CAPACITANCE vs. VOLTAGE for some common diodes.
The QRP-L thread on "what's the capacitance of a 1N4004," etc. made
me take the following measurements only moments ago. It is based on
a sample of one, but from experience, the 1N4000's and the 1N5232 is
very close to previous usage. See notes below.
I just finished building a junk-box VFO for the Winter QRPp "Hints and
Kinks" (hitting the streets next week I think) and used 1N4001's to
make a 2-3v poor-man's regulator as the "varicap" reference, and a
1N914 for the actual varicap, which tunes the VFO from 7.024-7.063 MHz.
As can be seen from the data below, while it is very practical to use
1N4000 series, etc. as a poor-man's varicap, it is also rather silly to
reverse bias them much more than 2 or 3 volts. Beyond that, the
capacitance change is miniscule, not to mention rather small!
Next project ... measure the Q's of the following diodes, as the Q
goes up as the reverse voltage goes up (higher Q at 4v than at 0v).
-------------------------------------------------------------
VOLT. 1N4001 1N4003 1N4004 1N4006 1N4007 1N4148 1N5232
-------------------------------------------------------------
0.00v 28.5pF 34 36 20 19.5 2 126pF
.25v 23 27 29 15.5 15.5 3 112
.50 20 23.5 25 13.5 13.5 5 104
.75 18 21.5 22 12 12.5 6 96
1.00v 16.5 19.5 20 11 12 8 92
1.50 15 17 17.5 10 10.5 12 84
2.00 13.5 15.5 15.5 9 9.5 16 78
2.50 12.5 14.0 14.0 8.5 9 32 73
3.00 11.5 13.0 13.0 8 8.5 50 69
3.50 11 12.5 12 7.5 8.5 72 66
4.00v 10.5 12.0 11.5 7.0 8.0 98 64
--------------------------------------------------------------
NOTES ON TESTS:
1. Diode capacitances measured with an H-P 4332A LCR Meter using a
Datel DC8500 Precision Voltage Calibrator as the DC source driving
the EXTERNAL DC BIAS VOLTAGE on the LCR Meter.
2. Below 30pF, accuracy +/- .5pF. Above 30pF, accuracy +/- 1pF
3. Capacitances measured with diodes REVERSED biased except 1N4148.
4. 1N4148 capacitances are for FORWARD biased voltages; reverse bias
capacitance on the 1N4148 is about 3pF below 10v.
5. The 1N4006 and 1N4007 are actually a PIN diode structure to
accomodate the larger current ratings, and thus do not have the
typical reverse bias capacitance of most switching/rectifier diodes.
6. Zeners actually make fairly good, linear varicaps ... but with a
fairly high standing capacitance (with high tuning voltage). Rule
of the thumb for linear region is to bias at around 1/2 the zener
voltage and below. In the case of the 1N5232, a 5.6v zener, the
capacitance gets fairly flat around 3v, or about 1/2 the Vz=5.6v.
7. It was pointless to reverse bias beyond 4v due to very little
additional change.
GL, Paul NA5N
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