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NCP81074 Driver Chip Input Resistor




 
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Author Topic: NCP81074 Driver Chip Input Resistor  (Read 1307 times)
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N4LTA
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« on: April 27, 2021, 06:19:52 PM »

All of my prototypes using the NCP81074 drivers were destroyed as well as all my notes. I used an input resistor to ground on the logic input. of the driver chip. I can't remember what the value was. Nigel used a 330 ohm resistor for his 20 meter Class E RF board. I assume that it is a pull down resistor and should be as small as the logic gate can easily drive . The data sheets don't show a resistor. Any info would be appreciated.

Pat
N4LTA
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vk3alk
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« Reply #1 on: April 27, 2021, 11:15:17 PM »

Hi Pat....

I did notice Nigel used a 330R resistor in that circuit.
To me it seemed rather low in value....
Perhaps something like 1K would be better as when no drive is applied that resistor would stop any pickup etc: and hold it at ground level...
Others may have ideas  Smiley


Wayne
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M0VRF
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« Reply #2 on: April 28, 2021, 02:31:46 AM »

Doesn't really matter, just depends on the drive level you have. Mine is marginal as drive direct from an AD9850 DDS PCB and there's a 220R Res on there so have ditched any input to gnd Res, no problems as yet!

However you may want to use a small value on the output. I do if driving SiC on lower frequencies but don't bother using anything and drive GaN FETs directly on 40m, again no problems seen as yet (3 years on).
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N4LTA
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« Reply #3 on: April 28, 2021, 10:43:07 AM »

Thanks everyone.

I may try 330 ohms first and see.  I'm driving with 74HC or HCT logic. Both can sink or source 20 mA. 5 volts across 330 ohms  is about 15 mA.  1 K might be better.

Pat
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N4LTA
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« Reply #4 on: May 05, 2021, 10:43:31 PM »

I ended up using a 1K resistor on the test transmitter.


Pat
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