Jaycar panel meter, QP5585

[Franc Zabkar]

I recently ordered the following panel meter from a Jaycar stockist:
http://www.jaycar.com.au/productView.asp?ID=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

 

[Mr.T]

I recently ordered the following panel meter from a Jaycar stockist:
http://www.jaycar.com.au/productView.asp?ID=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.

 

[Franc Zabkar]

I recently ordered the following panel meter from a Jaycar stockist:
http://www.jaycar.com.au/productView.asp?ID=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.

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

 

[Gadget]

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

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

 

[Andy Wood]

.. . .

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?

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




Andy Wood
[email protected]

 

[Mr.T]

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.

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.

 

[Jason Sobell]

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

 

[Franc Zabkar]

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

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
_|_
=

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
_|_
=

Fortunately 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

 

[Mr.T]

Ah, the brain to keyboard police are out in force

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

MrT.

 

[Franc Zabkar]

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

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

I recently ordered the following panel meter from a Jaycar stockist:
http://www.jaycar.com.au/productView.asp?ID=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

- Franc Zabkar

 

[Gadget]

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

MrT.

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

 

[jasen]

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
_|_
=

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