perplexing MOSFET + LED strip.

Other Hardware Motors, Mechanics, Power and CNC
1

I am using an ESP8266 NodeMCU to drive 4 outputs. Software all works perfectly as they drive 4 test LEDs on a breadboard perfectly.

Now I come to start wiring the device to the MOSFETs that will be used to drive long runs of (monochrome) LED strips.

I have RFP30N06LE MOSFETs

To test I know what I am doing with the MOSFET I connected one up on the breadboard.

The ESP8266 is powered by a USB connection. It is a 3.3V device.
I connect an output pin to the gate.
I connect the ground of a 5V DC supply to the source pin.
I connect a test LED with resistor to the drain
I bind the 5V ground with the ground of the ESP8288
I connect the output pin to ground with 10K Ohm Pull down resistor

It works. I can switch on and dim the LED through the MOSFET.

Now, I remove the LED and its resistor.
I remove the 5V Power supply

I replace the 5V supply with a 12 V supply (this is what my LED strips use)
I connect the drain of the MOSFET to the negative side of the LED strip.

The LED strip does not light up.

If I connect the strip to the 12V supply they all light up. LED strips works.

I cannot fathom why the 12V + LED strip circuit does not work and the single test LED setup does. Granted the single LED draws much less power....

I have checked the polarity - the ground of the 12V supply is at the same level as the ESP8266 ground.

With no LED strip connected, my voltmeter reads 12.00 Volts between the MOSFET drain and the positive. If I plug in the LED strip, it drops to 7.00 Volts

The 12V power supply has plenty of current to drive the 5m strip of lights. They light up if plugged in directly.

Any suggestions on what might be going wrong? It seems that something is sucking up exactly 5 Volts.

I get the same results with different MOSFETs and LED strips.

I have attached what is essentially the same wiring scheme.

rfp30n06le-arduino-lightbulb-400x218.png

2

The way you described your connections is for a P channel MOSFET, RFP30N06LE is N channel, you should have +V to LED +, LED - to MOSFET drain, MOSFET source to ground with power supply ground. Also, if ESP8266 only outputs 3.3V, that may not be enough to turn the MOSFET full on. AND you should have a 100 to 150 Ω resistor between output pin and gate.

3

OK, after re-reading, looks like the 3.3V is not enough to saturate the MOSFET, that's where the 5V drop is coming from, you need at least 4.5V, you need MOSFET drivers that will drive the gate from the 12V supply.

4

I don't see mention of commoning the 12V supply ground to ESP8266 ground.

5

as far as I have read these MOSFETs are claimed to be triggered by 3.3 Volts. If I read the data sheet correctly, 1 to 2 Volts is the threshold. (But I dont understand how 2V can be the maximum.) I will try a Voltage shifter but then I have to find 5V somehow. Not saturating it makes the most sense. It puts 12V out but only 7 under load. I don't know how the Voltmeter handles PWD current.

Yes, the ground of the power supply is bound to that of the ESP8266. I bound it at 5V to test and then replaced the 5V with the 12V leaving the binding in place.

I am wondering if different MOSFETs would work better.

6

I think @outsider is correct.

I applied 5V to the gate and then it works. Now I need to find out how to rig up a MOSFET driver from the 12V supply.

7

DaleSchultz:
(But I dont understand how 2V can be the maximum.)

Because that's the maximum trigger voltage. Aka, it's possible to have a mosfet that only starts to conduct at 2V.

But yeah, 3v3 can be a bit tricky at higher loads. You say 5m of strip but don't say which type or the current. So I assume 6A for that. With a gat voltage of 3V3 that should still work reasonable I would think. Ron is probably higher than 0,047Ω but even at 0,075Ω that would be <0,5V. But the mosfet would need proper cooling for that! Because power dissipation will be >2,5W with a static (no PWM) load.

So yeah, a mosfet driver like the TC4427 can solve that a bit. Although with a Ron of 0,047Ω it's still 1,7W without PWM which still would need a heatsink I think.

8

I post this regularly as its a very common misconception - a power MOSFET's threshold voltage is
nothing to do with using it as a switch, forget it, ignore it. Its only on the datasheet because
its easy to measure, its not actually of use to 99.9% of applications for a MOSFET.

[its often about 3 times lower than the proper gate drive voltage needed to turn on the device]

The place in the datasheet you consult is the Rds(on) specification, which will be given for
one or more values of Vgs. Those values of Vgs are enough to turn the device fully on to the
resistance values given, and voltage smaller than the minimum one of these is not guaranteed
to operate the device reliably (especially at high current).

If Rds(on) is given for Vgs=10V, but not for Vgs=4.5V, then the device is not logic level.

9

thanks @septillion that now makes sense to me (max threshold)

I did some more tests,

The LED circuit with the biggest draw is 5 Amps. (Others are 3.5A and 2.7A)
I have been running them through 7A fuses and never had one blow, (but they do each run through a manual PWD control, which I assume has a 100% duty cycle at the top end.)

For my initial test I connected one 5m strings of LEDs, probably around 1A, and it appears that the MOSFETs are actually passing current, but the demands of the MOSFETs are such that to pass the current I need (5A), I need a higher gate voltage. With the 3.3V it can only manage to pass enough current to bring the effective voltage down to 7 - insufficient to light the LED string.

So yes, it appears that I need some driver to increase the gate voltage.

As I understand it, I need to know how much gate voltage is needed for the RFP30N06LE to pass 6 Amps. After studying the datasheet datasheet for the RFP30N06LE I still cannot work out what needed gate voltage should be to pass 6A at 12V. I think I need to know that before I can start designing a gate driver.

I suspect I should start a new thread, now that the main bafflement has been solved. Now my question is how to get enough gate voltage to the MOSFET, and how much is needed.

10

Hi all,

I am trying to use some FQP30N06LE MOSFETs - which seem to be the recommended ones for driving high current loads with low resistance and gate threshold voltages.

I am driving my MOSFETs from an ESP8266 NodeMCU which has 3.3V output pins (and 12mA max).

The load I am applying to the drain is long strings of white LED strips. The largest circuit draws 5A at 12V.

My reading of the datasheet for the FQP30N06LE at http://www.farnell.com/datasheets/2299856.pdf
says the gate threshold voltage is 1V to 2.5V at 250uA.

Now, I believe that higher gate voltages are needed when there is a high current being passed by a MOSFET.

A test with a 3.3V gate voltage and a 12V source, with a 5m strip of LEDs, produces only 7 Volts on the drain - insufficient to light the LED strip.

I need to place at least a 5 Amp load on the drain, - about 6x the load in my test, so I believe that I need a gate driver.

In order to design the gate driver, I need to know what voltage it must supply. So how do I find out what gate voltage is needed in order to be able to pass (say) 6A?

Lets say we get that answer and it is 10 Volts... do you have any suggestions on how to shift the 3.3 logic to the desired 10V ? I have some logic shifters but they only talk about going from 3.3 to 5V. I have seen some 12V shifters are available too. I have 3.3V and 12V available.

Feel free to question my understanding of any/all of the above, I have not done this before and have spent days trying to read up on this stuff, but almost every thread has differing views or slightly different parameters.

If anyone knows of a 4 channel board that takes 3.3 PWD input and can drive more than 5A, I would like to know about it. I found some online but "out of stock". Given the popularity of 3.3V devices I would have thought this would have off-the-shelf solutions!

11

I had a long reply but then I see you're getting good help over on your other thread.

12

@DaleSchultz, please do not cross-post. Threads merged.

13

sorry, I did think the thread had shifted to a new problem.

14

DaleSchultz:
Lets say we get that answer and it is 10 Volts... do you have any suggestions on how to shift the 3.3 logic to the desired 10V ?

We only mentioned a MOSFET driver (which is a chip designed for that task0 a hand full of times :wink: And in reply #6 I gave an example off one :wink:

15

well yes, but what I am trying to find out is how a TC4427 is used. When one is new to all this stuff, one reads and reads and sees mention of numerous things and it is hard to know which are the gems and which are misunderstandings. Its a catch-22 if we knew, we would not be asking. So my understanding has progressed to understanding the gate voltage problem and has now moved (to what I though was a new topic) to how to connect what gate driver to the MOSFETs. I spent the evening last night googling that and there is surprisingly little information out there that describes what a TC4427 does in layman's terms. I think I am learning what it does by studying bridge control units but I still do not know.

16

So this is what my current understanding is.

If I connect a TC4427 up with +12V connected to the VDD and put a PWD 3.3V signal into IN B, then the TC4427 will add the 3.3V to the 12V and put out 15.3V on the OUT B pin.

That 15.3V PWM signal will then be able to saturate the FQP30N06LE and enable it to pass at least 5A from the source pin to the drain pin.

Grounds of everything tied together.

I have drawn a sketch of my understanding which is attached.

I have also read talk of capacitors being connected to the TC4427 but I do not know where they should go or if they are needed in my application.

Do I need a resistor between my ESP8266 pin and the TC4427?

Once I get this connected I will test to see how hot the MOSFET gets and determine if the heat-sinks I bought will be sufficient. I have read that one can use MOSFETs in parallel so I might have to do that for the circuit that has the 5A load if a single one gets too hot.

For my project I would need two TC4427s, each one driving two circuits. Does it make sense to balance the load between those two, running the biggest and the smallest load circuits through one, and the two medium loads through the other?

As always, I so appreciate all the help.

TC4427.png

17

Here is what I read about capacitors (in the data sheet)

The VDD input should be bypassed with local
ceramic capacitors. The value of these capacitors
should be chosen based on the capacitive load that is
being driven. A value of 1.0 µF is suggested.

Can someone help me understand what that means?

18

MorganS:
I had a long reply but then I see you're getting good help over on your other thread.

It appears that any responses you saw may have been lost in the merge, as all that came across was my post.

19

I found some excellent information on the TC4427 at the manufacturer's website

Once there, one can click the Documents tab and find links to the Application Notes

A lot of the technical docs are above me but I think I understand their recommendations on the capacitors
and a 10 Ohm resister between source and their Vdd pin

I have updated the diagram again. I have also ordered the additional parts and will report back test results.

TC4427.png

20

There you go :slight_smile: The datasheet and like you discovered app notes is a good source of information. And sometimes the manufacturer only gives you basic operation and connections and not all ins and outs. But for stuff like this, it's fine :slight_smile:

And sorry for the delay, but I do need to get some sleep once in a while, preferably every day.