Go Down

Topic: [Solved]Help a noob with transistors please. (Read 566 times) previous topic - next topic

larryd

#15
Aug 30, 2019, 01:02 am Last Edit: Aug 30, 2019, 01:10 am by larryd
@Smajdalf

From the OP:
"Additional info: Intended use for transistor is to act as switch for about 35 cm strip of led, it should be pulling under 0.2amp at 12v This BS170 resistor looked good enough for my needs but let me know if it has to be changed out."


"Here OP is switching hundreds of mAs or even amperes"

Amperes  :o  OMG, where are you get this from did you look at the data sheet for the BS170 ?


500 mA, 60 Volts RDS(on) = 5.0 W
1.8 - 5 ohms @ 10Vgs.

The OP's transistor isn't even a logic level MOSFET.

No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

JohnRob

OK...so here's what I know.

1) Within reason one cannot supply too much current to the gate.  Power MosFets are typically driver with 2 to 10 amps from a specialized driver.

2) The noise I was referring to is that created by a fast switching drain, resulting from a good amount of current supplied to the Gate.

3) As the Gate voltage rises from Zero, the gate voltage will rise as well until the drain starts to reduce in voltage.  For the time the drain is switching from off to on, the gate voltage changes very little.   The more current you supply to the gate, the faster the drain will switch. Conversly lowering the gate drive capability will slow down the switching time resulting in less generated noise (EMI)

4) The -3db point of the generated noise frequencies caused by a fast switching time at the drain is approximately 0.35/rise time.  So one can see fast switching times causes radiated noise that can be picked up by nearly anything close by.  With a reduction in switching time, the radiated noise can be reduced significantly.

For a simple test I would use a cheap AM radio (hard to find these days).  I would tune to the lower portion of the AM band and hold it near the switching device.  If the noise could not be heard above hiss on the AM band the design is close and worth testing in the screen room (an RF sealed room used for testing EMC emissions)

NEW SUBJECT:

Component datasheets are written under "ideal" or maybe "unrealistic" conditions.  A seasoned designer knows how to interpret these characteristics and can apply them to the design.  The novice will eventually learn but for the moment one should assume the MosFet should have a max current of at least 10X the expected current.   This 10X number is not cast in concrete but it gives the novice some place to start.

On your part (BS170) The OnSemi data sheet has a note by the max current.  "NOTE: The Power Dissipation of the package may result in a lower continuous drain current."  This means the heat generated in the MosFet trying to drive 500 ma will cause the MosFet to be damaged as the heat causes the device to exceed its maximum temperature.

This is just the tip of the iceberg regarding the application of  a MosFet. You should try to find a good application primer.
Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

Smajdalf

A gate resistor is used in these Arduino applications to protect the output transistors inside the Arduino/micro-controller.
This is one of the cancer-like misinterpretations spread commonly on this forum. There is loooong topic about this which IMHO proves a pin driver damage is not a valid reason for a gate resistor. Maybe if the MOSFET were really huge and it were switching very light load. Otherwise you damage the MOSFET by slow switching before any damage to the pin may be done.
How to insert images: https://forum.arduino.cc/index.php?topic=519037.0

JohnRob

This is one of the cancer-like misinterpretations spread commonly on this forum. There is loooong topic about this which IMHO proves a pin driver damage is not a valid reason for a gate resistor. Maybe if the MOSFET were really huge and it were switching very light load. Otherwise you damage the MOSFET by slow switching before any damage to the pin may be done.
My main reason for suggesting a series gate resistor is in the event the MosFet is wired incorrectly causing a 12V supply into an output pin of the arduino with no current limiting.

Secondly it (when chosen properly) can reduce slightly the rise and fall times and thereby reducing generated EMI a significant amount.  This has worked well for me in a number of automotive designs where the EMC requirements rival those of the Military.

You can find this information presented in a more formal manner in the Ott book on EMC.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

larryd

#19
Aug 30, 2019, 07:19 pm Last Edit: Aug 30, 2019, 07:43 pm by larryd
This is one of the cancer-like misinterpretations spread commonly on this forum. There is loooong topic about this which IMHO proves a pin driver damage is not a valid reason for a gate resistor. Maybe if the MOSFET were really huge and it were switching very light load. Otherwise you damage the MOSFET by slow switching before any damage to the pin may be done.
First you say "cancer-like . . .", you obviously have not had any exposure to cancer otherwise you would not have made this glib statement.

Second you say "Maybe if the . . .",  which says well maybe it is necessary in some situation. Realizing we get all kinds of applications proposed on this web site, best practises are given to new inexperienced users that they can use to succeed in their purpose.  Will adding a (example) 220R as a series resistor cause a problem, No!  Will this resistor prevent damage to the output to the Arduino, YES, (you, yourself said "Maybe. . .") Will this resistor lower the rise time of the driven MOSFET output, YES. This is not noise, it can prevent high frequency components from being generated.  Noise is viewed from the receiver.  Does this resistor dampen/stop the MOSFET from oscillating due to parasitic elements, YES. Does this resistor give 'some' sort of protection if the MOSFET shorts, possibly, but that's a possible side effect and just like a fuse, doesn't guarantee that protection will work.

Is the series resistor necessary in new user designs? Let's see, it protects Arduino output over driving, it limits the generation of high frequencies, it dampens/prevents oscillations and yes can give some limited protection when the new user miss wires a cct.
Yes, this resistor is necessary, highly recommended, is best practice for new users, can be used in 'most' MOSFET applications that a new user will be exposed to, has no negative side effects and all at the cost of a fraction of one cent.



"This is one of the, . . . , misinterpretations spread commonly on this forum" 
No one is forcing you to endure.



 Apologies to the OP :(







No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

Smajdalf

@cancer-like:
God knows we hate inaccurate information on the forum. It's like cancer , you have to remove it
before it spreads...
I want to avoid silly statements. So I said it is possible to damage an Arduino pin by driving a MOSFET gate without resistor. It is very unlikely - it is much more likely you damage the MOSFET by slow switching - but in theory it is possible.

Will adding a (example) 220R as a series resistor cause a problem, No!
No? IIRC internal resistance of Arduino output is about 40 Ohm. By adding a 220 Ohm resistor you slow down everything about 6 times. It means 6 times more time in the linear region, 6 times higher switching loses. You still don't see how this may cause a problem???
How to insert images: https://forum.arduino.cc/index.php?topic=519037.0

larryd

No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

MarkT

OK...so here's what I know.

1) Within reason one cannot supply too much current to the gate.  Power MosFets are typically driver with 2 to 10 amps from a specialized driver.
100mA to 1A is more normal unless talking _really_ big MOSFETs (with large screw terminals on them).  You don't want the gate spreading resistance losses to dominate(!).
Quote
2) The noise I was referring to is that created by a fast switching drain, resulting from a good amount of current supplied to the Gate.
With high power and PWM you need to keep switching losses under control or the device will overheat - tuning a gate resistor value allows that to be traded off against RFI - good layout and decoupling is also essential to controlling RFI.
Quote
3) As the Gate voltage rises from Zero, the gate voltage will rise as well until the drain starts to reduce in voltage.  For the time the drain is switching from off to on, the gate voltage changes very little.   The more current you supply to the gate, the faster the drain will switch. Conversly lowering the gate drive capability will slow down the switching time resulting in less generated noise (EMI)
There's something circular about gate voltage there!  As gate-source voltage increases a channel is induced, the _charge_ on the gate balancing the _charge_ of the channel.  The more charge on the gate the bigger the channel the lower the on-resistance.  A proportion of gate charge is just capacitive though.  You'll see the voltage plateau on the gate charge graph - this flat area is the channel forming, the slopes either side are mainly capacitance.  Drain current has some effect too.
Quote
4) The -3db point of the generated noise frequencies caused by a fast switching time at the drain is approximately 0.35/rise time.  So one can see fast switching times causes radiated noise that can be picked up by nearly anything close by.  With a reduction in switching time, the radiated noise can be reduced significantly.

For a simple test I would use a cheap AM radio (hard to find these days).  I would tune to the lower portion of the AM band and hold it near the switching device.  If the noise could not be heard above hiss on the AM band the design is close and worth testing in the screen room (an RF sealed room used for testing EMC emissions)

NEW SUBJECT:

Component datasheets are written under "ideal" or maybe "unrealistic" conditions.  A seasoned designer knows how to interpret these characteristics and can apply them to the design.  The novice will eventually learn but for the moment one should assume the MosFet should have a max current of at least 10X the expected current.   This 10X number is not cast in concrete but it gives the novice some place to start.
MOSFETs shouldn't have anything but a peak current rating really - the continuous current rating is simply a restatement of the abs max power - you'd never run a device close to that, water cooling is too expensive!

Size a MOSFET using the on-resistance.
Quote
On your part (BS170) The OnSemi data sheet has a note by the max current.  "NOTE: The Power Dissipation of the package may result in a lower continuous drain current."  This means the heat generated in the MosFet trying to drive 500 ma will cause the MosFet to be damaged as the heat causes the device to exceed its maximum temperature.

This is just the tip of the iceberg regarding the application of  a MosFet. You should try to find a good application primer.

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

JohnRob

Size a MOSFET using the on-resistance.
Remember we are trying to help the OP, who is having trouble with a simple circuit he/she doesn't yet understand.   I try to keep everything simple when trying to help these folks.  So if some/many of my responses do not exactly follow the physics.

In this particular case, if the load current is 0.5 A and they chose a (more or less) 5A Fet they should not have any issues.


Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

ConfusedNerd

Hi, first of all thank you for all the responses.

This thread has received way more attention than I had expected. While I do appreciate your assistance, most of the information has gone over my head. Electronics is very much out of my element, even though I am software engineering student... Ironic.

I will take my time to digest information here and hopefully finish my project eventually.

Thank you for your time kind strangers of the internet :)

Go Up