No current - Power LED Dimmer circuit using NE555P in astable mode with IRL540N

Thread Starter

dbinkhorst

Joined Jun 4, 2019
5
Hi all, this is my first thread here being new forum member and I'll try to be as accurate and explaining as possible.
Please do not hesitate to correct me on any shortcomings from my side in this thread. I am an absolute beginner when it comes to electronics.

The idea is to eventually power this circuit with 12V DC and attach 2-3 1W 350mA LED in series with an LM317T as a constant current source.

So, I set up the following test circuit and connected P1 to a 7V/2A rated DC power supply. P2 has a 1W 350mA LED attached in series with a 1 Ohm 1/4W resistor.

The datasheet for the used IRL540N can be found here: https://cdn-reichelt.de/documents/datenblatt/A100/IRL540N_IR.pdf

upload_2019-6-4_9-54-48.png

When powered on the LED is barely lit and turning VR1 does dim the LED. However it is fairly obvious I am unable to light LED to it's full brightness, by far.

The voltage across the LED is 3.1V, but when attaching the multimeter in series I get 0.012mA current flowing through the resistor and LED (Yes, that's 0.012 milliAmps).

As a test I connected the LED and 1 ohm resistor directly to P1+ and Pin3 of the NE555P and disconnected R3 and Q1 from the circuit. This way I am able to fully light the LED and dim it almost completely. Current is about 270mA.

The question is, why is there no current flowing when the IRL540N is connected?
 

ericgibbs

Joined Jan 29, 2010
10,470
hi db,
Welcome to AAC.
It appears that you are using the MOSFET to short out the power rail to 0V.?

Could you post an image showing the extra components that you cover in your text.?
E
 

Thread Starter

dbinkhorst

Joined Jun 4, 2019
5
Hi E, thank you very much!

As I am not at home currently I am unable to take any photos from the circuit and it's connected components.
I'll post some pictures later.

I am not aware of shorting out the power rail with the MOSFET...
The circuit above is based on figure 10a in the IRL540N datasheet, which eventually gave me voltage over the attached load and some kind of dimming.

But if this setup is actually shorting out, that explains why the LED -albeit dim-, lights up.

upload_2019-6-4_11-17-29.png

The original circuit I used, based on standard 555 high current circuits is the following:

upload_2019-6-4_11-19-45.png

This however gave me 0 volts over the LED connected at P2.
Even with P1 powered at 12V and 2 LED in series connected to P2.
 

Thread Starter

dbinkhorst

Joined Jun 4, 2019
5
Hi E,

That is pretty close to the circuit I want to set up, I see 4 capacitors instead of the 2 I use.
I made some adjustments to the circuit drawing.
That is the setup I have in mind.

My test setup differs from the drawing: P1 input = 7V DC and only LED1 is connected.
And R2 is actually 220 Ohms.

upload_2019-6-4_13-25-56.png
 

ericgibbs

Joined Jan 29, 2010
10,470
hi,
With only a 7V supply and a load of 6.4v the LM317 current reg will not work over that range, you need at least 2v across the LM317.

Ideally a minimum of 9V supply.
E
 

Daniel Sala

Joined May 28, 2015
61
Hi,

You can improve your original premise with little effort and perhaps only a few more parts if you feel like getting all fancy about it. A "real" LED dimmer would use pulse width modulation or another 555 variant circuit like pulse position modulation. Figures 18 (PWM) and 20 (PPM) may be helpful.

Bearing in mind that modulating the control pin voltage is limited to 1/3rd and 2/3rds Vcc, so can't get full range might want, but it is an option.

You would get a far greater range (much closer to from nearly 0% to around 100%) with a comparator + variable reference voltage or an op amp voltage follower driving your MOSFET (or the MOSFET + a feedback resistor as a current source). You could still inject a squarewave somewhere in there (like the supply line to the LED) to lower power dissipation considerably.
 

Attachments

Sensacell

Joined Jun 19, 2012
2,587
I see no purpose for D1 with an LED load.
That's only needed for an inductive load.

R2 should be less than 100Ω.
Indeed, but having a diode there protects the MOSFET from positive-going voltage transients on the drain.
Just having a long wire hanging off the drain is an invitation to ESD destruction, even a 1N4148 improves the situation dramatically.
 

Thread Starter

dbinkhorst

Joined Jun 4, 2019
5
The circuit works!

Thanks you all for your help so far!

I am now able to dim from approximately 25% to 95% taken from voltage measurements I did over one LED.

hi,
This circuit is a rough simulation of what you have posted.
Edit it if incorrect and repost.
E
Image updated:
Main problem was the IRL540N Source was not cpnnected to ground in parrallel opposed to P1 V+. Thanks, E, your attached circuit drawing led me to discover that error.

I see no purpose for D1 with an LED load.
That's only needed for an inductive load.

R2 should be less than 100Ω.
I changed R2 to a 22Ω 1/4W and removed the Schottky Diode D1 from the circuit. However, the circuit works with the Schottky Diode as well.

Indeed, but having a diode there protects the MOSFET from positive-going voltage transients on the drain.
Just having a long wire hanging off the drain is an invitation to ESD destruction, even a 1N4148 improves the situation dramatically.
Not sure what to do here, but I do have a few Schottky diodes, so I might as well use them just to be on the safe side.

hi,
With only a 7V supply and a load of 6.4v the LM317 current reg will not work over that range, you need at least 2v across the LM317.

Ideally a minimum of 9V supply.
E
I currently have 1 LED connected to the LM317T with R4 = 3.9Ω 2W and P1 fed by 6V DC.
Voltage over LED1 is now 2.9V @ 192mA. P1 at 6.5V gets LED1 at 3.3V @ 270mA.

As soon as I have a bit more time I'll do some experimenting with adjusted P1 (6.5V-7.5V) and adjusted R4 to see if I can get a little closer to 350mA.

Questions
1. As Duty Cycle comes closer to about 25% the LED starts flickering. C1 and C2 are both 100nF capacitors. I read somewhere if I set C1 to 1nF that will up the pulse frequency by factor 100. Might that solve the flickering issue?

2. Am I correct the total voltage drop before the LED in the circuit are the following values added?
- 0.7V for U1 (EN555P)
- 1.3V for Q1 (IRL540N)
- 1.25V for U2 (LM317T)
----------------------------------- +
3.25V

This suggests the voltage at the LED should be no more than 2.75V with P1 at 6V, which matches my multimeter measurements. Does this imply that ideally P1 should be set to 6.5V?

Thanks everybody so far, you've been a great help!
 
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