



Ohm's Law Answers Your Questions by Mark Persons 
Radio
World Article January 16, 2019 
Understanding these concepts will help you solve more than theoretical problems 

Fig. 1 is a list of simple formulas for using Ohm’s Law. Nothing complicated, just good answers to your questions. You don’t need to be a mathematician to run the calculations. The calculator on your Smartphone will handle this easily. P is for power in watts, I is current in amperes, R is resistance in ohms and E is voltage in volts. Solve for any of those knowing two of the other parameters.
OHM’S LAW ON
CURRENT 
Fig. 2: Bleeder resistors in a Gates BC1G transmitter 
OHM'S LAW ON
WATTS The bleeder in a Gates BC1G transmitter is R41, a 100,000 ohm/100 watt wirewound resistor. You see one handheld on the left side of the photo in Fig. 2. Ohm’s Law tells us that 2600 volts across the resistor squared (times itself) then divided by 100,000 ohms resistance equals 67.6 watts of power dissipation required on a continuous basis on a 100 watt resistor. You would think that the 32.4percent safety margin would be enough. This resistor typically failed after 10 years of use. The answer is in the ventilation the resistor gets for cooling. The 67.6 watts in heat has to go somewhere. This transmitter model has some, but not a lot, of air flow on the bottom where the resistor is located. My answer was to replace the 100 watt resistor with a resistor rated at 225 watts, as seen in the center of the photo. It gave more surface area so it ran cooler, thus longer. A 100 watt resistor is $15.14 vs $18.64 for a 225 watt unit. It is only a $3.50 difference for a huge increase in reliability and safety. The screw that holds it in place will need to be longer if you do this modification. No big deal. Yes, there is a meter multiplier resistor string next to the resistor and highvoltage capacitor. It samples the high voltage for the PA voltmeter. Dirt has accumulated on the highvoltage end of the string. It is high voltage that attracts dirt, requiring frequent cleaning to maintain transmitter reliability. It’s maintenance. The RF dummy load in this transmitter has six 312 ohm/200 watt noninductive resistors. The transmitter sees the 52 ohms because the resistors are in parallel. Simple math, 312 ohms divided by 6 resistors = 52 ohms. Yes, 52 ohms, 51.5 ohms, 70 ohms and other impedances were common in the past before solidstate transmitters more or less forced the standard to be 50 ohms. Tubebased transmitters will tune into almost any load while solidstate transmitters are designed to perform into 50 ohm loads….and don’t give me no VSWR!
OHM’S LAW ON
VOLTAGE
OHM'S LAW ON
POWER
HALF POWER? Think Ohm’s Law when you are on the job. It answers your questions and makes perfect sense. Mark Persons, ham W0MH, is an SBE Certified Professional Broadcast Engineer; he was named the Robert W. Flanders SBE Engineer of the Year for 2018. Mark is now retired after more than 40 years in business. His website is www.mwpersons.com. 
This is a reprint of the article which appeared in the online
version of Radio World at:
https://www.radioworld.com/columnsandviews/ohmslawanswersyourquestions 
Email: January 24, 2019: Great piece on Ohm’s Law in Radio World. Thanks for taking the time, and thank you for sharing. 73s Dolf Santorine, ham AD0LF Wheeling, West Virginia 
Questions? Email Mark Persons: teki@mwpersons.com 
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