Hi,
What is the purpose of the small value resistor R7 inserted on the mosfet source ? Schematic:
Hi,
What is the purpose of the small value resistor R7 inserted on the mosfet source ? Schematic:
source ?
Hard to say without seeing the whole schematic. Looks like a chunk of a simple boost converter. R7 doesn't make a lot of sense but since there is a 1N4148 for the diode efficiency seems not to matter. Maybe they wanted to prevent oscillations?
Or maybe it is there to reduce the chance of ice formation in winter :-)
-- Regards, Joerg http://www.analogconsultants.com/
source ?
Yes this is a boost converter. Full schematic is on Microchip application note #AN1209.
snipped-for-privacy@mid.>>> Hi,
source ?
#AN1209.
In case someone else wants to look:
R7 doesn't make a lot of sense. Other than to be able to measure the inductor current. It also protects (to some extent) against inductor saturation but why they put it in the source, beats me.
-- Regards, Joerg http://www.analogconsultants.com/
source ?
Where's the rest of the circuit?
Grant.
Probably to limit the inductor current, and keep the supply from collapsing during startup. (ports are high by default)
Cheers
They should most certainly not be high. Did the uC architects screw that up? 0.5ohms would draw quite some current.
-- Regards, Joerg http://www.analogconsultants.com/
snipped-for-privacy@mid.>>>> Hi,
source ?
note #AN1209.
Wow, what an insanely bad appnote!
John
snipped-for-privacy@mid.>>>>> Hi,
source ?
note #AN1209.
It sure ain't a pretty one, and that's when I really take schematics with a grain of salt.
-- Regards, Joerg http://www.analogconsultants.com/
snipped-for-privacy@mid.>>>>>> Hi,
source ?
note #AN1209.
Reduce R7 to zero and think fundamentals :-) ...Jim Thompson
-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et |
snipped-for-privacy@mid.>>>>>>> Hi,
mosfet source ?
note #AN1209.
But I am not a fundamentalist :-)
I never even have an R7 in mine unless I do current-mode control. Which I increasingly do because everything has to be so friggin' small these days.
-- Regards, Joerg http://www.analogconsultants.com/
"Martin Riddle" kirjoitti viestissä:i549hj$plv$ snipped-for-privacy@news.eternal-september.org...
Datasheet says they are high-impedance inputs as default. PIC are always that way.
"Joerg" a écrit :
What is wrong exactly with this schematic and appnote? I started making a simulation using Proteus/MPLAB and it seemed to work prettty well. It generated up to 75V with with 6V input and a 5k load.
The output will be current regulated and voltage limited.
I am just stuck now with the 18F14K22 A/D converter that always returns a null conversion result :-((
source ?
I do not think that we appreciate an ad for sports TV...
source ?
???
snipped-for-privacy@mid.>>>>>>>> Hi,
mosfet source ?
note #AN1209.
You mean your designs never ever had more than 6 resistors? I knew you were cheap, but not that much :-)
R7 is a pretty good and cost effective way of limiting startup & fault currents (uC controled, no current mode, boost,...) Of course that makes the MOSFET working linear under these conditions and it has to withstand the power...
-- Thanks, Fred.
And thereby hangs a tale ( or is it tail ? ). Relying entirely on simulations is very unwise and doesn't prevent today's 'design engineers' creating bad / unreliable products.
As for the function of R7? it is in a location that's typically used to monitor or limit current ( but there don't seem to be the relevant connection to other circuitry ). It would also limit peak current into a capacitive load but if the load is inductive it's redundant since current rises from zero when switching into an inductor.
Graham
source ?
"Fred Bartoli" a écrit :
currents (uC controled, no current mode,
has to withstand the power...
The system will be powered by 4 x AA batteries. Is their internal resistance not sufficient to limit the startup current, making the resistor redundant?
"Eeyore" a écrit :
is very unwise and doesn't prevent
Who said he relies entirely on simulation here? In fact this is the first time I use simulation. My goal was to simulate as much of the circuit as possible and see how far I could go. Now I am stuck with the A/D converter and about to build the hardware.
This is also a learning experience as I am not very familiar with PIC peripherals and have not programmed in C in more than 20 years.
monitor or limit current ( but there don't
peak current into a capacitive load but
switching into an inductor.
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