18 Pin FET Project board V3

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OzGrant

New Member
G'day
Am using the hi powered version (Version 3, with 4 Fets) to drive a load connected to 12V.
All works OK, but have found that nearly 4 Volts is lost across the FET when it is conducting, not only does it the fet get hot, but only 8 volts of useful voltage is across the load.
Is this normal, as all four fets drop around this voltage.
 
A

Andrew Cowan

Senior Member
What model are the FETs? It
may be that they are not logic level FETS, meaning the have an ohm or two of resistance.

A
 
O

OzGrant

New Member
Type of Fets

On the PCB they are IRF520 (as per the project circuit parts list). I think they now are replaced with IRF540 in later project boards.
 
B

BeanieBots

Moderator
What load current are you trying to switch and what voltage are you driving the PICAXE board at?

Despite the IRF520 being a 9A device, it can only handle such currents when the gate voltage is close to around 10v.

4v is the gate threshold. This is the voltage at which conduction starts, not the voltage at which the FET is fully on. If you are using a slightly low 3 cell battery pack, then the FET will hardly be on at all.

For more details, see the datasheet.
http://www.datasheetcatalog.org/datasheet/fairchild/IRF520.pdf
 
D

Dippy

Moderator
And, using the Data Sheet, the more enthusiastic people may be able to work out the approx MOSFET resistance in their app. - maybe even compare graphs against real-life measurements which would be interesting.

I haven't got one of these boards so a couple of questions.
1. Is it possible to replace the 'standard' MOSFETs with similar 'logic' level ones to improve performance?
2. Are the PCB track widths adequate to allow healthy currents. 4mm should happily handle 6A continuous on standard 1oz PCB esp pth.

I haven't looked in detail, but the FDP7030BL seems to have a good performance at 4.5V Vgs and, more importantly, is quite cheap - just over a quid for ones off.
http://uk.farnell.com/fairchild-semiconductor/fdp7030bl/mosfet-n-logic-to-220/dp/9845151
Data Sheet.
http://www.fairchildsemi.com/ds/FD/FDB7030BL.pdf

But there are oodles to choose from. And maybe some amazing quality deals on ebay.

I wonder why the choice of IRF520 for this board?
 
M

MPep

Senior Member
You need to look at the data sheet. Look for a graph that shows Vds vs Id, at various Vgs. You will notice that the higher the Vgs, the larger the Id.

In the datasheet posted by BB, look at Figure-6.
In fact at Vgs=4V, there a[ppears to be no hope of the FET actually working as a switch.

Hope this helps.
 
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