Comparator circuit thoughts....

[mackrackit]

Normally a PIC will only handle a maximum voltage of 5 volts. When you said

# Torch voltage is scaled from 120vdc to 1.2vdc.

I thought you were doing this with a voltage divider to bring the 120 down to 1.2, stating that the divider you planned on using was 100 to 1.
So...

I was thinking that being the expected voltages from the torch are 60 to 185 a voltage divider that would bring the 60 volts down to 1 volt for the low end and the 185 high end volts would then be ~3 volts. That is where the divide (scale) by 60 came in.

Many PICs have a 10 bit ADC. Even with a 10bit if you want you only have to use the first 8.
The difference is the resolution. There is also a voltage reference available when using the ADC. VDD can be used or some other voltage less than VDD. Some have a low end VREF but I figure on this one zero volts will be fine.

This all means that if an 8 bit (256 steps)ADC setup is used with VREF set for 4 volts, each step represents ~0.015 volts. In real life with the torch voltage being divided by 60, 0.015 would equal 0.94 volts. The torch voltage would have to change 0.94 volts before the PIC sees the change.

A 10 bit setup works the same but with 1024 steps. Now the PIC can see a 0.23 volt change on the torch.

This could be tightened up by lowering the VREF and/or scaling the 60 to maybe 0.5 volts, but I figure a little cushion would be good on both ends. I may have more than is needed...

Now using all of this. Say you want the torch voltage to be 120. With the 4 volt VREF and the voltage dividing, 4 volts at the ADC is equal to 240 at the torch, so we want to see 2 at the ADC to be good. The ADC will give a value of 512. If the ADC is + or - 512 the torch height will need adjusted.

Let me know how much I confused things

[kevlar129bp]

mackrackit,

No more confusion than before . Actually, the way you described the voltage scaling makes more sense to me now. As far as the torch voltage is concerned, maybe I added confusion from the beginning by not stating what would be a good baseline hysteresis to go with. After trying to get my head around the scaling, I started looking at the numbers more, and in doing that, I think a resolution of 1 volt change at the torch would be more than ok. That being said, that's where I figured using a torch value ranging from 0 - 256 Vdc...

8 bit ADC
4 Vdc Vref
Vdc per ADC count : 4 / 256 = .015625
Torch Vdc scale : 1:64
Torch Vdc per ADC count : .015625 * 64 = 1 Vdc

Ex: Torch Vdc = 115

ADC counts * Vdc per ADC count = ADC Vdc Now

115 * .015625 = 1.796875 Vdc

1.796875 * Torch Vdc scale = Torch Vdc

1.796875 * 64 = 115 Torch Vdc

So... for every 1 Vdc change at the torch, there will be a 1 count change at the ADC... correct?

If this is the case, coding for this arrangment should be way easier?
Am I making this easier or harder on myself? I think it's sinking in slowly but surely. Thanks for assisting me on my brain fryer!

Chris

[mackrackit]

I think you got it.

[kevlar129bp]

Thanks mackrackit,

I'll hack away at a bit of the code and see what I get... Will you be around for some assistance in that arena, when I get to that point?

Thank you again... TONS!

Chris

[mackrackit]

We are always here....

Start the testing with a POT to simulated the torch.

Which PIC are you going to use?

[kevlar129bp]

mackrackit,

In looking at my options, I think the 16F88 is going to do the trick. I'de like to plan ahead to do something like this on the front of the cnc pc. I dunno, let me know what you think... And again, thank you so much for your help. I'm working through the code as we speak

Chris

[mackrackit]

At this point I would say whatever you have laying around. The 88 is good for starters. Might run out of memory or pins later but might not.

But how are you writing code if you have not picked a PIC?