METER CONVERSION

[MARK J. HABIAN]

I AM BUILDING A PROJECT THAT USES A 0-100 UA DC ANALOG METER. I WOULD LIKE TO USE A DATEL/MURATA D20PC-1 LED 0-199.9 UA DIGITAL IN IT'S PLACE. I REALIZE THAT I WILL HAVE TO POWER THE DIGITAL METER. CAN THIS BE DONE? I DO NOT LIKE THE RESOLUTION ON THE ANALOG TYPE AND THE PRICE TAGS.

[sghioto]

MARK,

Short answer,yes. Data sheet shows simple connection as long as the power supply is a least 5 volts.

Steve G.

[Bigglez]

I AM BUILDING A PROJECT THAT USES A 0-100 UA DC ANALOG METER. I WOULD LIKE TO USE A DATEL/MURATA D20PC-1 LED 0-199.9 UA DIGITAL IN IT'S PLACE. I REALIZE THAT I WILL HAVE TO POWER THE DIGITAL METER. CAN THIS BE DONE? I DO NOT LIKE THE RESOLUTION ON THE ANALOG TYPE AND THE PRICE TAGS.

Yes. You haven't indicated if the original meter
is measuring current or voltage, only that it has
a sensitivity of 100uA (FSD).

Most likely there is a series resistor associated with
the meter, to scale the display. When replacing the
MC meter with a digital type you may need to add
a shunt resistor (to bring the digital's input resistance
down to match 100uA FSD) or change jumpers on
the actual digital meter module (varies by brand).

If you are still in doubt, can you post a link to the
schematic (or a screen capture)?

BTW, your CAPSLOCK key is ON.

[MARK J. HABIAN]

meter is used to measure voltage. it is wired into a bridge of (4) 1n914 diodes. third bridge connection goes to pin 6 of a 741 op amp. fourth bridge connection goes to pin 2 of that op amp and to a 2.2uf (+) electrolytic capacitor in series with a 10k resistor to pin 6 of a second 741 op amp.as previously stated, +meter goes to cathode's of (2) diodes, -meter goes to anodes of second pair of diodes. thankyou

[MARK J. HABIAN]

circuit is an impedance meter printed in popular electronics october 1972, author is charles d. rakes. was also reprinted in electronics experimenters handbook 1973. out of my hundreds of books and magazines which contain thousands of circuits, this is the only circuit of it's kind.

[Bigglez]

meter is used to measure voltage.

Not as described below. The MC meter is in the Op
Amp's feedback path, and therfore current driven.

The transfer function of the circuit is:

V(fsd) = 100uA * 10k = 1V.

it is wired into a bridge of (4) 1n914 diodes. third bridge connection goes to pin 6 of a 741 op amp. fourth bridge connection goes to pin 2 of that op amp and to a 2.2uf (+) electrolytic capacitor in series with a 10k resistor to pin 6 of a second 741 op amp.as previously stated, +meter goes to cathode's of (2) diodes, -meter goes to anodes of second pair of diodes.

Is this the circuit? Do you have the rest of it?
Please post (much easier to follow than text...)

[Bigglez]

circuit is an impedance meter printed in popular electronics october 1972

This one? (eBay Item number : 280262774083)

[MARK J. HABIAN]

your circuit is correct. the popular electronics issue that you have is also correct. i am not as good on the computer as you, but will try to show circuit as requested. thankyou.

[MARK J. HABIAN]

[MARK J. HABIAN]

transistor q1 and it's associated components form a simple 1-khz phase-shift oscillator that is buffered by q2. the output of q2 is taken from the level control, r9, and applied to the primary of t1. transistor q3 is connected as a constant current source, and its output (collector) is coupled to ic1, an op amp circuit having a high input impedance and a gain of about 10. the next stage is ic2, connected as an ac voltmeter. the output voltage of the constant current source depends on the values of the collector and emitter resistors. the base is held at a fixed voltage by zener diode d1. when an unknown resistance or reactance is connected to j1 and j2, the amount of voltage developed across it is read on the ac voltmeter.

[Bigglez]

your circuit is correct.
... will try to show circuit as requested.

Thanks for posting the schematic.

Back to your original request to substitute a
DPM (digital panel meter) for the MC type.

There are two problems to solve. Firstly, your
DPM has to be shunted to act as a 199uA FSD
ammeter (the MC is 100uA FSD). Assuming
the basic DPM is 199mV FS, a resistor of
0.199/0.000199 = 1k0 1% 1/4W is required.

The second problem is bit harder, depending
upon your DPM's circuit. In this application
both +ve and -ve meter terminals need to float
from ground for the rectifier circuit to work.
If the DPM has an internal connection from
either input to the DPM power input there is
a problem. You can check this with a DMM
on the low ohms range.

A work around is to power the DPM from a
floating power supply or battery.

[MARK J. HABIAN]

thankyou for all your help. my intentions for the use of the digital meter were to use a separate battery. following authors calibration instructions, i was only going to bring digital meter up to 100ua and not use full range.

[Bigglez]

i was only going to bring digital meter up to 100ua and not use full range.

Okay, but you still need to convert the basic DPM to
the same scale as the MC meter you're replacing.

Unless noted otherwise expect the DPM input to be
very high impedance (1Mohm is common). The
1000ohm 1% 1/4W resistor is a current shunt to
get a 100uA current meter to match the 100uA MC
meter. Whether you use it for 0 - 100 or 0 - 199 is
up to you, but at 100 reading it has to draw 100uA
from the original circuit.

Here is a typical DPM datasheet.

[DELETED]

DELETED

[MARK J. HABIAN]

thanks again for your efforts. i am constructing this now, and have found (2) mistakes. c10 is not electrolytic because parts list designates as 2.2uf, 50-volt polystyrene or mylar. secondly, c10 and r25 are in opposite connection on pc component layout diagram. in other words, c10 goes directly to pin 6 of ic1 and other end of c10 goes to r25. other end of r25 goes to pin 2 of ic2. future issues of popular electronics do not publish this error, so i cannot confirm it, but i think that the hookup i just described is correct. feedback welcome.