Need Help with LED Driver design

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
I'm trying to design an LED driver that takes a ground as ON, performs a logical NOT on it, and sends it into an N Channel MOSFET. The MOSFET will give ground to a color channel on an LED strip.

LED Strip Driver-schematic.pdf

I believe I have taken care of everything, but to be honest, this is my very first electronic design. Period. So I am sure I have made an error (besides not calculating the resistor size for Q1's base ... which I do not currently understand how to do).

This project is for a neat light display for my daughter's birthday, so I'm trying to get this done quickly, which is why I'm asking for help in this way (I'm not trying to be lazy, just don't want to start a fire).
 

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MisterBill2

Joined Jan 23, 2018
4,055
We are not given any hint as to what problem needs to be solved. Grounding the control lead will cut off the transistor and remove the gate drive from the FET. Is there a problem or are you just unsure? Without a specific complaint it is hard to suggest a useful solution.
 

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
Do I need a resistor between Q1 and Q2, to limit current through Q1?

Is my R2 value awful?
Will this circuit even work at all?
I plan to use a buck converter to provide the 5V power (pulling from the same 24V power supply that powers the LED strips), is there a good way to chain the grounds of the two power systems together?
 

Audioguru again

Joined Oct 21, 2019
272
I disagree that the Mosfet will heat with 1W.
The Mosfet has a maximum on-resistance of 0.016 ohms when Vgs is only 4.5V. But here the Vgs is higher at 4.8V or 4.9V so the maximum heating in the Mosfet is (1.1A squared) x 0.015ohms= 16.5milli Watts which is almost nothing.
 

eetech00

Joined Jun 8, 2013
1,726
I'm trying to design an LED driver that takes a ground as ON, performs a logical NOT on it, and sends it into an N Channel MOSFET. The MOSFET will give ground to a color channel on an LED strip.

LED Strip Driver-schematic.pdf

I believe I have taken care of everything, but to be honest, this is my very first electronic design. Period. So I am sure I have made an error (besides not calculating the resistor size for Q1's base ... which I do not currently understand how to do).

This project is for a neat light display for my daughter's birthday, so I'm trying to get this done quickly, which is why I'm asking for help in this way (I'm not trying to be lazy, just don't want to start a fire).
Hello

You could do something like this.

M1 holds M2 off, so the LEDs will not light.
If you ground the input, M1 will turn on and the LEDs will light.

Don't use 2N7002 though...they wont support the required load current. Actually, you could use it for M2 but not for M1.

eT
1575437655870.png
 
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ci139

Joined Jul 11, 2016
803
Ok. -- it's just 1 Amp -- the amount the AAA can output :rolleyes:
/// -- my schematic is just randomly compiled from loose memory clips that have applied somewhere = it's a fast collate -- without a deeper analysis = don't copy anything for "just in case"
/// -- it fits sharply with the Fig.1 and Fig.12 from prev. post !?

Random_LED_Test_10.png
 
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MisterBill2

Joined Jan 23, 2018
4,055
The revised cicuit shown in post #4 should work, and you can feed the 2N3906 PNP transistor directly with another 10K resistor right from the same +24 volts source.
, so there is no need for a separate 5 volt supply. That makes it a lot simpler. Then grounding the input point will switch on the LED load power. An added advantage is that there will be plenty of gate voltage to fully switch on the pass FET.
 

Audioguru again

Joined Oct 21, 2019
272
We do not know what provides the 0V input signal. It is probably from a logic circuit that has a rail-to-rail output and is powered from the +5V. So the input to this circuit is +5V when the lights should be turned off and be less than about +2V when the lights should be turned on.
 

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
I should have remembered that about PNP Transistors. Thank you! It's spread out because I was using TI's MultiSim to create and test the circuit. Which had a very strange amperage reading through the MOSFET, that makes absolutely no sense. I'm not sure how much I should trust that tool.

I didn't really have much lying around to test this with, so I was trying to get it working in theory, and order what i needed.

So, the full path of the circuit, which I could not draw using the schematic tool (MultiSim), is Arduino (which can later be modified to just an ATmega328 chip), using a 1-wire interface, to a TLC5971 LED Driver. That LED Driver is meant to directly drive a few LED's, but I want it to drive an entire strip. So I need to use the ground provided by four of the channels, to control MOSFETS which drive the LED strip. The goal of the project is a very high frequency LED strip driver, but for significantly cheaper than a "flicker free" DMX-controlled RGBW (four channel) LED Driver. Regular LED strips bother my wife and kids' eyes.

Due to the Arduino Uno and TLC5971, I need a Buck Converter (MP1584EN) to step down 24V from the power supply. I figured I could also use the 5V to drive Q1, and the gate of Q2.

I think that explains everything. Should the circuit in figure 4 work with this setup? I will be building this four times, in order to account for the four color channels of the LED strip.

Is this all a bad idea that won't work because the TLC5971 cannot be used in this way? As I said before I am completely new to designing circuitry. I've done a few simple arduino projects, and even used relays to do a bit of a Christmas display with 40 channels of lights ... but this is so much more complicated than any of that.
 

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
Hi

why not use circuit in post #7?
You don’t even a BJT.

eT
I was looking at the specs of a 2N7002. compared to a 2N3906, doesn't the 2N7002 have more power dissipation at 5V, than a 2N3906? I might be wrong ... probably am, as I'm not used to making the calculations needed. I've watched a couple YouTube videos on it, but I'm having trouble understanding.
 

eetech00

Joined Jun 8, 2013
1,726
I was looking at the specs of a 2N7002. compared to a 2N3906, doesn't the 2N7002 have more power dissipation at 5V, than a 2N3906? I might be wrong ... probably am, as I'm not used to making the calculations needed. I've watched a couple YouTube videos on it, but I'm having trouble understanding.
Mosfets are voltage driven devices (as opposed to BJTs which are current driven and consume more current to operate). They turn on or off based on the voltage level at the gate and consume extremely small amounts of current. The first 2N7002 (M2) is being used as a switch to invert the input signal and passes very little current through the drain. The voltage present at the junction of R1/M2 turns M1 on or off. M1 need to be of sufficient current capacity to pass the amount of current to light the LED string(s). The Mosfet you chose should work for M1 but M2 need to be chosen based on the required "turn on" voltage fed to its gate. The 2N7002 will turn on at about 2 volts (your mosfet will work also but is overkill).

This is a simple circuit and will work, but really should be designed for constant current and/or PWM driven.

eT
 
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Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
This is a simple circuit and will work, but really should be designed for constant current and/or PWM driven.

eT
If you read my post #12, I plan to control this circuit with a TLC5971, which uses PWM at 16 bit resolution, with a very high frequency. Is this what you meant, or no? I tried to select components that will be able to switch at a very low latency. I do like the MOSFET you chose.

Another question:
the TLC5971 has 60 mA max per channel. Will that be enough to overcome the pullup resistor R3 (10K Ohm)?

Initially, I thought of using optocouplers: the TLC5971 just drives four optocouplers, while the other side is used to overcome a pulldown resisitor at the MOSFET ... but I have no experience at all with optocouplers.
 

eetech00

Joined Jun 8, 2013
1,726
If you read my post #12, I plan to control this circuit with a TLC5971, which uses PWM at 16 bit resolution, with a very high frequency. Is this what you meant, or no? I tried to select components that will be able to switch at a very low latency. I do like the MOSFET you chose.

Another question:
the TLC5971 has 60 mA max per channel. Will that be enough to overcome the pullup resistor R3 (10K Ohm)?

Initially, I thought of using optocouplers: the TLC5971 just drives four optocouplers, while the other side is used to overcome a pulldown resisitor at the MOSFET ... but I have no experience at all with optocouplers.
hi

ok....lets step back a minute.

You want to drive three LED strips, right?

Specifically, what type of strips will need to be driven?

the type of strip will directly affect the voltage/current requirements of the driver.

a part number would be good..

why are you considering optos?

eT
 

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
You want to drive three LED strips, right?
Specifically, what type of strips will need to be driven?
the type of strip will directly affect the voltage/current requirements of the driver.
a part number would be good..
B00VIKAWWC
  • Color: RGB+Warm White in 1pc LED SMD
  • LED Quantity: 60LEDs/m, 300LEDs/roll(300pcs RGBW LED)
  • Voltage: 24V DC
  • Current: 4A/roll
  • Power: 96W /roll

One LED strip, with four color channels. Each Channel draws approximately 1 Amp. I tried to design for 1.1, just in case my numbers were off, I would have 10% leeway. I planned to design the system so that one driver (with four subchannels: Red, Green, Blue, Warm White) would control one LED strip. I am using a 24V strip, instead of a 12V or 5V, to reduce power loss as much as possible; The Power supply will need to be at least 12 feet away from the LED strip.

why are you considering optos?
The TLC5971 is designed to power LED's, so I thought that it would be easiest to turn the controlled ground of the TLC5971 into an optocoupler-activating signal, which would, in turn, enable a MOSFET, which would power the LED Strip. I scrapped this idea because:
1: I have never used an optocoupler.
2: there are very few articles I could find on using them, that explained it in terms I could understand without being an electronics engineer. IE: what resistors should I use with them? Can they handle the 5V necessary to enable my high-power MOSFET? Can they handle being driven at 24KHz PWM, without having pulses run into each other?
 

ci139

Joined Jul 11, 2016
803
Power supply will need to be at least 12 feet away from the LED strip.
↑ might require specific circuit protection elements ↑
I scrapped this idea because:
1: I have never used an optocoupler.
-- different optocouplers differ as a day and night the B817 can handle 100kHz (if i remember right - nope - it's below 80k about TPC817) . . . the ones designed for Hi-Speed switching applications may well do up to 2MHz . . . using the opto at photodiode configuration extends it's speed range but - requires a more precise design . . .
being driven at 24KHz PWM
-- in simple case the PWM frequency won't need to be too high - just about the eye won't notice it or the interference of (incase the multi channel/-color)
 
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eetech00

Joined Jun 8, 2013
1,726
One LED strip, with four color channels. Each Channel draws approximately 1 Amp. I tried to design for 1.1, just in case my numbers were off, I would have 10% leeway. I planned to design the system so that one driver (with four subchannels: Red, Green, Blue, Warm White) would control one LED strip. I am using a 24V strip, instead of a 12V or 5V, to reduce power loss as much as possible; The Power supply will need to be at least 12 feet away from the LED strip.
ok

The TLC5971 is designed to power LED's, so I thought that it would be easiest to turn the controlled ground of the TLC5971 into an optocoupler-activating signal, which would, in turn, enable a MOSFET, which would power the LED Strip. I scrapped this idea because:
1: I have never used an optocoupler.
Constant current drivers are designed to work with directly connected LEDs.
I would drop the opto idea.

Now that we have some design requirements...we can follow up with some suggestions.

eT
 
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