# Measuring Amps, Am I Reading This Right!?

#### MrClamperSir

Feb 3, 2016
72
I've taken apart an old power supply built on the LM338K circuit design and I am looking to record the values of each component. When I measured the the transformer I got 121VAC on the primary side, and 17.3VAC on the secondary side.

I now needed to measure the amps so I watched a youtube video on measuring current with my particular multimeter (FLUKE 23) and set up to get a reading. I was blown away to see the output reading 12.something amps. Could this be right?

With my other power supply's (LM317 circuit design) I was under the impression that I've been dealing with 0.5A-2A.

#### Arouse1973

Dec 18, 2013
5,178
Hello
Do you have a circuit diagram we can see?
Thanks

#### MrClamperSir

Feb 3, 2016
72
Hello
Do you have a circuit diagram we can see?
Thanks
Hi! I am researching that right now so unfortunately I don't have one I can post at this time. However I do have the values of the components used in this circuit.

Transformer - 120V-17V
Bridge Rectifier - unknown value
C1 - 104K 50V
R1 - 118Ω
R2 - 5KΩ
IC - LM338K
C2 - 10uf 35V
C3 - 105K 50V
C4 - 2,200uf 35V

#### MrClamperSir

Feb 3, 2016
72
Also I could upload a picture of the PCB if that would help?

#### Arouse1973

Dec 18, 2013
5,178
We speak circuits, a cicuit diagram is needed. We will have to wait.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
If you were trying to read the current by measuring across the output of the transformer then you were shorting it out.

Current measurement must be done in series.

#### BobK

Jan 5, 2010
7,682
Tell us exactly how you connected the ammeter probes to read 12A.

Bob

#### AnalogKid

Jun 10, 2015
2,674
A schematic is worth a dozen photos, but a picture still is worth a thousand words. Gimme.

Also, is there a label or sticker or something on the power supply with its input and output ratings?

ak

#### MrClamperSir

Feb 3, 2016
72
If you were trying to read the current by measuring across the output of the transformer then you were shorting it out.

Current measurement must be done in series.
Ahhh OK thank you

#### MrClamperSir

Feb 3, 2016
72
Tell us exactly how you connected the ammeter probes to read 12A.

Bob
I switched the lead probe from it's general position to the 10A position. I then connected the red probe to one wire on the secondary and the black probe to the other wire on the secondary side of the transformer.

#### MrClamperSir

Feb 3, 2016
72
A schematic is worth a dozen photos, but a picture still is worth a thousand words. Gimme.

Also, is there a label or sticker or something on the power supply with its input and output ratings?

ak
No labels on the power supply or the transformer. I'll upload the photo when I get home tonight

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,762
I switched the lead probe from it's general position to the 10A position. I then connected the red probe to one wire on the secondary and the black probe to the other wire on the secondary side of the transformer.

As Steve said and I agree ......
So what this does is essentially short-circuit the transformer secondary. The transformer probably doesn't provide enough current to blow the fuse in the Fluke 23, so (assuming you have the meter set to measure AC current) the reading you are getting is the short-circuit current capability of the secondary winding. The transformer should get very warm, hot even, as long as your meter is connected in this manner. Leave it connected too long and the fuse internal to the meter will blow.

Never connect a current measuring device in parallel with a voltage source. It will short-circuit the source. To measure the current provided by a voltage source to a load, you must open the connection to the load and insert the meter in series with the opened connection. Here is a pretty good video tutorial on how to use your multimeter to measure AC and DC voltages, resistance, continuity, and AC and DC currents.

Last edited by a moderator:

#### MrClamperSir

Feb 3, 2016
72
So what this does is essentially short-circuit the transformer secondary. The transformer probably doesn't provide enough current to blow the fuse in the Fluke 23, so (assuming you have the meter set to measure AC current) the reading you are getting is the short-circuit current capability of the secondary winding. The transformer should get very warm, hot even, as long as your meter is connected in this manner. Leave it connected too long and the fuse internal to the meter will blow.

Never connect a current measuring device in parallel with a voltage source. It will short-circuit the source. To measure the current provided by a voltage source to a load, you must open the connection to the load and insert the meter in series with the opened connection. Here is a pretty good video tutorial on how to use your multimeter to measure AC and DC voltages, resistance, continuity, and AC and DC currents.
Thank you very much!! I knew I was doing something wrong. I'm going to watch the video ASAP and try again tomorrow

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