EMRFD Message Archive 6377
Message Date From Subject 6377 2011-06-08 18:36:30 zl3ab Introduction & EMFD Power Meter question Hi all
I have been licensed since 1990 and apart from one inactive period my main interest to date has been DXing and antenna experimentation. I have built the odd kit in the past but I came to realise that simply putting components into holes didn't really help me understand what was actually going on in the circuit. To try and remedy this I recently bought EMRFD to help me learn some basic fundermentals. I was then really pleased to find this group existed.
Following the sage advice in the book to start with something simple, I thought I would give the power meter in fig 1.26 a go. Since I would rather not blow up my first project, the (probably simple) question I have is what wattage should the voltmeter 20kOhm and 1.5kOhm resistors be? I understand why the 50 Ohm load resistors need to be rated for the power supplied and my assumption is that by dissipating the power through the load the rating of the voltmeter resistors is not critical. Is that right?
Thanks in advance
PS the other thing I have discovered since I don't have a junk box of any description, is that in trying to assemble the parts you need to build something is not that easy in this day and age (in New Zealand anyway). As well as getting items from overseas, I am quickly learning the art of part subsitution!
6379 2011-06-08 23:36:00 Kerry Re: Introduction & EMFD Power Meter question Welcome to the EMRFD mob!
Let's find-out the answer to your question.
At full-scale meter deflection there is 1mA passing through the meter; it follows that 1mA is passing through either the 1k5 resistor or the 20k resistor, depending on which input is being powered.
Ohms Law (E=IR) and the power equation (P=EI) can be combined to produce P=IsqrdR.
In this case I is 0.001A (1mA).
So, for the 1k5 case, P (watts)=0.001sqrd x 1500; for the 20k case, P=0.001sqrd x 20000. I will leave the solutions to you and your trusty calculator.
So the answer to "What power resistors?" is "Anything"; if you do the above maths you will discover that. In future you will just use 1/8 or 1/4 watt unless it's a Sunday night, Jaycar is closed, you want to try your project and all you have is a ten-watter. :)
Note that the above equations require the use of the fundamental units of amperes, volts, ohms and watts; scientific notation can help with the "millis, kilos" etc.
This kind of maths will soon become second-nature to you.
Yes; parts for the junkbox are often difficult to find in the Antipodes (and elsewhere); US experimenters have it good!
It takes time and persistence. Jaycar are pretty good for the lower-tech stuff; they have packs of resistors & capacitors which are a good foundation for a junque box.
For less-common parts, haunt ebay and any hamfests you can get to.
6380 2011-06-09 12:46:22 Mark Sullivan Re: Introduction & EMFD Power Meter question Hi Kerry
Thanks for the reply and advice. I did the maths and yes that makes sense.
Funnily enough I was in Jaycar today and yes as you say they have fairly
reasonable range. Actually they have done pretty well to be open at all
since the area they are in got hit hard by the Christchurch Earthquake.