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Old 01-21-2021, 08:28 PM   #1
EasyGoing1
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Default How could I find a FET with specific specs?

I need a FET or a transistor ... don't care really, I'm going to use it to source current to three LEDs at three different levels of brightness (meaning I'm going to drive the transistor at three different voltage levels). It needs to be able to handle 850 milliamps and be fully saturated at around 3.3 volts ...

Is there a way to search for a FET or a transistor based on parameters like this?

Thank you,

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Old 01-21-2021, 08:35 PM   #2
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Default Re: How could I find a FET with specific specs?

well fets need variable voltage, and transistors need variable current.
most fets dont like being part-on(linear mode) and get hot.

you dont say your led drive voltage, but i think you need to use darlington transistors.
maybe a ztx602
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Old 01-21-2021, 09:44 PM   #3
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Default Re: How could I find a FET with specific specs?

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well fets need variable voltage, and transistors need you dont say your led drive voltage, but i think you need to use darlington transistors.
maybe a ztx602
This is a screenshot from a website where I put in the parameters and this is what it kicked out. I was presuming that I would build the circuit like this ... I just need to drive it...

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Old 01-22-2021, 08:41 AM   #4
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Default Re: How could I find a FET with specific specs?

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but i think you need to use darlington transistors.
maybe a ztx602
... so ... looking at the datasheet for that transistor ... am I reading this right? Would this not fully saturate long before it hits 3.3 Volts?

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Old 01-21-2021, 08:53 PM   #5
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Default Re: How could I find a FET with specific specs?

No transistor likes being part on (linear mode), they all get hot whether BJT or FET.
All depends on how the heat can and is being dissipated.

If you want (nearly) linear voltage (or current) to current translation, you have to use a BJT. MOSFETs are not linear to Vgs and... (broken record...) YOU NEED NEGATIVE FEEDBACK to get a linear response using a MOSFET. You can see people *do* use MOSFETs for audio amplifier final stages, and you do see they have negative feedback in order to get this to work.
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Old 01-21-2021, 11:35 PM   #6
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Default Re: How could I find a FET with specific specs?

But where's the transistorized portion, that's a huge part of the circuit missing!

Do also keep in mind that BJTs have a saturation voltage that may drop the voltage below 3.2 volts, and you may need to use MOSFETs with op amps, possibly with charge pumps, to get the gate drive high enough to fully turn on.

Last edited by eccerr0r; 01-21-2021 at 11:38 PM..
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Old 01-22-2021, 06:58 AM   #7
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Default Re: How could I find a FET with specific specs?

or use pwm.
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Old 01-22-2021, 08:44 AM   #8
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Default Re: How could I find a FET with specific specs?

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or use pwm.
PWM from a microcontroller would not have nearly enough current for this application, which is why I need to drive those LEDs through a transistor of some kind. I am going to use PWM to drive the transistor, however. An ATTINY85 to be exact.
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Old 01-22-2021, 11:13 AM   #9
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Default Re: How could I find a FET with specific specs?

i meant pwm and a fet

as for saturation, like i said - your driving it with variable current - not voltage.
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Old 01-22-2021, 01:53 PM   #10
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Default Re: How could I find a FET with specific specs?

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i meant pwm and a fet

as for saturation, like i said - your driving it with variable current - not voltage.
I'm not sure I understand what you're saying exactly.

Take for instance, this schematic, and the transistor is the ztx602...



Based on what I see in the datasheet, once the pin at the base of the transistor hits 1.7 volts, those LEDs will be on as bright as they ever could be in this circuit, meaning that my options for "medium" and "low" light intensity will have to be achieved by telling the attiny pin to drive the transistor at two different voltage levels below 1.7. Correct? Which gives me a lot less headroom for resolution, which is fine, as long as the transistor will do what I'm needing it to do. But I don't know how to drive a transistor with current because to me, current happens when you apply voltage to a circuit but it's not an option, in this case, to say "Im going to drive this transistor with x number of milliamps" because in an Arduino, I can only adjust voltage levels on pins.
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Old 01-23-2021, 04:12 PM   #11
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Default Re: How could I find a FET with specific specs?

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Originally Posted by EasyGoing1 View Post
I'm not sure I understand what you're saying exactly.

Take for instance, this schematic, and the transistor is the ztx602...



Based on what I see in the datasheet, once the pin at the base of the transistor hits 1.7 volts, those LEDs will be on as bright as they ever could be in this circuit, meaning that my options for "medium" and "low" light intensity will have to be achieved by telling the attiny pin to drive the transistor at two different voltage levels below 1.7. Correct? Which gives me a lot less headroom for resolution, which is fine, as long as the transistor will do what I'm needing it to do. But I don't know how to drive a transistor with current because to me, current happens when you apply voltage to a circuit but it's not an option, in this case, to say "Im going to drive this transistor with x number of milliamps" because in an Arduino, I can only adjust voltage levels on pins.
You are doing Emitter follower (Common Collector), so the Base Voltage with ref to ground will way higher than 1.7V Vbe junction. if you look at your circuit you will have Vbe, Vled, Vrs. The output Voltage on the Emitter will be = Vbase drive - Vbe.
You should have the Transistor or the MOSFET on the low side of the circuit and then PWM the MOSFET to control the current through the LED and you also need the current monitor feedback to maintain constant current through the LEDs.
Your circuit is the Voltage source not current source, so if you have fixed Voltage feeding the LED, but since LED Vf will vary with temperature so the current through the LED will change which will cause the LED brightness to change, that is why you need to use constant current source to drive the LED so if the LED Vf varies the circuit will try to maintain constant current through the LED thus maintaining the steady brightness.
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Last edited by budm; 01-23-2021 at 04:27 PM..
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Old 01-23-2021, 05:00 PM   #12
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Default Re: How could I find a FET with specific specs?

Correct, need a common emitter or common source for mosfets. So that means low side, or use a p channel/pnp transistor. OP can use n channel/npn on the high side if they make a driver for them, which may complicate the circuit considerably.

I'm facepalming because we're seeing the same problems that needs to be solved as the magnetic stirrer problem. Nothing learned... Insisting the final driver must be driven by an analog signal...
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Old 01-23-2021, 10:37 PM   #13
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Default Re: How could I find a FET with specific specs?

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I'm facepalming because we're seeing the same problems that needs to be solved as the magnetic stirrer problem. Nothing learned... Insisting the final driver must be driven by an analog signal...
I'm here to learn, not dictate. I only post what I know...
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Old 01-23-2021, 10:46 PM   #14
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Default Re: How could I find a FET with specific specs?

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we're seeing the same problems that needs to be solved as the magnetic stirrer problem.
And HEY - I got that pill to spin did I not? Gotta give me some credit... :-)
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Old 01-24-2021, 07:07 PM   #15
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Default Re: How could I find a FET with specific specs?

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Originally Posted by eccerr0r View Post
Correct, need a common emitter or common source for mosfets. So that means low side, or use a p channel/pnp transistor. OP can use n channel/npn on the high side if they make a driver for them, which may complicate the circuit considerably.

I'm facepalming because we're seeing the same problems that needs to be solved as the magnetic stirrer problem. Nothing learned... Insisting the final driver must be driven by an analog signal...
That seems to be the case, isn't it?

Does OP actually build the circut? If OP really wants to learn then build the circuit and report the results, the Voltage readings, the condition of the signals, etc. then you can analyze why the circuit is not doing what you want to do.

Last edited by budm; 01-24-2021 at 07:11 PM..
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Old 01-23-2021, 10:40 PM   #16
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Default Re: How could I find a FET with specific specs?

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You should have the Transistor or the MOSFET on the low side of the circuit and then PWM the MOSFET to control the current through the LED and you also need the current monitor feedback to maintain constant current through the LEDs.
OK, so in terms of learning, I have no idea what "on the low side of the circuit" means at all ... also, "current monitor feedback" - no idea what that is either.
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Old 01-24-2021, 07:18 PM   #17
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Default Re: How could I find a FET with specific specs?

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OK, so in terms of learning, I have no idea what "on the low side of the circuit" means at all ... also, "current monitor feedback" - no idea what that is either.
Well, computer and keyboard are your friend so just do the research on those subjects.
You have no idea how lucky you are have internet to access the knowledge world wide, in my days I would be lucky to find the info without waiting for months to get info on pieces of paper or spend lots of money at College book stores.
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Old 01-22-2021, 02:39 PM   #18
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Default Re: How could I find a FET with specific specs?


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Old 01-22-2021, 08:15 PM   #19
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Default Re: How could I find a FET with specific specs?

That circuit takes the small current available from the micro-controller pin, and amplifies it with the transistor to drive the LEDs.

This will turn them on full.

But PWM turns them on and off quickly, and varies the duration for which they are on vs off. By doing so it can control the brightness as we perceive it with our eyes due to persistence of vision. So there is no need to control the current linearly.

With a more advanced micro-controller, you can output an analog signal using the onboard DAC, then amplify that with a transistor to control the LEDs but then you will need a transistor with a heatsink so it does not overheat. PWM is more efficient as the transistor is either on or off.

The only other option is to have multiple transistors switching in different resistor values for your different brightness levels but this increases the cost and complexity greatly.
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Old 01-24-2021, 10:13 AM   #20
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Default Re: How could I find a FET with specific specs?

Since electricity can be interrupted anywhere in the circuit, there's a choice to be made. Low side control means controlling the "low" or ground/negative side of the device that needs to be turned on/off. High side = "high" or power/positive side of the device. Choice needs to be made, devices need to then follow suit.

If you chose devices before side then circuit topologies are more limited due to device physics.

Last edited by eccerr0r; 01-24-2021 at 10:14 AM..
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