in need of some help, basic questions - Page 1
Forum section: electronics / beginners

author: danz409
author_rank: Contributor
timestamp: January 03, 2012, 07:05:17 am
content: 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 inturns 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?

author: danz409
author_rank: Contributor
timestamp: January 03, 2012, 07:30:09 am
content: 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

author: IanB
author_rank: Super Contributor
timestamp: January 03, 2012, 07:32:46 am
content: Quote from: danz409 on January 03, 2012, 07:05:17 amAnalog 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.

author: danz409
author_rank: Contributor
timestamp: January 03, 2012, 07:59:05 am
content: Quote from: IanB on January 03, 2012, 07:32:46 amyea 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?

author: IanB
author_rank: Super Contributor
timestamp: January 03, 2012, 08:18:40 am
content: 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).

author: danz409
author_rank: Contributor
timestamp: January 03, 2012, 08:23:41 am
content: i'm still a little lost. i understand the part where you can't put all the amps through it. that would be over kill. but how i go about doing this is still confusing.

author: IanB
author_rank: Super Contributor
timestamp: January 03, 2012, 08:25:50 am
content: Do some research using the terms current meters, ammeters, shunt resistors and/or current shunts.

author: ipman
author_rank: Regular Contributor
timestamp: January 03, 2012, 09:07:23 am
content: Measuring current requires a shunt, a calibrated piece of wire which has a specific resistance. The instrument reads the voltage drop across this piece of wire.Shunt should provide the instrument the voltage differential needed to go to full scale ideally, but not above.

author: danz409
author_rank: Contributor
timestamp: January 05, 2012, 03:17:09 am
content: so i just bought a power supply. yea. building one from scratch is cool and all. but i wouldn't mind haveing something with safty into consideration. somethingthat has a few more options like1. Varable voltage without the use of resistors and all that jaz2. current limiting. prevent from blowing up expensive parts3. solid state. more robust reliable power source.however i'm not exactly good for cash and was able to get the absolute cheapest.and i can only find 2 video reviews by Dave on power supply.so i bought one for $51 that's included shipping from china..15v @ 2 amps. chances are those specs aren't accurate but i'm expecting that.chances are its a bad one. but its all i can affordsame goes for the DMM i ordered. its only a $17 deal but if all goes well ill be upgrading to a extech EX330 soonwas this a good move. or is just sticking with a ghetto computer power supply setup better?

author: ipman
author_rank: Regular Contributor
timestamp: January 05, 2012, 06:48:00 am
content: There is no right answer until you take apart the new power supply.The biggest concern is that an actual power supply with several tens of amps will not forgive any of your mistakes and will turn allmost all tiny electronics parts into fuses. Also, there are some complications with ground loops.Still, that new PS can be worse. Rating and certifications are not in vain, but chinese one are not the same with european and american ones.

author: danz409
author_rank: Contributor
timestamp: January 06, 2012, 10:08:59 am
content: so i went to go check the status of that order. turns out according to there web site. i have a "Recent purchases" of 0 items. and i never received a confirmation email that most places send to tell you order numbers info tracking and all that. (been 5 business days). so i sent a friendly reminder that i haven't seen any information on my order. even a tracking will do fine. and i got a reply from them... well... from pay-pal saying i was refunded the amount O.ohmm...any suggestions?

author: ipman
author_rank: Regular Contributor
timestamp: January 06, 2012, 11:30:15 am
content: Be happy that you received a refund and use another store!Try to find one that has good references from other users.

author: danz409
author_rank: Contributor
timestamp: January 08, 2012, 01:35:23 am
content: ok. since i don't have a power supply on the way. i should get working on home-made-ish onedid some work and its looking nice!who would of thought there was aluminum under that white gungehere is the allocation of all the features i plan to put in it.and a before and after of the insides.. (still work in progress)Before:After: