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Question about current and voltage.

mfo

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Ok, it's been a while since I studied the laws of electronics, so please bear with me here. I am trying to set the current on a driver board I have to 110mA. No problem, I simply connect the output of the driver to a Ampere meter, and turn the pot until it hits 110mA, correct?

Now the tricky part...I tested my results (The output of the driver) with three different 9V batteries I had laying around, all different ages & used more than others. All three batteries put out the same 110mA (give or take a mA or two). However, when I went to test the voltage across the same terminals, two of them registered as 7.5 volts, and the other registered as 8.02v. I was once told that this is an inaccurate way to measure voltage due to the fact that the driver isn't under a load when I do this, so the voltage will always read higher than what the voltage actually is. Is this true or false?

The reason I ask all of this is because I plan on driving a PHR @ 110mA, which I know is a safe current, but I know that voltage is too high. I believe the right voltage is around 5V or so? Thanks for reading this and for your help in advance.
 

billg519

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A current regulator will only allow enough voltage such that the load draws the set current and no more. Make yourself a test load (search test load) and try measuring the voltage then. The voltage measurements you took with no load are not meaningful. You also need to set the current using a test load. This ensures that everything will be spot on for your diode. Doi not forget to discharge the cap ...
 

mfo

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A current regulator will only allow enough voltage such that the load draws the set current and no more. Make yourself a test load (search test load) and try measuring the voltage then. The voltage measurements you took with no load are not meaningful. You also need to set the current using a test load. This ensures that everything will be spot on for your diode. Doi not forget to discharge the cap ...
As taken from the rkcstr driver manual....

"1. The most accurate method involves directly measuring the current with an ammeter (or
multimeter set to measure mA). Make sure your meter is rated to handle at least
500mA and that the measurement range is within your intended setting.
a. You can directly attach the multimeter leads to the driver output (red to positive,
black to negative) and power up the driver then adjust the pot to set your
current while monitoring the meter"

So I guess a test load isn't even necessary?
 
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For rkcstr's driver, it'll put out 110mA no matter what, so no test load is technically needed, FOR CERTAIN DRIVERS ONLY, including rkvstr's IIRC.

But it's good practice to always use a test load. For instance, powering up the lavadrive with no load will break the driver, and you're out >$20. So I just always use a test load along with a 1ohm resistor, so it's one less thing to worry about, no matter what driver it is.
 
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bobhaha

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The driver will always output a constant current (what ever you set it to). The draw from the diode will determine the voltage. Think of it this way, current as an amount of people, the diode is an open door and voltage is the amount of "push" or "power" exerted on the crowd to push them through the door. The door is always going to be the same size and only allows a certain amount of people through it. Thats how i used to remember it!

The only thing you have to do is make sure the current is set (use a test load! it is highly reccomended!) also make sure polarity are correct and that the input voltage is within accptable range! and the driver will do the rest!

Hope that helped! -Adrian
 




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