Electronics > Beginners

Getting confused with the basics and using my power supply

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HobGoblyn:
I'm getting a little confused with the basics, more the equipment rather than the result. 

I've managed to blow the mA fuse in both of my multimeters, but according to my working out, it should have been fine. 

I'm also getting a little confused as to how to set up my power supply (which might be the cause of most of my problems).

I simply wanted to set the power supply to replicate say 2 x 1.5v batteries.


Doing other experiments, I found setting the amps to 0.5 seemed to be a good place, as my experiments only seem to draw as many amps as they need.  So I set it like




and as soon as I turn the output on with nothing connected, the amps drop to 0




I create the simplest of circuits, 3v, 470 ohm resistor (measures at 469.2)




I turn the power supply output on and the amps say 0.004, I take that to read 4mA ?



So to find the actual current, 3v / 469.2 ohms = 0.0063, so the current is 6mA

I measure the current by putting the meter in series




I have to use the Amp input on my meter as I've blown the fuse on the mA

This shows as 0.006 which is what ohms law says it should be.




While I ended up with the correct result, I'm not sure why I blew up both mA fuses, and I'm not sure why my power supply only shows 4mA.  I just want to make sure I fully understand everything that's going on before I go onto the more complex stuff.

OK, I'm initially setting my power supply at 500mA, but the second I turn the output on, it drops to zero, and when  measuring the current, it was  zero until I touched the leads, then it went to 4mA.

One meter has a 250mA fuse, so if it somehow got the 500mA, that explains how it fused, but I don't understand how it would have got it, the other meter has a 1 amp fuse which shouldn't have blown, even if it did get the 500mA.

This was something I was reading in a book that I thought I'd spend 5 mins trying as I hadn't measured current before. Two hours of confusion, then I check fuses in both meters :)

rdl:
The current control on the power supply is a current limit control, the load is what determines how much current actually flows. If you set 500 mA, the the power supply will deliver up to that amount, but not more.

Multimeters are essentially a short circuit when measuring current. This is why the 250 mA fuses blew with a current limit setting of 500 mA.

ataradov:
Also, power supply measurement is just an indicator of a rough value. It is not a measurement tool. It is not accurate at all. So your results are pretty well aligned with what they should be.

And yes, when the output is off, you are setting the limit. When you turn the output on, it switches to showing the actual supplied current. So with no load it would be 0.

And 1 A fast blow fuse may blow before power supply would react to the over-current condition.

The limit you set is a safety feature designed to potentially limit the damage, not fully prevent it. Especially for sensitive electronics.

tunk:
A 2mA error on a 5A scale is a 0.04% error.
Also make this thought experiment: What would happen
if you instead used a (high-wattage) 2ohm resistor.

ArthurDent:
Note that when replacing the fuses you use exactly the same type and rating of ceramic body fuses as the original to maintain the DMM's safety rating. 

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