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Registered Member #1951
Joined: Sun Feb 01 2009, 01:59PM
Location:
Posts: 105
hello all, i have build the mini sstc: and the zener's that are in the recieving part of the circuit that are used are: 1n4148 so the problem is that i already replaced them 3 times. every time i had the sstc on, I only could have it on for about 10 minuts. there is somthing blowing up my zener's, cause if measure the zener's after they blew up, they would conduct if both ways.
can someone help me?? i don't know where to look or where to measure to find the problem. every tip is welcome.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hi Robert:
1N4148 is a fast switching diode, and not a zener.
In SteveC's circuit, these diodes are being used to protect the input of the first receiver stage from excessive antenna voltage.
I would suggest shortening the antenna, and/or moving it further away, so it picks up less of a signal to cook off those diodes. You could even put a variable attenuator in the antenna line if you want to get fancy. Then you'd be able to tweak it to just the right amount to make the circuit work reliably and no more.
Registered Member #1951
Joined: Sun Feb 01 2009, 01:59PM
Location:
Posts: 105
oops, i am feeling such a fool saying that it were zeners no antenna is 10 maybe 15 cm away. unless i even have to move it further away. i'll try diffrent distand. tell me what a variable attenuator is, ow wait that is where google is for :) thanks for tips. :)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
R v.d. Tuuk wrote ...
oops, i am feeling such a fool saying that it were zeners no antenna is 10 maybe 15 cm away. unless i even have to move it further away. i'll try diffrent distand. tell me what a variable attenuator is, ow wait that is where google is for :) thanks for tips. :)
That seems very close to me, son. Close enough for invisible streamers to reach out and grab it, and then it's more new diodes for you! Move it out to a metre, and if that doesn't then work, move it in closer 5cm or so at a time, until reliable operation is secured, but no more.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Conundrum wrote ...
Interesting. I'd have used something like a 1N4007, they are rated to 1A peak and 1KV PIV.
Also another idea would be to scavenge the hv diodes from old disposable cameras, they work well. -A
The famous 4007 isn't fast enough Andre, and it has quite a bit of capacitance too - as I it see it if a big signal comes in, then the diodes will go into varactor mode, which would mean that Zin woud be inherently unstable, and unpredictable tuning effects could occur.
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keep blowing up diode's
