Glad you are making progress.
Quote:
Originally Posted by Chris K
…and maybe when u get time Tom, for my education, can you explain how you knew the short was after the CO resistor when it got screaming hot? Some things about DC voltage I just don’t get.
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The short answer is doing a fair bit of ohms law and voltage divider math and seeing the principles in action under similar circumstances.
The short answer is that every resistor has a power rating (watts = amps X volts), and will smoke if that power rating is exceeded. The short will increase the load the resistor is feeding and thus the power it has to feed it's load. There are a few ways of looking at it mathematically...The resistor and the things loading it (the loads can be modeled by a resistance who's value can be obtained by ohms law and by measuring the load current and voltage drop) are basically a voltage divider. If you short the load resistance dead the voltage across the candohm resistor increases (as does current thanks to ohms law) so power through the resistor will increase even faster.
Another way to look at it mathematically if the short isn't a dead short, is to say that the load resistance has dropped which in turn lowers the total resistance of the divider...If B+ hasn't significantly changed then ohms law arranged amps= volts/ohms indicates that current will increase which will increase the power through the candohm.
Kirchoff's current law and voltage law are also helpful in understanding voltage dividers and other series parallel resistor networks.
If you want a fun/useful exercise in designing voltage dividers and have some tube battery radios designing LM317/LM337 voltage regulator power supplies for unregulated rectified AC is a interesting way to practice.
I remember some of my college EE labs where some transistor or voltage divider circuit was to be tested and a LOT of my peers not bothering to check power requirements in the math and smoking half watt resistors. In some cases you could adjust the math to get what you need within available wattage, in other cases the professor made the error and to do the assigned problem you had to know that the parts desk had some wire wound power resistors and be first to ask for them, or build your circuit with several series/parallel 1/4W resistors to make a single 1-5W part...often if you needed that many resistors you'd have to justify it to the parts room troll, or build the circuit with undersized resistors and collect the data before it burned open...Good times!