what I was looking for. Thanks John. > >Don --- My pleasure. :-) -- JF
no, not stuck, you can use the NPN in common-base mode.
+24 ---------+------. .--- | | | | | ==== | | | | | | |_|1.5K | | | +------' / +5 |/ -+----| | |\| | ~\ | | | | | | | | 470 Ohms | | | _|__ |_| | | mcu|----' ____| |--+--- 0V
The 470R puts about 9mA through the transistor, dependant on how strong the MCU output is, across the i.5K that's about 13V to turn the mosfet on, which sould be about right..
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what I was looking for. Thanks John. > >Don --- My pleasure. :-) -- JF
could have just used an n-type MOSFET and controlled it directly with the MCU.
you start thinking about using a PNP to switch it, you run into the same problem.
Nothing will put the emitter voltage higher than the base, the B-C diode is plenty strong enough. puwer up isnt going to be crazy. But you've got a base current problem when you try to turn the transistor on.
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what I was looking for. Thanks John. > >Don --- My pleasure. :-) -- JF
could have just used an n-type MOSFET and controlled it directly with the MCU.
you start thinking about using a PNP to switch it, you run into the same problem.
Oops! Needs a base or emitter resistor?
Grant.
what I was looking for. Thanks John. > >Don --- My pleasure. :-) -- JF
could have just used an n-type MOSFET and controlled it directly with the MCU.
as you start thinking about using a PNP to switch it, you run into the same problem.
For you, a Base R would be the simplest approach..
-- Using a E R is going to to force you to subtract the Vcc-.7 volts for example, from the upper divider.. I don't know what your VDD is but maybe it's 5 volts? If so, this would mean, you need to subtract ~ 4.3 volts from the 24+ up above which leads you to just under 20 volts to play with. This may work for you as a direct driving approach instead of using the voltage divider above. So select such a R for this, one suitable for putting the transistor into saturation would be enough I would say.. The one problem that may result from this is if the uP when in the high state does not meet the VDD of the uP. If it falls more than a diode drop below the Vdd, then it'll put the transistor into conduction state and slightly pull it down. That may not be a problem but something to look at.. If you want to experiment with emitter R's, try a 5k in yours.. Jamie
driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.
Simpler:
uP gnd port
where the uP port is active low, 3.3v logic.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.
-- Don specified that he wanted to drive a PFET gate with 24V and 19V and that his signal source was 5V logic. Your 3.3V logic ignores his 5V request and, with R2 in the emitter of the NPN, makes it impossible for its collector to get to his requested 19V.
driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.
I think he can handle that difference. 5 volt logic is getting pretty rare these days!
with R2 in the emitter of
Why impossible? This circuit allows any desired drop across R1, up until the NPN saturates. With a 24 volt supply, 5 or maybe 10 volts across R1 is easy.
The nice thing about this topology, besides the simplicity, is that the NPN is a constant current sink, so the drive to the upper pnp/pfet is independent of changes in the +24 supply.
To flip the control polarity, just do this:
The R1/R2 math is the same, but the uP port can be wimpier, since it doesn't have to sink the drive up into R1.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
I'm driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.
-- Nicely done.
Thanks John.
Don
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