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24VAC to DC

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joeyjoejoe:

--- Quote from: Paul Moir on January 22, 2020, 11:04:30 pm ---Presuming K1_EN is a 3.3v output from a uC, watch out:  that MOSFET has a Vgs(th) of up to 3.1volts.  The threshold means that's when it just starts to turn on.  Note that Rds(ON) is specified only down to a Vgs of 4.5 volts (3.3 ohms).  Check out the pg 3 top left graph with drain current vs Vgs.  Pretty marginal at 50mA eh?  Especially if it gets cold?  And what exactly is the voltage on K1_EN when loaded down with a LED there anyway? 
I'm pretty sure you can get a better part for less money.  Sorting by quantity in stock is a quick way of turning up the good ones.  ;)

The 10K resistor is presumably a pull down to keep the MOSFET switched off while K1_EN is booting up?  Maybe LED1 is enough depending on it's colour and the Vgs of the MOSFET you select.

--- End quote ---

Damn, good catch! Something like the DMN63D8LDW-7 might be better? (3rd cheapest too :) )

The 10k is indeed a pulldown during bootup/etc. Good point that the LED might take care of it. I think I'll go chinese on the LED's - I could always leave the footprint in place and see if the pulldown needs populating or not.

Paul Moir:
All old is new again:  Muntzing  And I'm sure Muntz got it from somewhere.

DMN63D8LDW-7 looks much better.  Much better specified too.  And a quarter million in stock.  :)

Zero999:

--- Quote from: joeyjoejoe on January 22, 2020, 11:41:43 pm ---
--- Quote from: Paul Moir on January 22, 2020, 11:04:30 pm ---Presuming K1_EN is a 3.3v output from a uC, watch out:  that MOSFET has a Vgs(th) of up to 3.1volts.  The threshold means that's when it just starts to turn on.  Note that Rds(ON) is specified only down to a Vgs of 4.5 volts (3.3 ohms).  Check out the pg 3 top left graph with drain current vs Vgs.  Pretty marginal at 50mA eh?  Especially if it gets cold?  And what exactly is the voltage on K1_EN when loaded down with a LED there anyway? 
I'm pretty sure you can get a better part for less money.  Sorting by quantity in stock is a quick way of turning up the good ones.  ;)

The 10K resistor is presumably a pull down to keep the MOSFET switched off while K1_EN is booting up?  Maybe LED1 is enough depending on it's colour and the Vgs of the MOSFET you select.

--- End quote ---

Damn, good catch! Something like the DMN63D8LDW-7 might be better? (3rd cheapest too :) )

The 10k is indeed a pulldown during bootup/etc. Good point that the LED might take care of it. I think I'll go chinese on the LED's - I could always leave the footprint in place and see if the pulldown needs populating or not.

--- End quote ---
Why not swap the MOSFET for a BJT and suitable base resistor?

joeyjoejoe:
Parts count mostly. 4 less resistors, 4 less manual placements.

Zero999:

--- Quote from: joeyjoejoe on January 25, 2020, 05:30:22 pm ---Parts count mostly. 4 less resistors, 4 less manual placements.

--- End quote ---
How did you draw that conclusion? A BJT would use exactly the same number of parts and small BJTs are normally cheaper, than MOSFETs.

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