avr controlling 30V hydraulic valve with Profet

OP #3238729 
hi!

I want to switch 2 hydraulic valves at the same time.
The valves using 30V and about 3A.

I'm using a Mosfet from Adafruit
http://www.adafruit.com/datasheets/irlb8721pbf.pdf
for switching a Profet from Infineon
http://at.rs-online.com/web/p/intelligente-leistungsschalter/7528265/

The problem is, that if I'm switching the Profet  OFF, the voltage only 
drops from 30V to about 28,8V not to 0.
So I added a pulldown-resistor at the output of the Profet, but I'm not 
sure about the right amount of Ohm.
If I'm adding a 10k pulldown-resitor I'm getting 0,162V
4,7k pulldown = 0,074V
2,2k pulldown = 0,035V

So my questions are:
x why do I need a pulldown-resistor in this case? (I know why I normally 
use one, but if I think about switching off a normal mosfet, voltage is 
0)

x and  what is the right amount of Ohm for the resistor for switching 
the 3A valves??

I have attached a small graphic of my circuit. Pins 1,2,6,7 of the 
Profet need to be shorted according to the datasheet.

thanks in advance
Attached files:
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Guest 
Hannes XXX wrote:
> The problem is, that if I'm switching the Profet  OFF, the voltage only
> drops from 30V to about 28,8V not to 0.

Why do you measure without the hydraulic valve?

Every FET has a leakage current in off state. In your case
at room temperature this seems to be around 16uA.

At high temperatures and maximum voltage this is usually
up to around 1mA.
And for a valve this is usually no problem.

Regards Anja
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OP 
thanks!

you were absolutely right, that I hadn't connected the valves because 
they are really expensive (500EUR each) and I wanted to test everything 
before.

thanks again!

regards hannes
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