Electronics > Beginners
Getting confused with the basics and using my power supply
HwAoRrDk:
The current readout on these Tenma (aka Korad) power supplies isn't very accurate below 10 mA. I have one too and it's the same. In a case like this, trust your meter rather than the PSU.
HobGoblyn:
I've replaced the fuse in my UT61E, this time I set the amps to 10mA and it worked perfectly.
As I bought a pack of 10 fuses, I tried setting it to 100mA as per ataradov's post, this also worked perfectly, so I will leave it set at 100mA for all my small experiments.
Many thanks for all your advice
HobGoblyn:
I'm doing (or thinking) something very very stupid, I thought I'd grasped this.
I repeated the experiment in my OP but using 1.5v and 10.02 ohms
The circuits the same, just different value resistor and voltage
1.5v / 10.2 = 0.147A = 147mA
So I set the amps of the power supply to 160mA
Turned it on. Amps on power supply showed 144mA which is about what my working out says.
I measured the voltage across the resistor
This was 1.46v
So far all looks fine.
Then I put my amp meter in series
This shows 74.23mA, the power supply shows 70mA
These appear to be the correct figure but divided by 2, and I can't work out why?
I've probably been at it too long and need a break :)
ArthurDent:
One thing to keep in mind if you are using your DMM in series with your power supply and your load to measure the current drawn is that your DMM is now part of the circuit and it will have a voltage drop so your load will see less voltage than your power supply indicates. This voltage drop caused by an ammeter in series with a load is called 'burden voltage' and varies with the range you're using and your load. The lower the voltage and the higher the current you use can cause the error caused by this burden voltage to be quite high. If you use both your DMMs in your circuit, one set to MA and in series, and the other set to voltage directly across the load, you can see how this burden voltage affects your readings. Here is a good video that shows the effects quite well.
HobGoblyn:
Thanks. I was only using one meter at a time.
It gets even weirder. If I use the Amp rather than mA part of my meter, it correctly shows 0.143A and my power supply shows 144,
but as soon as I go back to the mA, I get the wrong result
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