Bottlehead Forum

General Category => General Discussion => Topic started by: adeep42 on March 08, 2017, 04:04:08 AM

Title: Need some tech theory help
Post by: adeep42 on March 08, 2017, 04:04:08 AM

For another project I am working on, need to know what value resistor is required to bring 6 Volts dc down to 1.7Vdc-2.2Vdc.

Thanks in advance for help,


Alan

Title: Re: Need some tech theory help
Post by: Doc B. on March 08, 2017, 04:41:26 AM

You need to learn about Ohm's law to determine this. V=IR, so you have to know the current that will be going through the resistor to determine what the voltage drop will be across it.

Title: Re: Need some tech theory help
Post by: adeep42 on March 08, 2017, 05:32:10 AM

Thank you Doc. Found an Ohm's Law calculator on the web. Am I right that I need a 20 OHM resistor?

Thanks,

Alan

Title: Re: Need some tech theory help
Post by: Doc B. on March 08, 2017, 05:51:52 AM

You have not said what current (I) the circuit is drawing. You need that to use ohm's law. Not sure how you came up with 20 ohms for R.

Title: Re: Need some tech theory help
Post by: adeep42 on March 08, 2017, 06:26:30 AM

Found this calculator since.

http://www.ohmslawcalculator.com, then used their LED Resistor Calculator.  Source voltage is 6Vdc, Voltage drop is 2 (LED requires 1.7-2.2V), current through LED is .25 ma (20-30ma per Parts-Express).

Using this calculator, resistor should be 160 Ohms.

Your thoughts and experience much appreciated,

Alan

Title: Re: Need some tech theory help
Post by: Doc B. on March 08, 2017, 07:12:29 AM

Voltage drop across the LED is 2V, but the voltage drop across the resistor is what you are concerned with. That would be 6V_supply-2V_LED=4V_resistor. To solve this without an online calculator, V/I=R=4V/.025A=160ohms.

Title: Re: Need some tech theory help
Post by: adeep42 on March 08, 2017, 07:23:44 AM

Thank you Doc.

Alan