Rap 'n Tap
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From: Ben H. Tongue
Date: 3/5/01
Time: 9:16:48 AM
Remote Name: 209.236.135.47
Hi all,
Some of us have experienced very good results with the Agilent 5082-2835 Schottky dode, especially on weak signals. Others have had poor results. I recently started using a different soldering iron than usual, then noticed that my crystal set had less sensitivity and selectivity. Later I remembered something I had read many years ago, that some temperature controlled soldering irons tended to damage semiconductors because of some 60 Hz power leakage from the tip. I thought that maybe the soldering iron had this problem and had damaged the diode.
Agilent specifies that a '2835 should pass less than 100 uA reverse current at an applied voltage of 8V DC. I found that several virgin '2835 diodes passed only 2 uA at 8V. The one soldered into my crystal set passed 1000 uA at 9 V when taken out and tested. Its reverse leakage at crystal set operating voltages was probably much degraded. This test is very easy to perform. Simply connect in series a 9 V battery, a 1000 Ohm protective resistor, a DVM and the diode. Polarity of the battery, of course should be such that it back biases the diode. Read the current.
The soldering iron to which I had changed had a power transformer mounted in a small plastic housing, with a coiled iron holder attached. The AC plug was a three wire type. When I used an AC DVM to measure the voltage between my crystal set ground connection (a water pipe) and the iron tip, the reading was 61 V. Switching to the current scale of the DVM showed a current of 280 uA. I then decided to see if the diode would be further degraded if I momentarily connected it between the crystal set ground and soldering iron tip a couple of times. Wow! I now had a "diode' that was almost a zero ohms device. The voltage reading between ground and tip of the iron I had previously used, as well as another one ranged between 20 and 40 V, but the AC current from tip to ground was 5 uA or less.
What to make of all this? 1. I ruined the diode by soldering it into the crystal set circuit with the iron to which I had changed. BTW, the external crystal set ground was connected during the soldering. 2. When doing any soldering in a crystal set, protect the diode. The safest thing to do is to short the diode leads with a clip lead until you are finished working on the set. You can also try electrically grounding the iron tip to the crystal set circuit ground. 3. Disconnect the external antenna and ground connections to the crystal set when soldering anything, if the tip is not connected to the ground. I didn't do this and maybe if I had, the damage would not have occured. 4. Use a soldering iron with low AC voltage and power leakage to the tip. 5. Another caution: Static electricity can sometimes damage semiconductors. Moisten your finger and touch a ground connection while before up any semiconductor, especially in the dry winter climate. Most of the time this precaution will turn out to be unnecessary, but who can tell if maybe a static discharge damaged the semiconductor just a little bit? The damage will hurt DX.
The '2835 diode is easier to damage than a germanium diode because its reverse breakdown voltage is much lower. Maybe this is the cause of some of the performance problems reported by those trying a '2835. BTW, the SMD version of the '2835 is called HSMS-2820 and is specified to have a reverse leakage of less than 100 uA at the higher breakdown voltage of 15 V than the glass '2835 at 8 V. The reverse voltage ratings of germanium diodes I have seen is 60 V. or above.
I carefully soldered in a replacement diode and the old sensitivity and selectivity returned.
Best Regards,
Ben