Digital pot, voltage regulator or can software do the trick?

[Christopher4187]

What size capacitor did you try with your PWM and what frequency did you use?

I was able to go as low as 150uF. As far as the frequency, not really sure, I just varied the duty cycle between 230 and 255. I used DT's interrupt routine as a poor mans HPWM.

Your .58V is so incredibly close to the voltage drop across a standard diode. Perhaps you can experiment with the current through it to get .58V. Use it like a zener diode.

Good idea, didn't even think about that. I'll try that along with the solution below.

I think I may have found a workable solution. It looks like I was just using resistor values too high so I tried the 23ohm and 3ohm combination mentioned in my other post and it works perfectly. The only issue, which I think can be solved quite easily, is that the voltage divider draws about 185mA. Power consumption isn't really a problem because it's not being run from a battery but who wants to waste power?

This may be the bigger issue, I was going to "simply" use one output pin from an 18F4550 to turn the voltage divider on and off but that won't work as I have it now. Obviously disconnecting either the positive or negative leg of the voltage divider will throw off the thermistor value significantly. I was thinking about connecting one end of the voltage divider to one PIC pin and the other side to another PIC pin. I would make the PIC pins inputs when I don't need to force a certain temperature and then making one pin high and the other low when I needed it to work. Do you guys think this will work?

[HenrikOlsson]

I was thinking about connecting one end of the voltage divider to one PIC pin and the other side to another PIC pin. I would make the PIC pins inputs when I don't need to force a certain temperature and then making one pin high and the other low when I needed it to work. Do you guys think this will work?

Not if the voltage divider draws 185mA... Normally the PIC I/O's can source and sink 25mA.

Seems like you're getting there some way or another but if you need a regulator you could take a look at the SC4215, it'll go down to 0.5V and is available in SO8.

[Charlie]

I've read the thread several times, and I'm not quite sure what you are really trying to do.
Are you trying to measure temperature with a thermistor?
Do you need 0.58V for some reason, and if so what is that reason (this defines source impedance, etc.)
Are you trying to detect when something reaches a certain temperature, then take some action?
Depending on your answer, there are simple, and in some cases 0 extra component, solutions...

[Christopher4187]

The .58VDC represents an optimal temperature and I want to keep the thermistor voltage at that level while the 18F4550 continually checks the analog voltage once every 5 seconds. If the voltage is between .58 and 2VDC, the 18F4550 should force the voltage level (that is the issue I'm describing in this thread) to .58VDC. Below .58VDC and above 2VDC the thermistor value will not be forced. What did you have in mind?

I've gotten two methods to work, for the most part, but there are a couple of issues. The one I like the best is using a voltage divider from the PIC. I use two outputs and those pins are connected to the 18F4550 pins. It works really well and the voltage divider I'm using only consumes 10mA. Here's the problem and hopefully someone can point me in the right direction. When I put the pins as inputs, there must be a low impedance connection within the PIC because it pulls the voltage to ground. The pins I'm using are PORTE.1 and PORTE.2. I've tried PORTB.6 and PORTB.7. Can I configure something with the pins where they are truly in a high impedance state?

I've also used a zener diode (1N5352B) but only because it's all I had laying around. It worked better than I thought but it doesn't pull the voltage low enough. I'm going to look around, perhaps even Radio Shack, to buy some different types and play with them. I don't understand how to use a zener diode correctly in this application but I suppose it's not meant for it anyway. I will try the normal diode experiment when I get my hands on some. I know I have about 30 of them somewhere.....

[Christopher4187]

Ok, I have a workable solution. I still had a ground connection on the voltage divider, which pulled it to ground. So it looks like 2 pins and 2 resistors is all that is needed for this application. If anyone has some more ideas, I'd like to hear them.