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Registered Member #139
Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358
I just spent a while trying to work out why I couldn't get either a PLL or a 74HC14 circuit to work on feedback. I figured out that the only thing different in the circuit than the last time I built it is the fact I used 1N4148 diodes in place of my usual 1N60's, so I swapped them out and the circuit works. (I have always previously used 1N60's) Now I'm unsure as to why this happens, I am doing low power testing, so maybe the 1N4148's don't start to work properly until much higher power levels? It appears odd though that they seem to be interchangeable in this application, others have used them no problems, yet my circuit seems to function with one type and not the other. Maybe others who have tried to build these circuits for the first time have used 1N4148's and given up. There is always the chance I have got something totally wrong
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
4046 PLL has very sensitive input and you can get it working simply with antenna or base current feedback.
Two fast diodes of almosta ny kind are good for input clamping. You don't need germanium diodes for that, 4148's are most used and proved good for application.
With PLL and base feedback I use a lower value resistor (10k> , lowest that works is best) to minimise noise on highly sensitive input, and in place of antenan I put BCF transformer with DC blocking cap.
Also try swapping feedback, gate drive or primary if phasing is incorrect. (hope I didn't need to mention that)
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The issue is simply the junction capacitance of the diodes. Many diodes are several pF, and if your feedback impedance is too high, this capacitance can load it down too much. Im surprised the 1n4148's didnt work when the 1n60's did... i find them to be pretty much interchangable in these circuits. It could be that you are working at low power levels, so the feedback is accordingly weaker.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
That's scary, how long has the gate it is going into lasted? Seems like the chip is going to latch up sooner or later
Back on topic...
As to the diodes, I have even used uf40n60's on a simple antenna->gate driver->fet coil, so long as you tapped the antenna to get it going it seemed to work at pretty low power levels
Are you running at much higher of a frequiency? Could have something to do with it...
Banned on 3/17/2009. Registered Member #487
Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617
Well the coil is now dead but it was because I pushed it too far. I changed the on time and it couldn't handle it. It was working for about 2 weeks straight. What vltage rating of 1n60's do you use. I think I had 5.8v diodes in there. Maybe they were too low. I think I have 12 volt 1n60's too but I haven't tried them.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
On a small SSTC I once reversed the supply voltage (for very short period), clamp diodes went into conduction and one died. She saved the chip but since it died short output was held low.
I doubt that diode capacitance really means something, especially with 4046 wich is very sensitive and could acheive lock witj just hundred or two milivolts on input.
Antenna feedack s really simplest to get working and unless they are blown I wouldn't say 4148's are the problem.
I also ran him without diodes and there were no side effects, even with antenna pretty close to coil.
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1N60 V 1N4148
