Electronics > Beginners

in need of some help, basic questions

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danz409:
OK. hopefully ill get some help here. iv tried a few other forums with no luck. i was given a old as dirt. Hickok PS-505 from my uncle. he was using it for a -15v power supply for his sound mixer. went to go use it for  the first time and it worked for all of 15 min. before i started hearing a * sizzling* sound and smelling a foul smell. than it started crackling. at this point i quickly unplugged it and wandered why the fuses didn't kick in

turns out the transformers insulation layer had turned into goo over the years and shorted out. and the paper caps wasn't doing much better... needless to say it has seen the end of its operational years....


until i decided to put a computer power supply in it! (i'm cheap. can't afford a real bench top power supply)

and i wanted to utilize some of the features thats on it. like the analog volt meter and amp meter. i was playing around with a normal 12v wall wart to see how well it works. to my surprise it SLAMS the needle all the way to the 40/400 10/100 mark bending it. this  isn't doing what its suppose to... is it?

i'm not too keen on how analog meters work. is there some sort of stepping down you do with the voltage for them? is there some sort of "decoder" board that goes with them?

danz409:
turns out there was more resistance needed. there was a range selector knob that had a heap of em lobbed on it.
i kinda went nuts un-wireing everything and didn't see what was going where.

now to figure out the amp meter

IanB:

--- Quote from: danz409 on January 03, 2012, 07:05:17 am ---and i wanted to utilize some of the features thats on it. like the analog volt meter and amp meter. i was playing around with a normal 12v wall wart to see how well it works. to my surprise it SLAMS the needle all the way to the 40/400 10/100 mark bending it. this  isn't doing what its suppose to... is it?
--- End quote ---

Analog panel meters measure current, usually a very small current like 1 mA or so. So if you connect a 12 V supply to the poor meter you will break it.

To make the meter into a volt meter a suitable resistor is put in series with it. For example if you wanted to measure 12 V you would need a resistance of 12 000 ohms to pass a current of 1 mA through the meter. You subtract the resistance of the meter from 12 000 and whatever remains is the value of your series resistor. This resistor might sometimes be inside the case of the panel meter, but just as likely it is external to it.

danz409:

--- Quote from: IanB on January 03, 2012, 07:32:46 am ---
--- Quote from: danz409 on January 03, 2012, 07:05:17 am ---and i wanted to utilize some of the features thats on it. like the analog volt meter and amp meter. i was playing around with a normal 12v wall wart to see how well it works. to my surprise it SLAMS the needle all the way to the 40/400 10/100 mark bending it. this  isn't doing what its suppose to... is it?
--- End quote ---

Analog panel meters measure current, usually a very small current like 1 mA or so. So if you connect a 12 V supply to the poor meter you will break it.

To make the meter into a volt meter a suitable resistor is put in series with it. For example if you wanted to measure 12 V you would need a resistance of 12 000 ohms to pass a current of 1 mA through the meter. You subtract the resistance of the meter from 12 000 and whatever remains is the value of your series resistor. This resistor might sometimes be inside the case of the panel meter, but just as likely it is external to it.

--- End quote ---

yea i figured that out. the resistors was on the selector knob for selecting a range. and it works wonderfully. however im working on the amp side now. so i need to lower the amp sorce withen the meters range. how do i go about that, same thing? just throw resistors on it?

the original design is meant to only use 1amp max. i was going to put a fuse on my PSU for more. i could even go all the way to 10 amps easy.

so just put a 10k ohm resistor on it?

IanB:
To measure large currents you have to bypass (shunt) the excess current around the meter. For instance if the meter requires 1 mA full scale and you want to measure up to 10 A, then you need to bypass 9 999 mA around the meter so that only 1 mA goes through the meter. If the meter has a resistance of 500 ohms (for example), then you would need a shunt resistance of 0.05 ohms (since 0.05/500 = 1/9999 near enough).

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