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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Hi all. As many of you probably know, LEDs can be used as makeshift tuning diodes (varicaps).
In particular, this page suggests that blue and white (contains blue LED and phosphor) LEDs seem to be more stable and shift frequency linearly and reliably whereas red large areas work well for large shifts but are less predictable.
I acquired five fairly large blue SMD LEDs surplus from a defunct UPS front panel, and connected in series with a 680pF SMD capacitor which was then encased in Polymorph as a light shield. This was then connected so as to either forward of reverse bias the LEDs through a 470kiloohm resistor. Used Peak Atlas LCR as the measuring device connected directly to the "earthy" side of LEDs and 680pF capacitor.
So far the results look interesting. All measurements on 200kHz range according to LCR.
Used 470uF capacitor as an isolated DC source, discharging through its own internal leakage.
DC leakage measured at 1.5uA at -5V according to meter. DC leakage at -7.5V is 4.5uA
Interesting, thats a massive change for not much voltage bias.
Now to try it at -20V...
Wonder if these can be used as improvised mini DMMs with a negative resistance oscillator? They would certainly work, and if the frequency was kept low would probably be good for hobbyist use. Simple AM radio or frequency counter as the pickup
not least the possibility of using one or more of these to tune an induction heater "on the fly" using infrared thermometer made from pager motor+interruptor and PIR sensor to feed back temperature changes.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
The Q will be especially poor when the diode is forward biased because the resistance in parallel with the capacitance drops, though its capacitance should indeed continue increasing.
An LED is probably a cheap way to get a high capacitance varactor as the die areas are relatively large, but the Q and tuning ratio will generally be higher in a purpose-built varactor diode.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Yeah, reverse bias would be better here. The best bet would be to use a CMOS oscillator based around a 74HCxx as these oscillate with a wide range of voltages.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Interesting experiment! I'm interested to see that the variation is non-symmetrical each side of 0V. I've sucessfully used 15V zeners as the varicap diode in FM transmitter bugs before, but if they perform anything like this then there must have been more audio distortion than I thought! Looks like the ideal operating point is with a negative bias to keep the thing well and truely off, I suppose the leakage varies as you forward bias the diode even if you're below the critical voltage.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Yes, leakage will vary which is why for these circuits I now add a 470 k series resistor and ferrite bead with 10 turns in series with the diode-capacitor junction to prevent leakage affecting the tuned circuit.
Encasing it in Polymorph is IMHO the best method of lightproofing, plus its possible to remove it if something goes wrong.
See the multimeter thread, as there is more to this than a simple tuning diode. There are temperature effects but they can be worked around with a very simple circuit based on a variable error bias.
Interestingly the humble low emission blue LED even if wear has made it useless for light emission is still fine. Evidently the mechanism that causes the tuning effect is different than the light emission mechanism. I really wonder if this would work with OLED displays as well for the same reason, someone PLEASE test this if they have a minute.
This is basic physics 101, the tuning diode effect is caused by the barrier within the diode changing width which effectively changes the plate area and causes the capacitance change. Rumour has it that the now unobtainable MV series diodes used in early AM receivers used gallium arsenide IR LEDs doped with something else which amplified this change, possibly magnesium or some other reactive metal.
I am now working on harnessing this effect to make an ambient RF energy powered LED flasher..
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Hacking blue LEDs into tuning diodes
