lbenson
Senior Member
I'm trying to compare the usefulness with the picaxe of two MOSFETs, IRFZ44N and IRFL44N. The IRFZ44N is available from Jameco for $.61US; the IRFL44N is available from phanderson.com for $1.75US, and a note says, ""May be directly interfaced with PICAXE logic levels". I'm expecting to use PWM to drive 12
volts to LED lamps with a total draw of no more than 15 watts.
Datasheets are at: http://www.irf.com/product-info/datasheets/data/irlz44n.pdf
and http://www.irf.com/product-info/datasheets/data/irfz44n.pdf
In trying to determine what would make the "L" part preferable to the "F" part, I've looked at the datasheets, without entirely knowing what to look for. The
Drain Source Voltage, Drain Current, and Total Power Dissipation all seem to me close enough not to make a difference for my use. RDS(on) for the "F" part is
.0175 ohms, and for the "L" part is .022 ohms.
The "Power MOSFET Interfacing Circuit" in manual 3 shows no resister between the
picaxe and an IRF530. So how does one determine if a MOSFET can be directly driven by a picaxe, and is a resister desirable, and if so, what value?
These posts to a prior tread address general mosfet use without resolving the question of whether a series resistor is desirable between picaxe and mosfet, and what value.
inglewoodpete Posted - 23 October 2006 1:32
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"A resistor on the gate of a FET is of no real benefit. The impedance (input resistance) of the gate of a FET is very high (often mega ohms, even Tera ohms), so no current is going to flow."
[I assume he means a series resistor between the picaxe and the mosfet gate pin.]
premelec Posted - 23 October 2006 4:4
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"MOSFET gate resistance is indeed high however there is gate capacitance which is high when the MOSFET Rds ON is low... so sometimes to limit instantaneous
pulse current to gate [from PICAXE...] you would use a series R. This also lowers the rise time of the turn on which is or is not desirable depending on what you are doing... "
greencardigan Posted - 22 November 2006 0:10
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Can you give me an idea of a suitable resistor value [to connect a BUZ71 mosfet]? I'm switching 12v @ 1.75A.
premelec Posted - 22 November 2006 5:27
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If you can deal with a slightly delayed on and off time 1000 ohms should be fine limiting the 5 v peak current to 5ma. …
Don't forget to put a reverse diode across any inductive load and if you are worried about drain-gate short you could put a zener diode from gate to source [5v] and fuse etc... there are a variety of ways to protect the PICAXE from harm... [20K resistor drive would be easy..].
evanh Posted - 22 November 2006 11:36
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1: BUZ71 does not have logic level gate threshold. You'll prolly be okay at 2 Amps but be warned you are not turning the mosfet on very hard.
2: The series resistor is to protect the driver (Picaxe) in normal operation. In addition you also need a 5v1 zenor/tranzorb across the Gate-Source pins of the mosfet. As premelec has said the Gate is a capacitor and it is effective to both the Drain and Source so any sharp transitions on either are reflected straight back into the Gate.
volts to LED lamps with a total draw of no more than 15 watts.
Datasheets are at: http://www.irf.com/product-info/datasheets/data/irlz44n.pdf
and http://www.irf.com/product-info/datasheets/data/irfz44n.pdf
In trying to determine what would make the "L" part preferable to the "F" part, I've looked at the datasheets, without entirely knowing what to look for. The
Drain Source Voltage, Drain Current, and Total Power Dissipation all seem to me close enough not to make a difference for my use. RDS(on) for the "F" part is
.0175 ohms, and for the "L" part is .022 ohms.
The "Power MOSFET Interfacing Circuit" in manual 3 shows no resister between the
picaxe and an IRF530. So how does one determine if a MOSFET can be directly driven by a picaxe, and is a resister desirable, and if so, what value?
These posts to a prior tread address general mosfet use without resolving the question of whether a series resistor is desirable between picaxe and mosfet, and what value.
inglewoodpete Posted - 23 October 2006 1:32
--------------------------------------------------------------------------------
"A resistor on the gate of a FET is of no real benefit. The impedance (input resistance) of the gate of a FET is very high (often mega ohms, even Tera ohms), so no current is going to flow."
[I assume he means a series resistor between the picaxe and the mosfet gate pin.]
premelec Posted - 23 October 2006 4:4
--------------------------------------------------------------------------------
"MOSFET gate resistance is indeed high however there is gate capacitance which is high when the MOSFET Rds ON is low... so sometimes to limit instantaneous
pulse current to gate [from PICAXE...] you would use a series R. This also lowers the rise time of the turn on which is or is not desirable depending on what you are doing... "
greencardigan Posted - 22 November 2006 0:10
--------------------------------------------------------------------------------
Can you give me an idea of a suitable resistor value [to connect a BUZ71 mosfet]? I'm switching 12v @ 1.75A.
premelec Posted - 22 November 2006 5:27
--------------------------------------------------------------------------------
If you can deal with a slightly delayed on and off time 1000 ohms should be fine limiting the 5 v peak current to 5ma. …
Don't forget to put a reverse diode across any inductive load and if you are worried about drain-gate short you could put a zener diode from gate to source [5v] and fuse etc... there are a variety of ways to protect the PICAXE from harm... [20K resistor drive would be easy..].
evanh Posted - 22 November 2006 11:36
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1: BUZ71 does not have logic level gate threshold. You'll prolly be okay at 2 Amps but be warned you are not turning the mosfet on very hard.
2: The series resistor is to protect the driver (Picaxe) in normal operation. In addition you also need a 5v1 zenor/tranzorb across the Gate-Source pins of the mosfet. As premelec has said the Gate is a capacitor and it is effective to both the Drain and Source so any sharp transitions on either are reflected straight back into the Gate.