开云体育

-----Original Message-----
From: Monty N5ESE [mailto:n5ese@...]
Sent: Friday, July 09, 2004 12:56 PM
To: John Fisher
Cc: AQRP
Subject: Re: [AQRP] FW: [BITX20] Inexpensive capacitor measurements...


Gang:

Here's an ASCII schematic to replace the missing GIF image in the
posting below. If it doesn't look right in your mail client, cut and
paste it into Notepad or your favorite word processor, and set the font
to courier or courier new.

I highly recommend that you install the 1000 pF calibration capacitor in
the circuit with a switch to switch it in and out as required, in case
your original "calibration" drifts. There are lots of things that can
affect measurement accuracy and stability in this circuit: line
frequency; line voltage changes; R1's potentiometer stability,
resolution and settability; your meter's linearity, and your meter's AC
input impedance.

A wall-wart (AC, not DC) would be an ideal transformer, comes pre-fused,
and keep you away from line voltages. A nominal 9 or 12 VAC wall-wart
would probably end up providing 15 VAC in this application.

Your $9 Harbor Freight Digital Voltmeter probably won't be very accurate
at low readings (say, below 100-150 pF), but try it and see.

By the way, you don't have to have exactly 1000pF for your cal
capacitor.
If you have something close, and you know what it is by virtue of its
tolerance or because you can measure it with a lab instrument at work,
that's good enough. For example, if it's a 1000pF NPO ceramic and you
know that its actual value is 1090 pF, adjust the pot so the meter reads
1.09 VAC. As another example, suppose you have a 2% 910 pF silver mica
- just adjust the pot so that the meter reads 0.91 VAC.

I wouldn't trust measurement readings below 50 or so pF very much...
stray capacitances in the layout, meter probes, and meter input
circuitry will begin to contribute significant errors as the capacitance
approaches lower values.

Still, it's a great little circuit!

73,
monty N5ESE

-----------------------------
Capacitance Measuring Circuit
-----------------------------
(view with monospace font like Courier)

Cx
__o-F-o__
) ________o---||---o_____
)|( |
)|( __________|
)|( 15 | |
115VAC )|( VAC \|/ |
)|( R1 | |
)|(________/\/\/\/\_______|
) ^ ^
_________) | 180K (typ) |
| |
| ____ |
| | | |
|___| AC |___|
| VM |
|____|

I really like this BITX20 list on Yahoo groups. They're building up a
cheap rig and sending out great information :-)


Regards,
John

=============================================
email: k5jhf@...
photos:
files:
webpage:
callsign: K5JHF
=============================================

-----Original Message-----
From: Arv Evans [mailto:arvevans@...]
Sent: Thursday, July 08, 2004 3:35 PM
To: bitx20@...
Subject: [BITX20] Inexpensive capacitor measurements...


Hi

Since Farhan's stated original intent in designing the BITX20 was to
provide an inexpensive transceiver, maybe it would be appropriate to
show BITX builders an inexpensive way to measure unmarked or
questionable capacitors. The following circuit came to me via Don
Metzger - K8JWR, but I suspect the method has been around for quite
some time. The picture shown here is the relatively simple schematic
for this tool.



Use a mains transformer that delivers a few ma. at about 15 volts RMS.

Connect a known .001 mfd capacitor at Cx and adjust the value of R1 (
for R1 I used a 250K trim pot salvaged from an old circuit card ) for
an indication of 1.000 volts on your DVM. Now you can measure
capacitor values from a few pf up to 0.001 mfd by connecting them in
the place of Cx.

I used this method to "calibrate" the knob indicator on a 3X450 pf
variable capacitor that I regularly use as part of my test equipment
repertoire. Now I just set the dip meter to the desired frequency,
and use the variable capacitor to tune my inductor for a dip on that
setting. Once the coil is on-frequency I can read the capacitance
from my calibrated variable capacitor and substitute that value of
fixed capacitance into the circuit.

Part of the fun derived from building your own ham radio equipment is
in devising ways to perform complex design and testing by use of
relatively simple and inexpensive methods & equipment. This idea fits

into that category.

73's
Arv
_._

Before someone asks the question...No, this circuit will not measure
electrolytic capacitors, but it will get you close enough for most of
the small caps you will be using in designing resonant circuits for HF

applications.
---------------------------------------------------------







Via the Austin QRP Club list
Yahoo! Groups Links

Via the Austin QRP Club list
Yahoo! Groups Links