# Cannot read voltage on 240v to 12v transformer with multimeter

#### One23

Joined Sep 19, 2022
6
Hi all and merry Christmas,
I'm new at electronics and thought to use a 240V-50Hz 26VA ac to 12V Max 20.4VA dc transformer to power a breadboard to do some basic series & parallel circuit analysis. I wanted to test the voltage coming out of the transformer with a multimeter, expecting 12v or near. The meter just spikes and shows zero volts yet I plug the leads into the breadboard and it runs a 12v 1.1A fan no problem. Why is there no voltage reading?
When compared to a 12V AA battery configuration, the voltage drops across resistors and LED's differ wildly when using the transformer power supply.
Does the power coming out of a transformer differ from that coming from a battery supply?
Hey thank you to anyone that can enlighten me on the above.

#### sghioto

Joined Dec 31, 2017
3,940
The voltage coming out of the transformer is ac not dc

#### ronsimpson

Joined Oct 7, 2019
2,618
People often use the word "transformer" to describe a power supply. Can you post a picture of what you have?

#### WBahn

Joined Mar 31, 2012
28,182
Hi all and merry Christmas,
I'm new at electronics and thought to use a 240V-50Hz 26VA ac to 12V Max 20.4VA dc transformer to power a breadboard to do some basic series & parallel circuit analysis. I wanted to test the voltage coming out of the transformer with a multimeter, expecting 12v or near. The meter just spikes and shows zero volts yet I plug the leads into the breadboard and it runs a 12v 1.1A fan no problem. Why is there no voltage reading?
When compared to a 12V AA battery configuration, the voltage drops across resistors and LED's differ wildly when using the transformer power supply.
Does the power coming out of a transformer differ from that coming from a battery supply?
Hey thank you to anyone that can enlighten me on the above.
Simple test: Turn your meter to the VAC mode instead of VDC and see what it reads.

That the meter spikes and then reads zero is a pretty strong indication that you are trying to read and AC voltage while on a DC voltage range. The initial spike is due to the brief transient before things settle into steady state.

You need to be careful if you are powering LEDs with an AC voltage source since they usually have quite low max reverse voltage specifications, typically around 5 V to 7 V.

#### One23

Joined Sep 19, 2022
6
The voltage coming out of the transformer is ac not dc
Thank you sghioto, I didn't even think about that. The multimeter I have reads both AC & DC out of the one port but you have just taught me to have another look at it and I only now discovered the "function" button that alternates between AC & DC depending on what I am measuring. That's how much of a novice I am, thank you once again

#### One23

Joined Sep 19, 2022
6
Simple test: Turn your meter to the VAC mode instead of VDC and see what it reads.

That the meter spikes and then reads zero is a pretty strong indication that you are trying to read and AC voltage while on a DC voltage range. The initial spike is due to the brief transient before things settle into steady state.

You need to be careful if you are powering LEDs with an AC voltage source since they usually have quite low max reverse voltage specifications, typically around 5 V to 7 V.
Thank you WBahn, indeed I was reading DC when switching to VAC now read 13V. Thank you so much for getting back to me and also for the heads up on running LED's on AC, fantastic.

#### One23

Joined Sep 19, 2022
6
People often use the word "transformer" to describe a power supply. Can you post a picture of what you have?
Hi Ron, thank you for getting back to me. Thanks to the responses I have found out that I actually am getting AC out and not DC. I suppose I should have read that from the data plate on the transformer. I am still very much on training wheels when it comes to electronics and thank you very much for your time and help, I'll have plenty more dumb questions where that one came from

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#### MisterBill2

Joined Jan 23, 2018
14,260
That is the reason that a lot of us call those devices "wall warts", which is to avoid confusing them with other items. And I see that the produces of that version were too lay to even spell out "AC"
If you do not see a + or a - then it is probably AC. For DC, positive and negative mostly matter a lot.

#### One23

Joined Sep 19, 2022
6
That is the reason that a lot of us call those devices "wall warts", which is to avoid confusing them with other items. And I see that the produces of that version were too lay to even spell out "AC"
If you do not see a + or a - then it is probably AC. For DC, positive and negative mostly matter a lot.
Hey Mr Bill, LOL.......I love the "wall warts" analogy.............Sure thing, as I learnt to switch my multimeter to AC from DC it all became clear, man oh man I have a lot to learn and am loving it. That transformer ran a fan that blew up an inflatable Santa that had too many holes in it to rescue for another year. These days I just strip all the electric components I can and learn from them. Thank you for helping me out with my query.

#### Audioguru again

Joined Oct 21, 2019
5,577
The AC symbols are small and difficult to see.

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#### WBahn

Joined Mar 31, 2012
28,182
That is the reason that a lot of us call those devices "wall warts", which is to avoid confusing them with other items. And I see that the produces of that version were too lay to even spell out "AC"
If you do not see a + or a - then it is probably AC. For DC, positive and negative mostly matter a lot.
What's to spell out?

The use of the symbols

is widely used and internationally accepted.

There are, after all, many languages that don't even have the characters A, C, or D in them.

#### MisterBill2

Joined Jan 23, 2018
14,260
What's to spell out?

View attachment 283771

The use of the symbols
View attachment 283770
is widely used and internationally accepted.

There are, after all, many languages that don't even have the characters A, C, or D in them.
The symbols on this item are so poorly done that they are only valid to those well experienced in the field. Also, they could easily copied the "50 Hz from the line above. That would have been easier than inserting the poor graphic characters, and obvious to all who can read.

#### shortbus

Joined Sep 30, 2009
9,805
That would have been easier than inserting the poor graphic characters,
Those "poor graphic characters" have been around forever.

#### Audioguru again

Joined Oct 21, 2019
5,577
In Google Images, many transformers do not say AC or DC. I did not see one with the "poor graphic character" that looks almost like a straight line.

#### MisterBill2

Joined Jan 23, 2018
14,260
The posted image is a poor image, and the quality of the photo is adequate to see that the image is a poor image.

#### One23

Joined Sep 19, 2022
6
The AC symbols are small and difficult to see.
Hey Audioguru, thank you for your reply. I think what really threw me was that the fan being run by the transformer outputting AC is marked 12VDC 1.1A. I didn't think 12VAC would run 12VDC so originally thought the output was also DC. Would Ohm's law still apply to AC as it does with DC? Thanks again Audioguru

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Joined Jul 18, 2013
26,683
Hey Audioguru, thank you for your reply. I think what really threw me was that the fan being run by the transformer outputting AC is marked 12VDC 1.1A. I didn't think 12VAC would run 12VDC so originally thought the output was also DC. Would Ohm's law still apply to AC as it does with DC? Thanks again Audioguru
Those BLDC fans have internal electronics for commutation, it is quite possible there could be an internal bridge to make it Universal i.e. AC or DC supply !?
I have seen the exact same practice with other devices, DC solenoids etc.

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#### Poor old sod

Joined Jul 25, 2017
193
the output of ANY transformer only is ALWAYS AC, it takes other parts to change that,
with the very rare exception of an auto transformer with dc biased primary.

#### MisterBill2

Joined Jan 23, 2018
14,260
I have seen a number of devices that include reverse polarity protection diodes. That would explain it adequately.