I should say, the current is the third root. From: Chuck McCown Sent: Thursday, March 09, 2017 8:46 PM To: af@afmug.com Subject: Re: [AFMUG] Ohms law
The solution is the third root of: 0=ax^3 + bx^2 + cx + d Where: Where A = Resistance B=0 C = - Voltage D = Power This is a third order polynomial or cubic function. It produces complex roots in areas where there is no solution. Like my example if you go over 12 watts on the load the equation goes complex. Like I said, it looks simple ... looks simple. Once I realized it was not linear it was much easier to arrive at the solution. Initially I thought quadratic but when I could not get rid of the current cubed term then the light bulb went on. https://en.wikipedia.org/wiki/Cubic_function From: Chuck McCown Sent: Thursday, March 09, 2017 8:35 PM To: af@afmug.com Subject: Re: [AFMUG] Ohms law Constants are the resistor, the 48 volt power supply, and the power consumed by the load. Yes, the load resistance changes. Many devices are like this. Canopy SMs are like this. 48^2/6 only works if there is zero voltage drop across the loop (100 ohm resistor). It is 48-(I^2R)/6 From: David Milholen Sent: Thursday, March 09, 2017 8:20 PM To: af@afmug.com Subject: Re: [AFMUG] Ohms law Ah.. Now I get it.. So, the constants are the resistor and the Power used. source E^2/P=R Load 48^2/6=384 Ohms So, if the load power remains the same throughout any source voltage changes then the Load resistance will change. Bill, is on the same rail I am there has to be a dynamic whether it be loss in the loop or loss in the load. In dc I would believe that the load resistance has to change as copper wire may change in extreme conditions like lots of heat or cold. On 3/9/2017 8:34 PM, Chuck McCown wrote: The voltage of the power supply is constant.� 48 volts. The resistor is a fixed and constant 100 ohms. The load is a fixed and constant 6 watts load. � But the current of the load will change depending on the voltage applied to the load. And any change in current causes a change of voltage applied to the load due to a change of voltage dropped across the resistor.� � The VDSL2 ethernet range extender uses 6 watts.� It can be powered from 10 to 48 volts.� � This is a real world application, not just an academic exercise to tease everyone.� I really thought I would have the solution knocked out in 3 minutes.� Took about an hour. � From: David Milholen Sent: Thursday, March 09, 2017 7:29 PM To: af@afmug.com Subject: Re: [AFMUG] Ohms law � The idea would be to remove the power source entirely and find the thevenin resistance. So is the resistance changing or is the voltage? � On 3/9/2017 7:29 PM, Chuck McCown wrote: But what is the Thevenin equivalent of a constant POWER component? � Read my reply to Bill, there is feedback.� The voltage drop of the resistor changes the current drawn by the load which changes the voltage drop of the resistor and so on.� � From: David Milholen Sent: Thursday, March 09, 2017 6:24 PM To: af@afmug.com Subject: Re: [AFMUG] Ohms law � This is straight out of my Dc circuit analysis book from college. https://www.allaboutcircuits.com/textbook/direct-current/chpt-10/thevenins-theorem/ I kinda thought my voltage was a bit off but the 100ohm is not a fixed value. I just used Watts/volts to get my current. W/V=I I then used the current to get the voltage drop across the loop I*R=V drop Its been a while for the thevenins therom but if i do a little study I think I could get it. � On 3/9/2017 5:28 PM, Chuck McCown wrote: But what is the formula? � From: Dave Sent: Thursday, March 09, 2017 4:20 PM To: af@afmug.com Subject: Re: [AFMUG] Ohms law � Current =.125A at load Voltage=35.5 at load If my current is correct then I should be on point. Otherwise I would use Thevenins Therom to get closer. On 03/09/2017 05:08 PM, Chuck McCown wrote: The questions are: What is the current and voltage on the load.� � From: Chuck McCown Sent: Thursday, March 09, 2017 4:05 PM To: af@afmug.com Subject: [AFMUG] Ohms law � Had a fun afternoon.� � Solve this.... and give the general formula... � 48 volt power supply 100 ohm wire resistance to the load. 6 watt load. � Took me some time.� Not trivial.� -- -- -- --
