How could I find a FET with specific specs?

[eccerr0r]

Re: How could I find a FET with specific specs?

The OP was using PWM except was not letting it through to the end device, insisting that the conversion happen early in the chain, thus wasting energy as well as increasing the dissipation requirements of the switching elements. The same problem was encountered in one of the previous projects the OP was working on yet persists on here.

In any case, proper drive needs to be figured out. OP apparently still hasn't figured out how to properly turn LEDs fully on and off - a requirement for PWM.

[sam_sam_sam]

Re: How could I find a FET with specific specs?

This might seem like a stupid suction but why not use PWM function ( if this controller has this option) and
only have one output

You do have use the right setup for the transistor or mosfet that you would using

I have done this a few time with a Basic Stamp and using the PWM function but you have to have output setup correctly I have even done this with an optic sensor ( like a 4N25 or something similar )

For give me for saying this but you might be making this hard than it needs to be
unless you have a special reason for doing it this way

One note you do have to set parameters correctly with the PWM signal for this to also work correctly
You might not be able to to go all the way down to 0% duty cycle and see the LEDs light very dimly but maybe down 10% depending on the controller and maybe as high as 75 to 80 for full on brightness in appearance to naked eye

One other note you might even have to put a 1uf ( or less ) capacitor on the output pin of the controller and the negative side of the power supply to smooth out the pulses coming from the controller if they flicker to much ( see below )

I once had to use this method for a solenoid valve coil hook up to Basic Stamp Controller using the PWM output to control a portioning hydraulic valve coil because the controller board for this machine took a crap for controlling this valve and to replace it would have cost to much money

One final note this valve portioning power amplifier version has a feature that you can hook pot control to but because of the way valve works it would not always work correctly all the time and this was the reason for using a controller with a PWM controller function so it would work correctly most of the time

The company that I worked for at the time really appreciate the fact I was able to fix this machine issue without having to spend a lot of money to do it ( this machine was about 20 years old )

[eccerr0r]

Re: How could I find a FET with specific specs?

Before you test that circuit and find it may not work as well as planned, do recall one LED turned on is dimmer than 2, and 2 on is dimmer than 3... so you may also be overthinking the problem.

[EasyGoing1]

Re: How could I find a FET with specific specs?

What do you guys think about something like this ... where picking different values for RO1, RO2 and RO3 as current limiting resistors for high, medium and low brightness in the LEDs, turning on only one of the three transistors at any given time?

[EasyGoing1]

Re: How could I find a FET with specific specs?

I have to leave it up as an exercise to the reader to convert this to a 'low' side control.

Low side control?

Thank you for putting the correct polarity/channel semiconductor in the schematic. Very important when dealing with the drive needed to turn on low/high side semiconductor switches.

You're still upset over that little mistake? That was like ... 5 topics ago wasn't it?

:-)

[EasyGoing1]

Re: How could I find a FET with specific specs?

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.

Hey I was there too ... I remember when Al Gore proposed his bill to fund the "Information Super Highway" - which was a federal budget created specifically to expand the Internet from the privileged few (governments and public education) out to the home via the Telco's. I was in my second year of college back then and didn't actually see Internet access until two years after that when I was a tech at Edwards AFB ... then AOL tapped in and started separating the men from the boys... lol

And for the record, I did immediately look up current monitor feedback and was reading eagerly until I realized I started getting completely lost .... what I got from it though, was that a voltage feedback monitor basically takes a voltage and feeds it back to an opamp using a high impedance input on the opamp so that it would be the small voltage differences that guide the output of the opamp causing it to provide a more stable voltage, and current feedback would do something similar only it would be fed back into a low impedance on the op-amp so that it would be the current changes guiding the output of the opamp ... that's about as much as I was able to register before the propeller head tore off into all kinds of hieroglyphics ...

[budm]

Re: How could I find a FET with specific specs?

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.

[budm]

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...

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.

[eccerr0r]

Re: How could I find a FET with specific specs?

Well, clearly now you don't have a low side control of the load. What you do have is 'high side' where the transistor switch element is controlling the positive rail to your device as it blocks the electrons after it flows through your load(LED) (or described by classical electrical flow, current is blocked before it goes through the load).
I have to leave it up as an exercise to the reader to convert this to a 'low' side control.

Thank you for putting the correct polarity/channel semiconductor in the schematic. Very important when dealing with the drive needed to turn on low/high side semiconductor switches.

[EasyGoing1]

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.

OK, then when budm says, "You should have the Transistor or the MOSFET on the low side of the circuit" how does that change this schematic? What would be different?

[eccerr0r]

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.

[EasyGoing1]

Re: How could I find a FET with specific specs?

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... :-)

[EasyGoing1]

Re: How could I find a FET with specific specs?

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.

[EasyGoing1]

Re: How could I find a FET with specific specs?

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...

[eccerr0r]

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...

[budm]

Re: How could I find a FET with specific specs?

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.

[Agent24]

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.

[eccerr0r]

Re: How could I find a FET with specific specs?

[EasyGoing1]

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.

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.

[stj]

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.