Is there a way to digitally set resistance? To control curr

[leesing2k3]

I am wondering if there is a chip that can control resistance. Like i can digitally control the chip to set resistance so that I can generate a constant current with any value that i want.

By the way I actually starts doing electronics from learning PIC. The only electronics forum I am familiar with is ccs forum. Is there any more nice forum that I can go to and learn some new thing?

Thank you in advance. Thank you very much.

[PCM programmer]

Digital Potentiometers:
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=11026&mid=11&lang=en&pageId=79

Price and availability:
Go to here: http://www.digikey.com/
Search for: Digital Potentiometers

CCS drivers:
c:\program files\picc\drivers\mcp41010.c

[newguy]

If you're familiar with Circuit Cellar magazine, http://www.circuitcellar.com/, their site hosts a forum as well: http://bbs.circuitcellar.com/phpBB2/

You can get quite a bit of electronics advice there.

[Ttelmah]

Also, you can control current using a voltage. Get the Siemens application note 54, which shows how to make a votage controlled current loop. The circuit given is 'aimed' at controlling a 4-20mA loop, but with modifications can control almost anything. You can feed this type of circuit, from a PWM output, with an integrating capacitor, or from a proper D-A converter, and (depending on how accurate the voltage rail feeding the PIC/D-A is), give something over 9bits of accuracy. Generally digital potentiometers commonly only come in 8bit versions (there are a couple of higher accuracy units, but these are more expensive than DAC's).

Best Wishes

[nurquhar]

You should check out the MAX6966 or MAX6967 chip. I am using these to drive LED's with a constant current. ie no bias resistor required. Each chip has 10 outputs, each individualy programmable with a current of between 2.5 and 20mA. You can also use the PWM feature of each output to give further control. If you need more current you can common up more outputs so 2 will give a maximum of 40mA, 5 will give 100mA, etc. You could even add a drive transistor to the output to multiply the current capabillity.

Have a look at the datasheet
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4539