mosfet
- Thread starter vultac
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What part is confusing? You will find we are willing to help.
I had similar questions, which is why I directed you to my original thread in post #16. Being a hands on kinda guy I also outlined some simple experiments for my own benefit.
I have written an article on simple usage of MOSFETs, hence the post #20.
No one is born knowing this stuff. I've personally had a lot of help from my friends.
Stock MOSFETs must have 10V on their gate, but there are some specialty parts that can use a lot less, called logic level MOSFETs. They aren't common, and can be a little hard to get a hold of.
I had similar questions, which is why I directed you to my original thread in post #16. Being a hands on kinda guy I also outlined some simple experiments for my own benefit.
I have written an article on simple usage of MOSFETs, hence the post #20.
No one is born knowing this stuff. I've personally had a lot of help from my friends.
Stock MOSFETs must have 10V on their gate, but there are some specialty parts that can use a lot less, called logic level MOSFETs. They aren't common, and can be a little hard to get a hold of.
Correct my understanding, so basically the output from the 555timer has a maximum of 5V? and thus this wont drive the Mosfet im using? and if i use a pmos it would work? Sorry if im asking stupid questions.... 
... do you have any suggestions on how i can fix it so that i can provide the gate with 10V? amplifier?
... do you have any suggestions on how i can fix it so that i can provide the gate with 10V? amplifier?ive gone through the data sheet... so many parameters... which 1 is the key parameter that i need to know so that i can properly switch on n off my switch? skory.gylcomp.hu/alkatresz/irfz44.pdf
For eg how did u know i need 10V on the gate? or for this type of mosfet it's fixed?
For eg how did u know i need 10V on the gate? or for this type of mosfet it's fixed?
Your Mosfet is not a switch. Instead it is a follower. If its gate voltage is +10V then its source voltage will be 0V to +1V. But the 7404 has a max output of only about 3.5V so the Mosfet will do nothing.
R77 applies 18V to the output of the 7404 which will destroy it.
If you used a high voltage inverter instead of the 7404 then R77 draws 360mA when the inverter output is low. You don't need the very high current.
Why not use a 12V supply then the 555 and a P-channel Mosfet can both use it? The output of the 555 can drive the gate of the P-channel Mosfet.
R77 applies 18V to the output of the 7404 which will destroy it.
If you used a high voltage inverter instead of the 7404 then R77 draws 360mA when the inverter output is low. You don't need the very high current.
Why not use a 12V supply then the 555 and a P-channel Mosfet can both use it? The output of the 555 can drive the gate of the P-channel Mosfet.
sorry, but cud u explain to me what do you mean by follower? what if i remove the 7404? the output of the 555timer will only give 5V isit because its TTL? I cant set it to 12V because the DC voltage actually represents the solar panel, which outputs 19Vdc and 0.3A... and could you explain why p-mos can be used? i know im asking a lot of questions but really.... the datasheets are very confusing ! sorry....and high voltage inverter.... meaning like 7406? !
! sorry....and high voltage inverter.... meaning like 7406? !The output of your Mosfet is its source, it is a source-follower. For the source to go from 0V to +18V then its gate must go from 0V to +28V.
But it is driven from an old fashioned 7404 inverter that has a max output of only 3.5V.
A switching Mosfet has the drain as the output. Then a 0V to 10V input to its gate switches it completely on and completely off.
Since the load for the Mosfet connects to ground then a P-channel Mosfet should be used.
The output high of your 555 is also only +3.5V so it cannot drive a Mosfet that needs +28V or +10V.
I haven't seen a 7406 IC for about 40 years and can't remember it.
But it is driven from an old fashioned 7404 inverter that has a max output of only 3.5V.
A switching Mosfet has the drain as the output. Then a 0V to 10V input to its gate switches it completely on and completely off.
Since the load for the Mosfet connects to ground then a P-channel Mosfet should be used.
The output high of your 555 is also only +3.5V so it cannot drive a Mosfet that needs +28V or +10V.
I haven't seen a 7406 IC for about 40 years and can't remember it.
sorry, but cud u explain to me what do you mean by follower? what if i remove the 7404? the output of the 555timer will only give 5V isit because its TTL? I cant set it to 12V because the DC voltage actually represents the solar panel, which outputs 19Vdc and 0.3A... and could you explain why p-mos can be used? i know im asking a lot of questions but really.... the datasheets are very confusing
The 555-timer will only output because its a TTL device, correct. That is a logic level standard output.
http://www.interfacebus.com/voltage_threshold.html
You could use a linear regulator to create a +12V supply for other electronics, assuming they dont draw too much current.
If all you need them for is gate drive this shouldnt be an issue at all.
Look into the LM317 device, it should be able to do what you need with minimal biasing components and calculations. Since you will be dropping 7V at low current, power dissipation should be OK as well.
PMOS can be used because it is essentially inverted logic when compared to an NMOS device.
CMOS technology is complementary MOSFET technology, which means that each 'side' of the output (high or low) has its own device to pull up or pull down the output. This is more efficient and allows for cleaner (read: more closely approximated) high and low values due to the absence of extra voltage drop.
Read about CMOS here:
http://en.wikipedia.org/wiki/CMOS
It should help explain why PMOS is a better device to use in this case.
hmmm... "Since the load for the Mosfet connects to ground then a P-channel Mosfet should be used" what do u mean by that? so wad the disadvantage of using pmos? our lab helper asked us to use nmos.... last time... and do u have any suggestions on how can i achieve the switching? if i replace a pmos.... will it work?
A 555 is not a TTL device, though at 5V it strongly resembles one. I suspect this isn't a coincidence.
The thing to know about a 555 is it's power supply range is 4.5VDC to 15VDC. So a 555, with the right power supply, will handle MOSFETs just fine.
Most CMOS 555's will go from 2V - 18V, and since their output (while weak) will go rail to rail they are excellent at driving MOSFETs, especially P-channel (which a standard 555 chokes on).
Under very few circumstances will a TTL handle a MOSFET. A general rule of thumb would be never.
So if right now i remove the 7404, n change the Vdc of the 555timer source to 15V, it will be suffieicient enough to drive the NMOS?? and also erm.. i havent gotten a reply yet on why we need 10V on gate of NMOS.... where in the datasheet does it say ^^
i suppose i was sticking to the rule of thumb of 'never' bill.
instead of asking us if it will 'just work' with PMOS, try running some simulations.
do you have access to a circuit simulator?
regarding your question - refer to the table at the top of the NMOS FET datasheet - it says that with a Vgs (which means with a voltage differential from the gate to the source) of +10V you will have a low resistance across the Drain-Source path of 0.028 Ohms.
If you do not meet this criteria, the device is not acting as a switch, but it is acting as a follower - meaning that the source follows the drain voltage.
Also, later in the datasheet the Vgs threshold voltage is given as 2-4 VDC, this voltage must be added to the +10V to ensure a full opening of the D-S channel.
Ideally, you would then drive the gate with 15VDC greater than the voltage on the source node.
instead of asking us if it will 'just work' with PMOS, try running some simulations.
do you have access to a circuit simulator?
regarding your question - refer to the table at the top of the NMOS FET datasheet - it says that with a Vgs (which means with a voltage differential from the gate to the source) of +10V you will have a low resistance across the Drain-Source path of 0.028 Ohms.
If you do not meet this criteria, the device is not acting as a switch, but it is acting as a follower - meaning that the source follows the drain voltage.
Also, later in the datasheet the Vgs threshold voltage is given as 2-4 VDC, this voltage must be added to the +10V to ensure a full opening of the D-S channel.
Ideally, you would then drive the gate with 15VDC greater than the voltage on the source node.
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