The LM317 is quite the hot head ...

[delaware74b]

Re: The LM317 is quite the hot head ...

Both Digikey and Mouser will ship priority mail (1 pound max, no more than 3 business days after order is processed). Mouser charges $7.99 and Digikey charges $7.50. They will ship just 1 item but it makes sense to make a larger order so your parts total is more than the shipping. I always try to spend at least $30 before shipping.

[EasyGoing1]

Re: The LM317 is quite the hot head ...

So if the Vbe is around 0.9V then what do you think the Voltage will be at the Emitter of that Transistor based on your last diagram?

OH I think I see grasshoppah .... Should I remove the 220Ω from base to ground? cause like that, it should about 1 volt minus Vbe - right?

And possibly lower that resistor value so that it would give me like 1.7 volts cause the Vbe should be roughly .7 volts?

[EasyGoing1]

Re: The LM317 is quite the hot head ...

May have found a better solution ... was scouring through my regulators / mosfets and found an IRFP460 ... look at the graph on this bad boy...



I think we might have a winner...

[EasyGoing1]

Re: The LM317 is quite the hot head ...

So with that, I'm thinking this is how its gonna work... maybe?

[eccerr0r]

Re: The LM317 is quite the hot head ...

Will 1A be sufficient? At 3.3V it's below the rated threshold though slightly above the minimum spec (used for Ids leakage estimates). You'll need a level shifter or find another FET. Or you could use a power darlington but the characteristics might not be sufficient...

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Will 1A be sufficient? At 3.3V it's below the rated threshold though slightly above the minimum spec (used for Ids leakage estimates). You'll need a level shifter or find another FET. Or you could use a power darlington but the characteristics might not be sufficient...

The way I read the graph, its good for 10 amps at 3 volts

See?

[mariushm]

Re: The LM317 is quite the hot head ...

May have found a better solution ... was scouring through my regulators / mosfets and found an IRFP460 ... look at the graph on this bad boy...



I think we might have a winner...

Don't know what you attached but doesn't seem to be related to irfp460

here's datasheet for irfp460: https://cdn.badcaps-static.com/pdfs/...7e5d86dcce.pdf

note Vgs up to 4v (min 2v, max 4v), your mosfet may or may not turn on (fully) with 3.3v and 4200pF input capacitance seems huge.

also strictly related to that graph picture, those values are for Tambient of 25c which won't ever happen during operation

[mariushm]

Re: The LM317 is quite the hot head ...

The way I read the graph, its good for 10 amps at 3 volts

See?

Vgs are those lines, the graph shows only Vgs starting from 4v, your 3.3v Vgs isn't even in that graph picture.

something cheap and good for around 5A continuously : https://www.digikey.com/product-deta...ICT-ND/2052800

or something for ~12a : https://www.digikey.com/product-deta...DKR-ND/3458181

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Don't know what you attached but doesn't seem to be related to irfp460

here's datasheet for irfp460: https://cdn.badcaps-static.com/pdfs/...7e5d86dcce.pdf

note Vgs up to 4v (min 2v, max 4v), your mosfet may or may not turn on (fully) with 3.3v and 4200pF input capacitance seems huge.

also strictly related to that graph picture, those values are for Tambient of 25c which won't ever happen during operation

This is what I was using

[EasyGoing1]

Re: The LM317 is quite the hot head ...

What would use one of these for? Something called a “Power Schottky rectifier”??? I've got a few of those.

[budm]

Re: The LM317 is quite the hot head ...

3.3V Vgs is not going to cut it based on your graph in post 66. BTW, how did you come up with 220 Ohms GS resistor? You know what Vds, Vgs are, right?

[eccerr0r]

Re: The LM317 is quite the hot head ...

Power Schottky Rectifiers are for the output stages of forward switching regulator converters.

And here's a suggestion for the level shifter (I did not do much validation on component selection but at first glance it should work and saturate the mosfet. That +12V must be clean and not exceed the maximum gate voltage of the mosfet or you'll kill it.) This is the same old schematic as before except I drew more on it...

[EasyGoing1]

Re: The LM317 is quite the hot head ...

3.3V Vgs is not going to cut it based on your graph in post 66. BTW, how did you come up with 220 Ohms GS resistor? You know what Vds, Vgs are, right?

I just arbitrarily chose 220 because from what I’ve read, the only thing that matters, is that the gate be able to go ground should the arduino pin ever float for whatever reason. Because from my experimentation, you can apply a voltage to the gate and if you take it away, the mosfet stays ON ...

Vds = Voltage between drain and source
Vgs = Voltage between gate and source

I took two years of electronics and even passed my CET ... but that was in 1992. Since then, Ive done nothing with that education, but rather became a Novell CNE in 1995 and have been a network engineer ever since. Only recently have I begun to experiment with electronics again. I remember doing calculations for transistor circuits, I remember doing nodal circuit analysis, Thevenoms theorem, Kirckhoffs laws etc., I even took a year of electrical calculus ... I just seem to have forgotten most of it. BUT, what I haven’t lost, is my ability to re-learn, nor my curiosity for it.

[budm]

Re: The LM317 is quite the hot head ...

Well it is never too old to learn, you are so lucky that you have internet to get all kind of information at your finger tips right away I did not have that over 40 years ago.

[PeteS in CA]

Re: The LM317 is quite the hot head ...

You need to understand the device a bit and think about what that graph tells you. And remember that ohm's law still applies. Each curve on that graph tells you what a typical device will do with a particular Gate-source voltage applied with a junction temperature of 25C. If you apply 4V G-S, the Drain current will never be greater than about 1.5A, no matter how high your Drain voltage is. The 10A, 3V D-S point you picked is with 5V G-S, if the silicon inside that nice TO-247 package is 25C. Well with 3V across the D-S and 10A Drain current, that silicon is going to be much warmer than that, even with a massive heatsink and a lot of air.

Now, if your available G-S voltage is just 3.3V, you're going to need a logic level MOSFET, which may turn with 3.3V

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Well it is never too old to learn, you are so lucky that you have internet to get all kind of information at your finger tips right away I did not have that over 40 years ago.

My first exposure to the Internet was my first job as a paid computer tech. I worked at Phillips Laboratories which could be considered the “North Campus” of Edwards AFB. Its the area where they did all of the Apollo rocket tests and a lot of astronaut training etc. Very VERY interesting facility complete with underground command centers and cable tunnels that went for DAYS AND DAYS ... data acquisition systems that were interconnected using these tunnels. In some cases, a tunnel would be far too long to see the other side, and barely wide enough for one person because on each side of the tunnel, there were sometimes 5 cable trays running the length of the tunnel, each COMPLETELY PACKED with cable of all kinds. I can't imagine how much weight those trays had to sustain ... not that it mattered since the trays were welded from some very rigid metal. They weren't going anywhere anytime soon.

But I digress...

I used the Internet for the first time on that job back when Mozilla / Yahoo was the only GUI browser in town. Google was non-existent, and our LANs were interconnected using RG-59 coax or “Thinnet” ... we had special cables that connected the computers using something called a Vampire Tap which would leave a port in the wall that these special cables would use. The coax would be connected through the tap in the wall and when you plugged one of these jumpers into the tap, it was wired in such a way that it would break the connection in the wall, but then feed it through the jumper, leaving the entire run still connected in series. Not sure how much you know about that stuff, but at each end of one of those runs (which could include I believe up to 255 computers), the coax needed 50 ohm terminators and each computer was slaved off the coax in series (like the way they wire telephone in a house). They ran Ethernet when I was there because they had just upgraded from Token Ring. They also used TCP/IP ... but thats right when CAT-5 started making a name for itself as the better option - the government seemed to always be years behind everyone else when it came to adopting new technologies in areas that didn't require clearance, and especially when they had to provide infrastructure to contractors fulfilling contracts.



There were 7 of us techs supporting the whole area, and we had to rotate help desk duties (cause it was boring) and I would often be at the helpdesk and in between calls, researching UFO's, or looking at porn (no filtering back then nor any kind of fancy firewall stuff like they have now)...

Amazing how much the Internet has exploded in the last few years.

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Well it is never too old to learn, you are so lucky that you have internet to get all kind of information at your finger tips right away I did not have that over 40 years ago.

I sill have my original Electronics textbook by the way ... it's in storage, but I took a look at it a few years ago and was quite impressed with how much I apparently knew at one time... lol

[EasyGoing1]

Re: The LM317 is quite the hot head ...

You need to understand the device a bit and think about what that graph tells you. And remember that ohm's law still applies. Each curve on that graph tells you what a typical device will do with a particular Gate-source voltage applied with a junction temperature of 25C. If you apply 4V G-S, the Drain current will never be greater than about 1.5A, no matter how high your Drain voltage is. The 10A, 3V D-S point you picked is with 5V G-S, if the silicon inside that nice TO-247 package is 25C. Well with 3V across the D-S and 10A Drain current, that silicon is going to be much warmer than that, even with a massive heatsink and a lot of air.

How did you determine that at 4 Vgs, there will never be a Drain current larger than 1.5 amps?

25C is 77F ... and if it can sustain 10 amps and remain at that temperature, then I can't see why it would be improbable for it to sustain 3 amps at room temperature, unless the amount of current it can handle significantly decreases with each degree of temperature increase, but in most cases, the temperature that the device will be in (in this case anyways) wont be much higher than 77F.

Now, if your available G-S voltage is just 3.3V, you're going to need a logic level MOSFET, which may turn with 3.3V

Do you have any in mind that I can get at Amazon?

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Now, if your available G-S voltage is just 3.3V, you're going to need a logic level MOSFET, which may turn with 3.3V

What about something like this? FDP7030 N-Channel Logic Level Power MOSFET?

[mariushm]

Re: The LM317 is quite the hot head ...

Open your datasheet:



Determine your Vgs value.

Page 2, in the Electrical Characteristics (note above the table it says Tj = 25C unless otherwise specified) :

Vgs (to) GATE THRESHOLD VOLTAGE : min 2.0v , typ 3.0v , max 4.0v

This tells you that typically , the mosfets will turn on fully at 3.0v but they don't guarantee ALL will turn on at 3.0v or that the ones that turn on at 3.0v will turn on at 3.0v under all circumstances.
Basically, if you buy 100 of them, some won't turn on fully with 3.0v, but all should turn on fully with a voltage above 4.0v (the maximum value in that table).

Also, note again that all values are for Tj = 25C. The mosfet has some internal resistance so as soon as there's some current flow through it, it will slowly warm up.

The mosfet has some mass to it so it won't heat up instantly, it will slowly warm up and some heat will be radiated to the air around it, and some heat will also be radiated through the leads in the circuit board ... but basically you know that if you started the circuit with the circuit board and the mosfet at a temperature of 25 C, after a while that mosfet won't be at 25 C

Now go down in the datasheet and look at Fig.5. Typical output characteristics. on page 3 :



Here's how you read this chart :
On vertical (left side) you have the maximum current that will flow through the mosfet (in amps)
On horizontal (bottom) you have the Vds (voltage between drain and source pins)
Each of those lines (curves, whatever) tells you how the mosfet behaves for a specific Vgs value. Note that they don't even bother to use the typical Vgs of 3.0v, they use the 4v value as minimum because that's the one guaranteed to work for any transistor at the default 25c temperature (see chart corner)

So with a Vgs of only 4v, you're looking at the most bottom line: regardless of how much the Vds will be, the mosfet will only allow less than 2A of current to flow through it

With a Vgs of 5v, you're looking at the top line from the set of 6 lines. So you can see that with a Vds of only 1v, the mosfet may only allow up to about 5A to flow through it, but if you design the circuit properly to allow at least 3v between Drain and Source pins, then the mosfet will allow up to 10A. For anything above Vds of 3v, you can see that mosfet will basically "stabilize" itself to somewhere below 10.5A of current flow.
If you want more than 10.5A you need to have a Vgs higher than 5v - for the maximum of 10v (top line) you can see the mosfet scales almost linearly with the Vds voltage : it can do over 12A with a Vds of 3v and almost 20A with a Vds of 5v

Now I have to note again that values are valid only for 25 C : think of the chart being made with the mosfet at 25c , then power the circuit for 1-2 seconds (so that the mosfet won't have time to heat) and literally take a snapshot of the values and power off the circuit. As the mosfet and the circuit board will heat up, those lines will drift a bit (but overall the numbers should still be "in the ballpark"

You can scroll down in the datasheet some more and see how the mosfet will behave with heat (on page 4, Figure 9 and Figure 10 in the picture below) :



So you can see that the warmer the mosfet will be, the easier it will turn on fully.
It's important to know these, because for example you may say "oh, i'll just use 4v and then I'll know for sure the mosfet will turn on fully" but what if your project/design will be outside in a winter when the ambient temperature will be -20c .. 0c ? You can see that the 4v max is only guaranteed at 25 C - at 0C you already need about 4.2v for the mosfet to turn on fully.

The other way is also important .. for example you may say "I'll use Vgs of 4v because then according to the chart above I'll know for sure the current between drain and source will be limited to less than 2A so basically my mosfet will limit the current through the LEDs and I won't need a led driver or a resistor to limit the current"

BUT as the mosfet heats up, you can imagine it behaving like having 5v on the gate, it will basically let more than 2A flow between drain and source pins so in the theoretical example above you'd damage your leds.
Also you can see in figure 9 how with Vgs of 4v, it barely lets 2A at Tj of 25c but at 150c it almost crosses the 5A threshold.

Something else you can see from the datasheet, scroll up to page 3 and look at Figure 6 Typical on-state resistance :



You can see that with a Vgs of 4v even with as little as 1A of current between drain and source, the internal resistance will be somewhere around 0.4 ohms
So you have P = IxIxR = 1x1x0.4 = 0.4 watts of heat dissipated in the mosfet.
But if you use a Vgs of 5v then even with as much as 10A of current between drain and source, the Rds (on) resistance will be below 0.3 ohm and at 1A it will be below 0.25 ohm ... basically with higher Vgs, the mosfet will heat less.

So basically, this mosfet is really designed for a Vgs of around 5v, even though it says it typically can turn on at 3v... if you want the advertised currents and rds(on), you drive it with at least 5v.
To drive a mosfet directly with 3.3v, you would normally look for one mosfet that has less than 3.3v on the MAX column for the Vgs threshold.

The input capacitance is important as well.
Microcontrollers can only output a limited amount of energy per pin, and then a limited amount of energy over all the pins - for example a PIC chip may output up to 15-25mA per pin but only 100-150mA through all of its pins.

If the gate capacitance is too high, in those microseconds when you want to turn on the mosfet the gate capacitance will suck a huge amount of current from the microcontroller and in some rare cases that could cause glitches in the microcontroller or even resets. With such mosfets you either accept a slower switch time by limiting the charge with a resistor, or you use a mosfet driver between your micro and the mosfet itself.

Also high gate capacitance means the mosfet will discharge slower or not discharge at all when you stop sending voltage to the gate pin .. that's why you typically put a resistor between the gate and source pin, so that the capacitance between gate and source will discharge into the resistor.
The resistor will quickly discharge the gate and bring the voltage down to below the threshold required to keep drain-source link "active".

But the resistor will then be also between your output pin and ground, which means while you keep the mosfet on you'll also waste some power in the resistor that's on the gate-source ... ex with a 1kOhm resistor you have V=ixR .. 5v = i x 1000 = 5mA of energy being wasted on a resistor between mosfet gate and ground.
Mosfet drivers would have circuitry inside to discharge the gate and then disconnect that discharge mechanism while the mosfet is kept ON.

For example, see LTC1154 datasheet , especially at the bottom of page 6 where you see the block diagram, which shows you how the IC has a gate charge and discharge control logic and fast/slow gate charge mechanisms.