So i have to check all mosfets datasheet for the VGs Gate-To-Source voltage and it should be 5v or lower? Correct?

Sort of.

Often it's stated in the description at the top of the first page of the data sheet that it's a "logic level" device.

Otherwise you look for the Vgs value used where the Rds(on) value is given (not the Vgs(th) threshold voltage which is the point where the MOSFET just starts to conduct) .

 

Okay; so the MTP50P03HDL should be fine? It states "logic level" in the titel on the first page.
Can I use this one in your circuit? Together with a BC547

 

Also, something complementary. I need to switch a 1-6amp load with the picaxe too. I'm using the IRFB4110 for this now but it's not logic level and I'm told it's probably not full on. So I probably better get the same mosfet a couple times from farnell and use a logic level one for switching my load.

Can you please confirm if the MTP50P03HDL is good for your circuit and if it's a good one for switching a load between 1 and 6 amp without producing heat? The irfb4110 stays cool, not even a little warm. It's pretty much a requirement that it's not getting hot when switching the load.


Thanks in advance crutschow! You've been very helpful already!!

 

Okay; so the MTP50P03HDL should be fine? It states "logic level" in the titel on the first page.
Can I use this one in your circuit? Together with a BC547

Yes to both.
If you notice, the data sheet specifies the value of Rds(on) at Vgs = 5V.

 

Thanks crutschow! I'm going to place my order now

 

I created a complete circuit diagram of the device. Does it look correct to you crutschow?

Some explanation:

Q1 + Q2 + R1-R5 + D1 + S1 should reassemble the circuit you provided me, to power on/off the picaxe. The picaxe will have to hold C.5 high as long as it requires power. And is able to read C.2 for pushbutton status.

R7-R10 + L1-L4 are just 4 leds used to show status of the picaxe's program. No big deal.

R6 + Q3 +R11 +Q4 are used to switch on/off a load between 1-6 amp. Does this look good? The picaxe can high/low pin C.1 to activate/deactivate the load

R15 + Q6 + R12 + Q5 are used to switch on/off a voltage divider (R13 + R14) and I'm hoping this is correct too?

I want the picaxe to be able to measure the battery voltage on pin C.6. But this should only happen when the load is off so I wanted the picaxe to be able to activate the voltage divider using ping C.0 and then read the battery voltage on pin C.6. But I'm not sure if such a mosfet will impact the voltage reading negatively? I will have to calibrate things anyway... I'm just not sure if this is a bad idea? Maybe there are better ways for the picaxe to measure a voltage higher than 5V?
The reason for adding a mosfet there is lower power usage when the device is off.

Looking forward for your advice

 

I see no problems with your circuit and it should work as you want.
Q5 will have a negligible effect on the voltage you measure with the voltage divider R13 and R14.
Of course the size of Q5 is overkill for the small current through the resistors but that is okay.

 

Thanks crutschow!

Yes, I was also thinking on replacing Q5 with a smaller one because I'm also pretty space limited inside this enclosure.
But I'm having difficulties finding a smaller logic level mosfet.

Maybe I should try to find the IRF7404 SMD one, the one you used in your first circuit, and solder a couple legs to it. Shouldn't be too hard..
Or are you able to find a good alternative on www.rs-online.com maybe ?

I ended up ordering the other parts from rs-online.com because farnell.com did only have american express payment options and I don't have american express cards. So I search for other sellers and found rs-online.com. The parts are already shipped and should arrive tomorrow.

 

Also one more question.

The mosfet has an internal diode so I'm wondering. Can I charge the batteries on the same pins where the load is connected?

I think like this:

The load is connected between ground and the mosfet. When I put 8.4V there the mosfet 's diode should allow it to go to the battery.
Is this correct?

I don't actually need this in my circuit but it could come in handy, if it's possible