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Generic electronics question, check if a source is AC or DC


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#1 Stefan

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Posted 07 February 2012 - 03:37 PM

Hi :) As most know by now, I'm mostly a software guy. For electronics I need to ask the experts around here. Now I am working on something, and I have a power input. But I don't know the exact source, I just know, from two wires there's power on it. I won't tell anything about the project until it's completed, I think I gave all relevant details ;) Now I don't know if the power source is AC or DC, nor do I know how much volt it gives. I need to figure both things out by measuring. I'm a bit afraid if I put my multimeter in the wrong mode, it can get damaged. So is there a reliable way to find out what kind of power source it is? Following wires is no option, believe me. I wish it was that simple ;)
"Fact that I'm a moderator doesn't make me an expert in things." Stefan, the eternal newb!
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#2 Mario Vernari

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Posted 07 February 2012 - 04:03 PM

Not sure to understand: which one? - you have two wires, and you want to know (1) how many Volts, and (2) whether AC/DC? - you have two wires as supply, and you want to pull out "safely" the DC for some circuit? Cheers
Biggest fault of Netduino? It runs by electricity.

#3 Stefan

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Posted 07 February 2012 - 04:05 PM

- you have two wires, and you want to know (1) how many Volts, and (2) whether AC/DC?

I have an input signal over two wires. I want to convert it to a true or false. I wil probably end up adding a relay on the two wires. But I need to pick up the right relay.
"Fact that I'm a moderator doesn't make me an expert in things." Stefan, the eternal newb!
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#4 Mario Vernari

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Posted 07 February 2012 - 04:22 PM

You still don't give enough parameters.
Just (try) answer to these:
  • min and max (approx) voltage? (e.g. of sure above 1V and below 10V);
  • supposed current flowing/available from the wires? (e.g. coming from a TTL, or from a transformer, or else);
  • "AC" means "mains", that is 50 or 60Hz sine?
  • If not, what could be the supposed waveform? (e.g. pulses, random data, pwm, etc)
  • what would "mean" this signal? (e.g. an edge, a slow-changing source, etc)

I do *NOT* want to know your goal, but I ask these questions because a relay could be not possible, or not proper.
Also, this kind of problem could be common to other users, and these question should be answered.
Cheers
Biggest fault of Netduino? It runs by electricity.

#5 Stefan

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Posted 07 February 2012 - 04:29 PM

  • min and max (approx) voltage? (e.g. of sure above 1V and below 10V);
  • supposed current flowing/available from the wires? (e.g. coming from a TTL, or from a transformer, or else);
  • "AC" means "mains", that is 50 or 60Hz sine?
  • If not, what could be the supposed waveform? (e.g. pulses, random data, pwm, etc)
  • what would "mean" this signal? (e.g. an edge, a slow-changing source, etc)

1. I have no clue but it won't be more then 230V. My multimeter has Auto-Ranging though.
2. It's an ON/OFF signal. No PWM, or anything else.
3. If it's AC, it'll be 50Hz
4. Just ON/OFF.
5. Think in the line of: button pushed, take action!

I'm sorry for being cryptic. I love open source soft- and hardware. But I have reasons of keeping this a bit silent until/if it's completed ;)
"Fact that I'm a moderator doesn't make me an expert in things." Stefan, the eternal newb!
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs

#6 Mario Vernari

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Posted 07 February 2012 - 04:45 PM

No prob for secret-ness!

Anyway, you can use the multimeter without problems: it won't be damaged.
Test as it were AC first: AC has no polarity, thus if you read something "believable", the signal is actually AC.
If you read zero Volts, probably it's DC. In such a case, the polarity matters, but the multimeter simply put a minus when reversed.
Even if you try DC first, you'll read zero if the signal is AC. But, no damage.

Multimeter can't read AC other than the mains waveform. Also can't read TTL data (e.g. MOSI), or any "squared"/pwm signal. You may read a number on the display, but it's without meaning.

Be careful to avoid touching any of the wires with hands (or body in general).
Cheers
Biggest fault of Netduino? It runs by electricity.

#7 Stefan

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Posted 07 February 2012 - 06:02 PM

Thanks. I won't lick the wires, I promise :D Fabien also pointed out to me that my multimeter can measure frequency. So that's also a good one...
"Fact that I'm a moderator doesn't make me an expert in things." Stefan, the eternal newb!
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs




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