Author Topic: Reg PCB replace with mosfet?  (Read 250 times)

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Offline harps

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Reg PCB replace with mosfet?
« on: December 03, 2017, 06:45:53 pm »
hello

i bought a pcb that is meant for an lm317t regulator. (the PCB schematic picture is attached)

I hope to put an IRFZ34 n-channel mosfet in its place for 48v (pic of mosfet attached)

the 3 pins of the mosfet match up with the footprint so that is good. 

I wonder do i need to include the two diodes in the schematic and what values would r1 and c2 be ? 

thank you for any help.
 

Offline Ian.M

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Re: Reg PCB replace with mosfet?
« Reply #1 on: December 03, 2017, 07:19:22 pm »
A MOSFET + a few resistors is *NOT* a regulator - it can be used as part of a regulator circuit but without other semiconductors it will NOT stabilise the output voltage.

There are regulators with a higher input voltage than an ordinary LM317, e.g the LM317HV which is rated for up to 60V input.   However in that circuit it will blow at switch on if the transformer output voltage exceeds 42V RMS, and as there is only 12V headroom between 60V and the desired 48V out, and the LM317HV will drop out at a bit under 3V, that means you've only got 9V headroom for ripple, transformer regulation and tolerances and mains voltage variations.    That isn't anywhere near enough for reliable operation.
« Last Edit: December 03, 2017, 07:24:21 pm by Ian.M »
 

Offline rob77

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Re: Reg PCB replace with mosfet?
« Reply #2 on: December 03, 2017, 07:19:50 pm »
you simply can't replace a linear regulator with a mosfet.
it's like replacing a transmission box in a car with a single plain shaft and still expecting it to shift gears :D
 

Offline Ian.M

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Re: Reg PCB replace with mosfet?
« Reply #3 on: December 03, 2017, 07:23:44 pm »
Not even that good - its more like bolting a boat propeller to the crankshaft and expecting it to push you along the road!
 
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Offline harps

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Re: Reg PCB replace with mosfet?
« Reply #4 on: December 03, 2017, 07:44:28 pm »

darn i was looking at the wrong pdf.  got the parts mixed up from another project.

I have the part here it is a tl783c to be put in the PCB.

it has the same pin footprint pin order as an LM317.





 


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