EMRFD Message Archive 8346

Message	Date	From	Subject
8346	2013-03-16 16:33:44	Jerry Haigwood	W7ZOI/W7PUA Wattmeter current
Hi All, As you know I am building my version of the W7ZOI/W7PUA wattmeter. I want to add an LED so I can see that it on since I have a bad habit of not noticing I left something and end up running the battery down. Does anyone know how much current the meter draws from the 9VDC battery? I am thinking I might put an LED in series with the battery if the current is something like 10mA or so. Using this method, and a low overhead voltage regulator, I might still get some decent battery life. Of course, I may not end up with enough voltage to feed the LM358. Has anyone else added a LED or some other indicator? Jerry W5JH "building something without experimenting is just solder practice" [Non-text portions of this message have been removed]
8348	2013-03-16 17:09:06	Kerry	Re: W7ZOI/W7PUA Wattmeter current
If it's the common LCD meter it will draw about 1mA. Most LEDs will give some light at 1mA; a high-brightness one will probably produce enough light to serve as an indicator. Series connection is not usual; you will have to contend with a voltage drop of about 2 volts (+/- depending on colour) across the LED. Kerry VK2TIL.
8349	2013-03-16 17:50:23	Jerry Haigwood	Re: W7ZOI/W7PUA Wattmeter current
Kerry, It is the original meter from June 2001 QST . It doesn't have any LCD meter. It has the 78L05 voltage regulator, an AD8307, a LM358 dual op amp, and a 0-1mA analog meter. As far as LED color. I will go through my junk box to find the lowest LED voltage drop. The extra voltage drop could partially be made up with a low overhead voltage regulator instead of the 78L05.
8350	2013-03-16 18:10:24	Kerry	Re: W7ZOI/W7PUA Wattmeter current
I haven't built the W7ZOI/W7PUA one; mine is based on Bob Kopski's design. In your case the AD8307 draws about 10mA; the LM358 1 - 2mA. I assume the LED will be in series with the LM324 supply. I've never seen it done that way but I can't see why it won't work; let us know how it goes. I used six AA cells in a holder for my build; they seem to last forever. Kerry VK2TIL.
8351	2013-03-16 18:25:04	Jerry Haigwood	Re: W7ZOI/W7PUA Wattmeter current
Hi Kerry, I think if I put the LED before the voltage regulator and change the regulator to a low dropout type, it will work. I think I will leave the LM358 at the 9V. Jerry W5JH "building something without experimenting is just solder practice"
8352	2013-03-16 19:39:39	Kerry	Re: W7ZOI/W7PUA Wattmeter current
Sounds OK; you will have perhaps 15-20mA through the LED. It's a novel idea; you may be onto something new!
8354	2013-03-16 21:20:19	Jerry Haigwood	Re: W7ZOI/W7PUA Wattmeter current
Kerry, It is not a new idea. It has been done many times and I am not that smart. ;-) Jerry W5JH "building something without experimenting is just solder practice"