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Old 12-31-2010, 04:29 PM #1
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Question Basic electronics question regarding DC power supplies/output voltage

I'm in the process of building my first laser and I'm trying to learn a little about electronics as I go...

I have a bunch of DC adapters from defunct electronics that I no longer use and I'm trying to determine if any of them would be a good power source for a rkcstr driving a 6x BluRay or 20x DVD burner diode (bought from Modwerx after killing the PHR I was originally using). I was messing around with my DMM, testing the various adapters and I noticed that every adapter I tested was putting out more voltage than it was rated for, sometimes by a lot.

For instance:

Rated 7.5v 1A outputs ~11v
Voltage selectable 1.5v - 12v 300mA outputs >15v on ANY voltage setting
Rated 9v 210mA outputs >10v

My thought was that if the 7.5v adapter was putting out 11v that it would work fine for driving a diode, since the rkcstr can handle up to 12v of input? But why is it rated as 7.5v when it puts out ~11v? And also, why the heck does the voltage selectable adapter ALWAYS put out over 15v no matter what the voltage is set at? This makes no sense to me, what am I missing here?

I've been reading up on basic electronics on this site: basic electronics and am learning how to calculate voltage, current etc. using Ohm's Law but I haven't yet had that "AHA" moment where it all makes sense.


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Old 12-31-2010, 04:42 PM #2
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Default Re: Basic electronics question regarding DC power supplies/output voltage

It is 11V without load. It drops closer to the rated voltage under load. I don't think any of those would work with a 6X bluray since the current is too low on all but the first, and the voltage might be too low on the first. The first, however, would work with a 20X red.
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Old 12-31-2010, 04:43 PM #3
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Default Re: Basic electronics question regarding DC power supplies/output voltage

It might be that the voltage is higher than rated only when the transformer
is running idle, with nothing attached that pulls a current (DMM in voltage measuring
mode doesn't do that)
As soon as you hook something up to it, the transformer should deliver the correct voltage.

EDIT: dammit too slow
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Old 12-31-2010, 05:18 PM #4
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Default Re: Basic electronics question regarding DC power supplies/output voltage

Ah, that makes sense... since it's the amount of resistance applied to a voltage that determines the current? Um, I think?


EDIT: ...and when you measure output voltage with a DMM, the DMM doesn't technically COMPLETE the circuit so there is no resistance hence no current. I think I got it...

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Old 12-31-2010, 09:08 PM #5
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Default Re: Basic electronics question regarding DC power supplies/output voltage

The DMM does complete the circuit. The impedance of the meter is just so high that no current flows through it.. Therefor it doesn't load what your measuring.
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