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Topic: PWM Controller (Read 2154 times) previous topic - next topic

elemakil

Hello,

I'm building a LED workspace light spotlight which consists of ~60 LEDs which are arranged in parallel Groups of three serial LEDs (and a resistor per group of course). I'm using a supply voltage of 12V and those LEDs will draw a current of approximately 500mA.


I'd like to be able to dim the LEDs using a PWM controller, but I don't want to use a microcontroller (i.e. Arduino) but a dedicated PWM IC instead. I've been googling for some time now and so far I have found the TL494 IC which provides about 400mA max but it comes in a DIP-16 package and has a lot of options which I do not need, so it's a little oversized.

Essentially I'm looking for a PWM IC (DIP-8 preferred) with an adjustable duty cycle, which can be supplied by and can supply (as U_OUT) 12V. Does anyone know of an IC capable of this (and maybe has a schematic for wiring it)?

CrossRoads

How about just a 555 timer set up as astable oscillator, driving an transistor to turn the 500mA on/off?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

elemakil

I found a schematic of that too, but I'd like to use as few parts as possible and therefore thought about an IC designed for only that purpose. I guess if there's no other solution I'll go for the 555, but still, maybe someone knows an IC that gets it done in one chip.

salsaman

I found this circuit documented all over the place, very nice general purpose PWM dimmer, simple and inexpensive.  I'm using one for an LED light strip, works like a charm.



Is 8 parts (excluding connectors) few enough?
1 555 IC
2 capacitors: .1uF and 1uF
2 1N4148 diodes (could use others)
1 100K potentiometer
1 resistor (4.7K for no good reason)
1 MOSFET (I used an STP16NF06, overkill)

This page is pretty sweet if you're curious about 555's:
http://www.kpsec.freeuk.com/555timer.htm#astable


My Arduino blog: http://jmsarduino.blogspot.com
Comprehensive (?) Arduino-compatible board list: http://tinyurl.com/allarduinos

CrossRoads

Another option is to use chip like ATTiny25V/45V/85V, a potentiomter, and a transistor.

Use ADC to read voltage from the pot, use PWM to control the resistor.
Internal oscillator, just need 100nF cap on Vcc.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

elemakil

First of all thanks for your responses!

@salsaman: What is the input header in the schematic used for?

@CrossRoads: Can I program the ATTiny using an ICSP-Header and the Arduino IDE?

I guess I'll be ordering both of them and fiddle a little to see which one works better and fits into the enclosure.

CrossRoads

#6
Nov 11, 2011, 05:44 pm Last Edit: Nov 11, 2011, 05:51 pm by CrossRoads Reason: 1
I believe it is possible to program the ATTiny using an ICSP-Header and the Arduino IDE, with some additions to boards.txt so the ATTiny shows up as a part.
Probably have to download some cores from avr-developers.com, or you can search for Teensyduino & see how that is loaded, I think that uses ATTiny also.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

salsaman

The input and output on the 555 board are for 12V input power and output PWMed 12V power.  I picked screw terminals for flexibility.

The ATtiny solution is cool, I new to start using those for small projects!  Note that you'll need to get it 2.7-5.5V from your 12V source, so an LDO regulator and a cap or two in addition.
My Arduino blog: http://jmsarduino.blogspot.com
Comprehensive (?) Arduino-compatible board list: http://tinyurl.com/allarduinos

elemakil

So the input header is essentially the VCC, correct?

I guess for thee ATTiny solution I need a MOSFET as well, I doubt that the ATTiny can source 500mA.

I'll be ordering both parts and look into it, maybe borrow an oscilloscope from the university where I study to check the PWM capabilities of the 555-circuit. But while digging some more I found the UC 3842 IC (datasheets http://www.datasheetcatalog.net/de/datasheets_pdf/U/C/3/8/UC3842.shtml ). I don't know whether it can be used to PWM some LEDs because there is no similar example in the datasheet but it's description says it's a "Current Mode PWM Controller" so I guess this could be used for PWMing the LEDs as well, right?

CrossRoads

Depending on who you get it from, the UC3842 will have a fixed switching frequency of 50 KHz, 400KHz, or 500 KHz.
Your current drive transistor will need to match up with that.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

salsaman

Quote
So the input header is essentially the VCC, correct?


Um... both headers are polarized, + and - leads on both.  Check out the schematic ;)

Really, the 555 is just the timer outputting a PWM signal, with the MOSFET doing the power switching.  The same is the case with the UTtiny approach-- it can output a PWM signal and you can use that however you like.  I use my 555 board with an LED strip that draws ~400mA at 12V, but it could handle much more than that.
My Arduino blog: http://jmsarduino.blogspot.com
Comprehensive (?) Arduino-compatible board list: http://tinyurl.com/allarduinos

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