PWN Dimmer with NE555 and Mosfet

[Ian.M]

If the MOSFET heats up and it has good gate drive, at a low enough frequency for it to fully switch in <<1% of the period, its a sign it may be oscillating at RF.  This is more usually a problem if there is a drain resistor e.g. for current sensing or limiting. A resistor in series with the gate damps any oscillation, and should be fitted as close to the MOSFET as possible.   Values between 10 and 100 ohm are usual.  In some applications you'll also see a ferrite bead on the gate lead.

[alsetalokin4017]

One more: mosfet IRF3205 Drain trace at close to maximum duty cycle setting. Nice clean switching, almost no linear region operation or other bad effects. This is with the 12v incandescent bulb as load, drawing a bit less than 250 mA average.

[alsetalokin4017]

That's right, and with the IRF3205's whopping 8 milliOhms R_ds(on) and the circuit's maximum drain current of 300 mA pulled by the LED strip load .... well... let's just say "heatsink optional"......   

[Forser]

Been building after Schematic from Alsetalokin.

Everything starts up fine but very bright already on the lowest potentiometer selection but that isn't a issue.

Issue i have is after a second or so, it autodims the Leds and requires to turn off / on the power and power supply reports a Current of 0.03 only even at start.

Got a Oscilloscope that i could hook it up to and see if 555 is doing something odd but don't really know where to put the ground from the probe and don't want to blow anything up.

If there are any questions about the wiring or something else, just ask.
Will tinker around and see if i can figure it out.

[Forser]

Solved it, I had misplaced the 1 k resistor and noticed it when i looked more closely at the image of the build Alsetalokin had done.
Also changed the wiring so it matched the image, aka, Pin 4 to Pin 8 and Pin 2 to Pin 6.

Now it works, also changed the Potentiometer to 200k to dim it more, don't know if that is the correct way of doing it or not.

[alsetalokin4017]

Glad you got it working.

You must want really dim LEDs! You could try increasing the value of the LED series resistor, but this will also make them dimmer at the bright end too.

The scope probe ground goes to the circuit common ground. Same as Mosfet Source pin, 555 pin 1, etc.

[Forser]

So, spent a few hours on getting the setup moved from breadboard to a perfboard.

It works but for some reason, it seems like it isn't dimming but increasing the light instead.
Been trying to figure out if i put something wrong and such but can't figure it out.

Any clues on how to do a fault search on it?
Got access to an Oscilloscope, Multimeters and power source.

Sadly, it's a bit of a mess on the backside but i have looked closely with magnifier and don't see any shorts or similar.

Thanks in advance for any tips on how to try and solve it, don't need a straight up answer but tips on how to do a fault search so i learn more.

[Yansi]

Swap the leads going to the ends of the potentiometer resistance track.

Hope I guessed your problem.

[Forser]

@Yansi, When you mean the Leads, you mean the Diodes (1N4148)? I tried that and didn't make any difference.

I built up the circuit on the breadboard again (not with the soldered parts) and it works just fine.
Can a broken Mosfet be the reason for it to behave like it does?

On the breadboard i can go from 300 mA to 100 mA but on the soldered board, it only goes from 300 mA to 200 mA and it looks brighter also so something is off but i can't figure it out, been looking around to see if the wires are correct and such and only thing i can think of is the Mosfet.

[alsetalokin4017]

No, I don't think your mosfet is broken.

It's very difficult to tell from your pad-per-hole construction but it looks to me like you may not have the center pin (wiper) of the pot connected to 555 Pin 2 (and 6) . Correct me if I'm seeing it wrong but it looks to be connected only to the capacitor but not to the Pin 2+6. The capacitor should go between Pins 1 and 2 (or, equivalently, Pin 2 and Ground) and the pot wiper to the Pin 2 side of the capacitor.

I think if I were you I'd start over on the circuit board, using thinner wires, more direct routing and a finer tip on your soldering iron. Chalk this one up to practice.

Also, if you have a 3-pin version of the blue screw connector, these make very good mosfet sockets so you don't have to solder directly to the mosfet (you have to spread the legs of the TO-220 package a bit). I use them all the time in my own constructions.

[Forser]

I rechecked the wires.
From center pin on pot, it goes to cap which is connected to pin 2 and there is a wire between pin 6 and 2.

On the breadboard, I changed the cap from center pin / pin 2 to cap on pin 2 to ground and center to pin 2.
Which resulted on the breadboard a even dimmer led strip.

Will rebuild the circuit on a stripboard instead to decrease the number of cables I need to use.

[alsetalokin4017]

I rechecked the wires.
From center pin on pot, it goes to cap which is connected to pin 2 and there is a wire between pin 6 and 2.

Note that on the schematic, the center pin of the pot is connected to Pin 2+6, and the capacitor is connected between Pin 2+6 and Pin 1 (ground). 

It looks to me like on your circuit board, the center pin of the pot goes _only_ to one side of the capacitor, and the other side of the capacitor is connected to Pins 2+6. If you do have it wired this way, it's not what the schematic shows.

On the breadboard, I changed the cap from center pin / pin 2 to cap on pin 2 to ground and center to pin 2.
Which resulted on the breadboard a even dimmer led strip.

I'm not following you here, I don't understand what you mean. The schematic shows center pin of pot > Pins 2+6, and capacitor between Pins 2+6 and Pin 1 (ground).

Will rebuild the circuit on a stripboard instead to decrease the number of cables I need to use.

That's probably a good idea.

[alsetalokin4017]

Also, check the value of your "1k" resistor. Never mind, I was probably reading it from the wrong end (110R 1% vs. 1k 1%) 

Just for fun I built up the circuit on a pad-per-hole board like yours, although I rarely use these for prototyping. Had to dig up my last genuine Radio Shack board!

I wanted to see if I could improve your routing, while still using the same basic component layout that you used.

[Forser]

Did a recheck today of the circuit, checked that pin 2 and 6 connected right, the center pin on pot and the cap and so all ground and such.

And now it works

From full blast to the lowest.

Thank you all for your help!

[Forser]

After a long time since last post, i am now doing the schematic in KiCad to make it into a PCB.

Difference is i want to add two 2-pin connectors for two led strips and i am not sure if the connections on the schematic is correct or not. Each length of strip will be the same length and amperage and power supply will handle the extra amount of amps.

Attached the schematic to this post.

[Zero999]

Use your NE555, no need for the CMOS version _in this application_. Use the schematic below. Note the Pin 3 and Pin 7 connections; Pin 3 is almost always used as the Output from a 555 astable circuit. Use almost any mosfet; I would prefer IRF3205 for this application. Substitute your LED strip plus any needed dropping resistor for the 12v bulb shown in the schematic. The mosfet will not heat up.

Tested and running on my breadboard right now. Provides a pulse width from under 2 percent Hi to about 99 percent Hi at around 215 -230 Hz. Pin 3 output shown at the extremes of the pot setting in the scopeshots below. I used a 50k pot on my breadboard but 10k works fine too but at a higher frequency.

Yes that works much better, than charging & discharging the capacitor, from the output.

You'll find it's better with a 50k pot. than with a 10k pot. A 100k pot would be better, with a smaller capacitor.