K _|_ =3D

=46ortunately my application only requires a 2V range, so I'll just add an extra 1Mohm to Rb making it 1090K. The panel meter has a trimpot which will hopefully account for any tolerances.

- Franc Zabkar

--=20 Please remove one 'i' from my address when replying by email.

Frank,

One of those meters (can not remember if it this one) had an error in description. Input signal is differential, not a common ground!

Rudolf

The catalogue describes it as "not just a digital display, but a fully functional high impedance digital panel meter that only requires power to operate. It will measure voltages from 200mV to 500VDC (jumper selectable) and has a maximum count of 1999".

Based on the above description it would be reasonable to expect that the meter would have 5 decadic ranges, namely 200mV, 2V, 20V, 200V, and 500V (2000V), as do all (?) other panel meters and multimeters. But, inexplicably, this meter does not have a 2V range. There doesn't seem to be any logical reason for this because the ranges are selected by a single potential divider - an additional range would have required only an extra resistor and an extra couple of PCB pads.

You can guess which voltage range my application requires. :-(

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

If you are not bothered about the loading then just bridge the 500v link and put a suitable (lowish value) resistor across the 200mV link.

Alternatively select the 200mV range and put an external resistor (approx 100M) on the input to increase the range.

Or select the 200mV range and put an external voltage divider on the input.

Lots of possibilities!

Alan

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setting?

for

itself

At least the first time you had the grace to excuse your ignorance :-)

MrT.

On Thu, 16 Nov 2006 11:56:50 +1100, "Rudolf" put finger to keyboard and composed:

This is the "B" version which has a common ground. The "A" version is differential. Thanks anyway.

- Franc Zabkar

--=20 Please remove one 'i' from my address when replying by email.

Sorry, it's always nice to see a "actually, I think you meant this..." rather than a "you idiot..." style response. There are many ways to define grace.

Still, it's nice to see Alan giving a constructive and helpful response! Thanks Alan :)

Cheers, Gadget

how to get a 2V range:

Vin o----|----o o---| 200mV Ra 9M9 | | |----o o---| 20V Rb 90K |---------> to A/D chip ( >100Mohm ) |----o--o---| 200V Rc 9K | |----o o---| 500V (2kV) Rd 1K _|_ =

Bye. Jasen