hello again. Ive done a little reading up on mosfets and have attached a simple schematic I drew up along with a MOSFET datasheet. I understand a little better how they work but am confused as far as the datasheet is concerned. I have a few questions so hopefully you can point me in the right direction.

starting information: 5 ohm solenoid at 13.8vdc (car voltage) would mean the solenoid draws approx 2.76 amps and dissipates approx 38 watts of power. Correct?

1. in the schematic the MOSFET I picked is capable of 32 amps at 10volts Vgs at 25 degrees C but has a maximum rating of 20 Vgs? Thats based on the current draw though right? So how would I figure out what the Vgs should be for a 2.76 Amp load?

2. The resistor (I picked one megaohm) pulls the gate to ground with the switch off to increase the MOSFETs resistance to practically open and "switch" the solenoid off. Would I need a resistor between the switch and the gate to lower the current?

3. I see the "drain to source on resistance" is typically .032 ohms. How would I figure out how hot it would get?

4. Would this circuit work or am I missing something else? Please feel free to modify the schematic and get me in the right direction.

THANKS AGAIN!!!

edit: This is probably not the MOSFET I would be using. Just trying to understand how I would set this one up correctly so when I make a schematic with a better suited one I will know a little more. Then you guys can give me the yay or nay (helps me learn to do it myself)

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FDP3682.pdf (278.7 KB, 4 views)

 

Correct.

It cannot pass 32A because that is if its case is cooled to 25 degrees C which is very difficult. The case is attached to a heatsink that gets hotter than 25 degrees C. Maybe it can pass 25A with a pretty big heatsink.
Use a gate voltage from 6V to 10V to turn it on. Then it will have a low resistance and will not get too warm.

1M is too high. It might take too long for it to discharge the high gate capacitance (1.25nF) to turn off the Mosfet. Use 10k.

It will heat a little which will increase its resistance to maybe 0.04 ohms. P= current squared times R. It will heat with 0.3W.
Its junction to ambient thermal resistance is a temperature rise of 62 degrees C per Watt for the TO-220 package so with an ambient temperature of 25 degrees C its junction will be at 43.6 degrees C without a heatsink.

The gate pin of a Mosfet should have a low value resistor (10 ohms to 100 ohms) in series mounted at the pin to prevent the Mosfet from oscillating at a VHF frequency.

__________________
Uncle $crooge

 

here is another attempt at making this work! What do you think? Ive also attached a possibility for the 25 ohm (false solenoid) as well as a possible MOSFET. Will these be up to the task?

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	resistor.pdf (170.1 KB, 5 views)
	MTD3055V.pdf (238.6 KB, 5 views)

 

would this be a good substitute for the 2n2222 in the above diagram if I replace the 220 ohm 1 watt with a 500 ohm 1/2 watt? THANKS!!

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FJX2222A possible.pdf (71.2 KB, 5 views)

 

Why is your schematic as big as, no bigger than my entire neighbourhood?
I cropped it to make it a little smaller.

You had your transistor connected to +12V but not connected to the Mosfet.
I corrected it.

You can use a tiny little surface-mount transistior if you can handle it.

I would not use the tiny little surface-mount power resistor because it is difficult to cool it.

The tiny little surface-mount Mosfet will probably melt if it tries to drive 2.4A. Its resistance will be about 1.2 ohms and it will try to dissipate 6.9W. But it cannot be bolted to a heatsink, it is cooled a little by the copper on a pcb.

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__________________
Uncle $crooge

 

Ooops...thanks for the correction Is it always better to "over-do" the ratings rather than get close to them as far as MOSFET's go? Will the higher rating produce an overall lower temperature? In you calculations above, what information on the data sheet led you to the 1.2 ohms and 6.9W? (just so I can look and figure it out myself) Ive attached another MOSFET...This one should be plenty big enough. Last one, there are several different types of resistors and Im not to sure which type I should be working with (for the 25 ohm 10 watt one) should it be a standard power resistor? THANKS AGAIN!!!

EDIT: would this be a good canidate for the 25ohm resistor? ((Attached))

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	IPP50CN10N_Rev1.03.pdf (490.3 KB, 2 views)
	resistor.pdf (123.2 KB, 5 views)

 

The tiny little Mosfet has a pretty high on-resistance. With 2.4A through it (12V/5 ohms= 2.4A) then it is going to get hot.
Its tiny case is difficult to cool.

Its on-resistance is 0.15 ohms max when its case is cooled to 25 degrees C somehow. Its on-resistance is 1.9 ohms max when it is hot.
Assume its chip is halfway between 25 degrees C and its max temperature of 175 degrees C. then its chip is 100 degrees C. Then its on-resistance is about 1.2 ohms max.
Its power dissipation is then 2.4A squared times 1.2 ohms= 6.9W.
Without a heatsink then its chip is (48 degrees rise per Watt x 6.9) + 25= 356 degrees C which is way more than enough to destroy it.

The problem is that it is an old Mosfet with a very high on-resistance and it is in a tiny case that is difficult to cool.
The other Mosfet is almost the same.

A good Mosfet is the IRF3711Z but its max voltage is only 20V. Mosfets with higher voltage ratings are available.
Its on-resistance is only 0.006 ohms max when it is cold and is probably 0.01 ohms when warm. It will heat with 2.4A squared times 0.01 ohms= only 0.06W. 100 times less than the Mosfet you first selected. It won't even feel warm.

I found a power resistor in a small aluminum case with mounting holes for attaching to a heatsink and little cooling fins. Its max power rating is 25W. Ordinary 10W power resistors are in a pretty big cement case. They get extremely hot.

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Uncle $crooge

 

Your amazing Audio. Thanks for all the help and not just giving me the answers. Im learning a lot. Not to keep beating a dead horse but in the datasheet for the MOSFET you recommended (IRF3711Z) I still can't figure out how your getting .006 ohms cold and about 0.01 warm? I dont see either of these anywhere on the data sheet. Can you guide me one more time and help me figure this out? Thanks again...your extremely helpful!!!

Strokedmaro

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irfr3711zpbf.pdf (262.9 KB, 9 views)

 

You found the tiny little surface-mount Mosfet. It should work fine if its 20V max voltage rating will be OK in your circuit.

All its details are on its datasheet.

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__________________
Uncle $crooge

 

Once again your awesome! Ive attached a 25W resistor I found based on your recommendations. Since the load will be almost 25% (7.6W) the rated wattage, will I even require a heatsink? Without a heatsink it says it would be good up to 12.5W @ 25 degrees. In the graph it shows it would be about 20 degrees C but thats with a "standard heasink" right? Also, the max voltage this circuit should have is 13.8vdc so Im guessing the 20V max rating WILL be ok? THANK YOU, THANK YOU, THANK YOU!!!

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25 ohm 25 watt.pdf (142.3 KB, 5 views)

 

one more....why in the world would'nt you use a MOSFET over a bi-polar like a TIP41 for example? Is it just the price? It would seem to me that they operate much cooler in a smaller package and able to carry many times the current..I dont understand.

in the schematic you posted do you add the resistors up in series to figure out there wattage? I understand the 1k (.19W at 13.8Vdc through the 2n222) but what about the 100 and 10k? The total resistance to the source of the MOSFET would be 11100 ohms (.017W at 13.8) so would 1/8W work for these?

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The HS25 resistor is fine without a heatsink. with only 7.6W it will be pretty warm but not too hot.

__________________
Uncle $crooge

 

We talked about using a cool little Mosfet instead of a big hot TIP41 transistor.

The transistor saturates with a voltage loss of about 0.2V so the 1k resistor will have 13.6V across it. Its power dissipation is (13.6V squared /1k ohms)= 0.185W.
A 1/4W resistor is fine.
The gate of a Mosfet draws no DC current. When the transistor is turned off then the 1k, 100 and 10k resistors have a current of 1.2mA. Then the 100 ohm resistor dissipates (1.2mA squared x 100 ohms)= 0.00014W and the 10k resistor dissipates 100 times less.
Any size resistor is fine.

__________________
Uncle $crooge

 

Here is my new schematic. Using what Ive learned from Audioguru (Thank you, thank you!) Ive put this together. The text on the schematic didnt come out very clear so I blew it up into another pic. Will this work and are my pull up values on the 4028 ok? Thanks for the help everyone!!

EDIT: Oops..the MOSFETS are IRF3711Z's. I know these are overkill for the solenoid's which draw only about .5 amps but they are what I have (and necessary for the 5 ohm solenoid).

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4028.PDF (122.7 KB, 2 views)

 

Your schematic is huge. It is the size of my neighbourhood. It is covered in dots (can't you turn off the dots?).

It is not clear because you saved it as a fuzzy JPG file type instead of as a very clear GIF or PNG file type.

The 1k pullup resistors might not be able to be driven by the computer because their current is high. Use 22k instead.

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Uncle $crooge