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Registered Member #58118
Joined: Sat Dec 12 2015, 12:34AM
Location: Sanger, Texas
Posts: 14
Just wondering if anyone had added the phase lead inductor / burden resistor to an older style UD board that uses the 74HC14? I noticed looking through the UD schematics that the TL3116 started being used around the same time that phase lead inductors were. My question is this... Can you just simply install the inductor and resistor on the old style board? Is the 74HC14 sensitive enough, or would I also need a comparator for better gain?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The 7414 has a wide hysteresis band which will give a variable delay to its output when feeding in a sinusoidal voltage signal like that from the LR filter (phase lead circuit).
My version of the phase lead circuit relies on a very high voltage across the LR filter, 50 to 70V peak. This is of course way too much for a 5V comparator, so this signal is fed through a resistor into a diode clamp network to make a sorta square signal of just a few volts (limited by the diode drop). The subtle part is that the feedback voltage is not constant, and starts off very small, and so the comparator needs to have a small hysteresis band (to avoid noisy switching) so that the delay between feedback zero crossing and comparator output change is small. The reason i let the CT voltage get so high is so that the variance in switch delay would be minimized, basically, the input signal would be way bigger than the hysteresis band.
You could probably just stick the LR + series R in front of the CT input on my UD1 and get OK results. I didnt like the idea of wide variation in switching timing at the start up, but practically speaking, the current is "small" there anyway, so switch timing is less important.
Registered Member #58118
Joined: Sat Dec 12 2015, 12:34AM
Location: Sanger, Texas
Posts: 14
Steve,
Thanks for the reply. I fiddled around with it a bit, but in the end I just decided to build up another one of the UD 2 boards I have lying around. No sense trying to solve a problem that's already been solved...
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Adding phase lead to older UD board
