8ch PWM

[Markdem]

Hi Everyone,

Quick question. I need to control 8 0-10v dimming ballasts. Is there a free running PWM driver out there with 8 individual outputs, or does any PIC have 8 individual channels? I know the dsPIC have 8 outputs, but you can't have (as far as I can tell) have 8 different duty settings.

OR do you think it is a better idea to do this with a digital pot?

Thanks

Mark

[Ttelmah]

NXP, do an I2C controlled PWM chip with I think 16*12bit channels. PCA9685. It is designed as an LED controller, but could be used fairly easily for other applications. Texas do a similar chip as well (TLC5940), but this has current sink outputs, which might be harder to interface.

Might well be simpler just to use a couple of PIC's. The PIC18F2331 for example, has the four channel power control PWM module, offering up to 14bit resolution. Two of these as I2C slaves, and "Robert's your parent's brother".

Best Wishes

[Markdem]

EXCELLENT! That is exactly what I need. It even can even have pins configured as ON\OFF only. Perfect.

Now just to write the driver.

Thanks

Mark

[temtronic]

Depending on the speed you require (and other features..)

You could do it with just one PIC and create a software 8 channel PWM.

I've run 16F84s this way for(sigh) decades.....

[Markdem]

I think I have lost my mind here (and 2 PCA chips).

I have written the driver (took me 10 min, which I was quite happy about) and it works like a charm.

However -- I would like to use it with a PNP transistor as I need to push 0-10v thought it. I have made it work with a NPN transistor, but can't work out how to connect a PNP. The PCA has been configured in open drain mode, so I would of thought I can just connect the base leg of the PNP to the PWM pin, and bob's your mum, 0-10v output. Each time I do it, all I end up doing is blowing the PWM driver (Lucky the chip has 8 of them).
If I put the resistor in the base leg, the transistor always stays on, and I get 10v out of it.

I am no superstar when it comes to transistors, but have tried for the last few days to get this going and cant seem too work out what I am doing wrong. Has anyone got a circuit for a PNP PWM driver, or any hints on what I am doing wrong?

Thanks

[temtronic]

One (easy) way is to have the NPN drive the PNP. I've done that for almost 30 years using MPSA92/93 transistors to control +60 volt logic lines. Cost is maybe 5 cents...

Another is to use an optocoupler(4N26 ?).This one also allows you to isolate the power supplies,if necessary.

I've also used opamps (in diff mode) to 'level shift / polarity control' in telecomm circuits...

so grab a breadboard and see which works best for you !
(wished I'd invented those things.......)

[Markdem]

thanks Temtronic,

Yep, I also thought of NPN-PNP back to back. This will, and has in this case, worked. I was just wondering if there is a cleaner way of doing it. The datasheet says you can use a PNP, I just don't know how.

Sounds like I need too blow some more PCA up

Thanks

[freesat]

Hi, do a PWM with software using an interrupt, its easy and u dont need buy more chips, works great with me on a 8 pwm channels on 16F628 runnibg at 4Mhz.

im tested it only with DC PWM for Leds and Motors, at AC PWM im working, but is dificult without have a scope.

see an example.

[Markdem]

Hi Freesat,

Thanks for that, but I can't do software. The pic is quite busy as it is.

I don't mind using the extra hardware, and it is working fine. I just want to connect a PNP transistor to the PCA.

I have now also tried PNP-NPN Sziklai pair, but can't seem to get 10v out. It only gets to about 4.8 then stops.
I think I might just use a voltage doubler.

Thanks

Mark

[RF_Developer]

[Markdem]

Thanks for that RF. That is what I think was going one too. Still a little confused as to why the datasheet is saying that I can use a PNP.

No matter, I have gone down the voltage doubler path. Works well and smooths out the PWM perfectly too.

Thanks for all the help.

Mark

[asmboy]

If you mean the 0-10v DC dimming controls so long as you dont need to update each one every FEW milliseconds -
and it is a CLEAN DC value for each control ( my but thats an OLD dimmer system ;-)))

As i think you mean
then, i did this YEARS ago as an early PIC project - first on a 16f877
and OP AMPS are your friend too you know ;-))

This can actually be done with a SINGLE PIC PWM channel - and a second order, fast settling SK LP filter - one or more DG406's and and a bunch of
of +12v/-5V fet input OP amps - configured as S&H buffers ( the DG406's charge - discharge 2200 PF metal film caps and you simply step thru your
dimmer values while addressing the right mux channel using an ancient NE5532 - one half as the PWM SK filter to dc circuit and the other half to charge the 2200pf caps through the MUXES as the capacitor charge control amp.

With 2200 PF GOOD Metal film caps and TL074 quads - i could go for several MINUTES without having any noticeable droop in the stored dimmer values. The pic program would update ALL dimmers when ANY was changed and also update every appx 60 secs using an 8 mhz pic clock. This also takes VERY little pic overhead to do to.

BTW: the time per channel for charge to 'take' was allocated very nicely at just 16 msec each SETUP ( + 10 ms filter settling)

Who says the answer has to be all digital ??

[aaronik19]

Dear asmboy,

I just read all the datasheet of the multiplexer you used and seems that the MUX you used does not hold the value of the dimmer. What I can't understand in your design, where you put the 2200uf capacitor? What is the voltage used? You need to wipe the values of all the channels everytime to keep the dimming settings?

Thanks

[asmboy]