# DAC Problems

Discussion in 'Electronic Basics' started by Immo Birnbaum, Jul 19, 2004.

1. ### Immo BirnbaumGuest

Hello everybody,

after spending two semesters listening to electronics lectures, I've
recently started assembling some boards with simple circuits on them
and I've also hooked some of these up to a microcontroller evaluation
board based on a Motorola HC08. Recently I bought some parts for my
next little project, but I just can't get the job done. I'd greatly
appreciate if anyone around here could help me with that little
problem.

I've got several old speed/traction gauges which were once used in the
dashboard of a locomotive. Operation of these gauges is really simple:
they have a + and a - connector, so just hook them up to a PSU, set a
voltage and the gauge will show an according value. The gauge reaches
"top speed" at approx. 3.4 Volts, measured with the multimeter
parallel to the gauge's connectors. Here's a picture:

http://immo.birnbaum.bei.t-online.de/t1.jpg

At that "speed", the current is approx. 1.5 mA, gauge and multimeter

http://immo.birnbaum.bei.t-online.de/t2.jpg

Great, I thought, this thing could be computer-controlled - it would
make a great CPU load meter or something similar (of course replacing
the km/h scale with something else). So I went out and bought a few
DAC chips, type AD7524, an 8-bit DAC, and did the following:

- connected D0 to D7 to the microcontroller board
- connected WR and CS to GND, as those are low-active
- connected VREF to the board's power supply, which is the output of a
7805, with a potentiometer inbetween so that I can regulate the
reference voltage.

When I started testing the board, I had D0 to D7 connected to the
HC08, but later just set all data inputs to high, so that VOUT is
always =VREF. I'd later reconnect the data inputs to the
microcontroller. So far, the entire thing worked. I set the
potentionmeter so that the voltage above it was approx. 3.5 V.
With all the data inputs high, the output voltage (OUT1 to GND)
also was 3.5 V as long as I didn't connect the gauge to the DAC's
output. As soon as I did that, the output voltage dropped to 0.9 V and
the needle of the gauge didn't move any further than 40 . When
reducing the resistance of the potentiometer, the needle would move a
little further, but not too much.

Can anyone tell me what's wrong here? I hooked up two of these gauges
to the output of the 7805 with potentiometers inbetween to check if
the 7805 could supply the two gauges at all, and it worked fine. Could
it be that the DAC cannot supply the required current? Do I need an
op-amp?

Thanks in advance if anybody could help me with that problem.
Immo Birnbaum

P.S. I also tried the DAC's "voltage mode" which was described in one
of the datasheets, which means connecting VREF to OUT1 and getting the
output from VREF or something like that, the effect was the same.