How to measure current to not burn out supply or components

Odiesdad

Joined Aug 7, 2015
8
Hi everybody
I have a DMM it's VC9805. For measuring the output current of this Buck module, I used it this way:


Then I connected the red test lead to 20A and set it to DC mode(it is in DC mode itself) and connected the test leads to the output of the module. and when I turned the supply on, BOOOOM!



Burnt out.:(
Why? (I think there is a big load in DMM to draw current)

I want to measure the current on my board. I want to know Is my MCU(STM32F103) drawing current or not(want to know it's functioning/working or not). but I'm worry that maybe it would burnt out my debugger(it's J-Link) or my MCU. Should I be worry?
 

Odiesdad

Joined Aug 7, 2015
8
Remember the definition of current. It is the FLOW not the voltage. You must intercept the flow with the meter, in other words it must be in series with the load and source. Always connect the plus lead where the voltage is being supplied and the negative lead where the plus voltage enters the circuit. Other caveats are to be observed; i.e. max settings on the meter, etc. Good luck. . . .
 
At the risk of repeating some of the above info, here is a summary:

1. Note that you can't measure current "at" a power supply output (in parallel). Current is "drawn" into a load, so it must be measured in series with a load. If you want to measure if your power supply can put out 3A, measure your voltage at the output, calculate the resistor for a 3A @ whatever voltage using Ohm's law, and then put it in series with your meter set to the 20A scale as PeterCoxSmith describes above. But start by calculating a resistor for 1A at your output voltage, and then move to the 3A resistor, so you don't overload the buck regulator right off.

2. You may have blown out your buck regulator. It has a series switch which may not have tolerated the zero ohms your ammeter put on the output. To see if it's still OK, Set your meter to the voltage scale (with leads in the voltage sockets) and measure the voltage across the output terminals of your buck regulator (red to +, black to -, remember this is voltage measurement now). If it reads the voltage you desire, try putting the PeterCoxSmith load resistor on it and see if it still shows that voltage. Put a resistor calculated for 1A on the output and see if it holds the voltage. If the voltage drops then your power supply is damaged (if you've calculated the resistors correctly).

3. If the buck regulator is OK, you can proceed to the current experiment as PeterCoxSmith described it.

- The Inspector
 

Odiesdad

Joined Aug 7, 2015
8
Hello booboo;
Following is from the instruction manual for the VC9805:

From the instruction manual


4-3.Current measurement VC-9805

1. Apply the black test lead to “COM” terminal and the red one to “mA” or “20A” terminal.

2. Set the knob to a proper range A. If the current under tested is unsure beforehand, please set it on the max., then measure diminishingly to reach a resolute value.

3. Press the DC/AC key down to AC mode to measure AC current. Oppositely, resetting the key to DC mode to measure DC current.

4. Connecting the leads with the tested circuit in series, the current value is displayed on LCD. While testing a DC current, the reading is the value of current and polarity of the point connected by the red lead.

NOTE: 1.If the LCD displays 1 or OL, it means the current is over range. Now you need to set the knob to the higher.

2.Max. input current is 200mA or 20A(subject to where the red test lead apply to), too large current will blow the fuse. Be sure the test is less than 10 seconds. Please keep the leads off the circuit while switching the function and range knob.
 
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