Getting More Amps from a MOSFET
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I have a motor that draws 10 amps with it connected straight to a battery.
I bought a motor controller
( uses IRF7805 MOSFET ) that will handle up to 30 amps, and it powers my 10
amp motor great. I want to make
my own controller. I found a schematic on a site (
http://www.capable.ca/rcstuff/esc.htm ) that uses IRFZ40
MOSFETs - and claims that the circuit will handle up to 20A.
After building part of the circuit, I am not getting the kind of Amps I want
from the IRFZ40 MOSFETs. I am only
getting 6.5 Amps from my partial circuit. Digikey's catalog shows that the
IRFZ40s are rated at 35 Continuous
Amps.
What controls the amount of Amps I get out of the IRFZ40 ?
What is preventing me from getting the full 10 Amps when I use the IRFZ40 ?
Note: I've tried using a IRFZ44, and get the same results as the IRFZ40.
The specifications for the IRFZ44
can be found at:
http://www.irf.com/product-info/datasheets/data/irfz44n.pdf
The Specifications for the IRF7805 can be found at:
http://www.irf.com/product-info/datasheets/data/irf7805.pdf .
I bought a motor controller
( uses IRF7805 MOSFET ) that will handle up to 30 amps, and it powers my 10
amp motor great. I want to make
my own controller. I found a schematic on a site (
http://www.capable.ca/rcstuff/esc.htm ) that uses IRFZ40
MOSFETs - and claims that the circuit will handle up to 20A.
After building part of the circuit, I am not getting the kind of Amps I want
from the IRFZ40 MOSFETs. I am only
getting 6.5 Amps from my partial circuit. Digikey's catalog shows that the
IRFZ40s are rated at 35 Continuous
Amps.
What controls the amount of Amps I get out of the IRFZ40 ?
What is preventing me from getting the full 10 Amps when I use the IRFZ40 ?
Note: I've tried using a IRFZ44, and get the same results as the IRFZ40.
The specifications for the IRFZ44
can be found at:
http://www.irf.com/product-info/datasheets/data/irfz44n.pdf
The Specifications for the IRF7805 can be found at:
http://www.irf.com/product-info/datasheets/data/irf7805.pdf .
Comments
MOSFET's are resistive devices, which means that you can parallel them
to provide larger currents to your output device. That said, what you
want to look at is the RDs on value...
IRF7805 --> .0092 Ohms (typ)
IRFZ40 --> .0280 Ohms
IRFZ44 --> .0175 Ohms
The circuit you show has 4 IRFZ40's in parallel, effectively producing
a RDs value of .007 Ohms... This seems ok, unless you have 4 IRF7805's
in parallel, in which case you would effectively have an RDs value of .0023
Ohms.
If your circuit does not go to a full 100% duty cycle, which I doubt, then
you will not see the maximum power on the output. Try adjusting R8 for
better efficiency or disconnecting pin7 of Z1 and temporarily take it to
CH+ to see if you have any dramatic increase of power.
If your voltage at CH+ is not adequate for transistor saturation, you will
also have problems... Note: the Gate turn on voltage in the datasheets to
determine if your turn on voltage is high enough.
>I have a motor that draws 10 amps with it connected straight to a battery.
>I bought a motor controller
>( uses IRF7805 MOSFET ) that will handle up to 30 amps, and it powers my 10
>amp motor great. I want to make
>my own controller. I found a schematic on a site (
>http://www.capable.ca/rcstuff/esc.htm ) that uses IRFZ40
>MOSFETs - and claims that the circuit will handle up to 20A.
>
>After building part of the circuit, I am not getting the kind of Amps I want
>from the IRFZ40 MOSFETs. I am only
>getting 6.5 Amps from my partial circuit. Digikey's catalog shows that the
>IRFZ40s are rated at 35 Continuous
>Amps.
>
>What controls the amount of Amps I get out of the IRFZ40 ?
>What is preventing me from getting the full 10 Amps when I use the IRFZ40 ?
>
>Note: I've tried using a IRFZ44, and get the same results as the IRFZ40.
>The specifications for the IRFZ44
>can be found at:
>http://www.irf.com/product-info/datasheets/data/irfz44n.pdf
>
>The Specifications for the IRF7805 can be found at:
>http://www.irf.com/product-info/datasheets/data/irf7805.pdf .
>
>
>
>
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Beau Schwabe Mask Designer IV - ATL
National Semiconductor Wired Communications Division
500 Pinnacle Court, Suite 525 Mail Stop GA1 Norcross, GA 30071
device. The only thing that limits the current is the device itself, the
Power Supply, and the motors you are using.