# Sensitive voltage detector questions

#### atrumblood

Joined May 13, 2012
59
I am reading through the experiments Volume of the E-book and I built the Sensitive Voltage Detector with the parts I had available to me. It kinda works, but I have a question.

The question I have is how to read the specs on a audio transformer.

The link to the experiment page is : http://www.allaboutcircuits.com/vol_6/chpt_3/12.html

The parts lists mentioned a 120:6 Step down audio transformer.
I went to radio shack and purchased the only thing labeled "audio transformer".

I am trying to figure out what the ratio is on it but the packaging doesn't tell you what the ratio is as plainly as I would like.

It says the input is 70Ω and the output is 0.7Ω.
When I measure it with my meter I read ~63.5Ω on the input and ~0.5Ω on the out.

How do I figure out what the ratio is or put it in a format that resembles X:Y ratio.

I understand that to get the ratio you divide X by Y.

I get 127, but I am having a retard moment and don't remember how to put it in terms of a ratio.

The reason I am posting this is because I am not getting the sensitivity the experiment suggests. This makes me think the ratio is way off what I need.

Specs for the Transformer:

Input: 1Lohms +-10%
DC Resistance (input):70Ω +-20%
DC Resistance (output):0.62Ω +-20%
Frequency Response: 300-10,000Hz +-3dB
Insulation Resistance: > 100MΩ @ 100VDC

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#### #12

Joined Nov 30, 2010
18,210
Your main problem is that DC resistance is not impedance. To find the ratio, you put a 1KHz signal in the input side and measure the voltage on the output side. The voltage ratio is the ratio of the transformer.

#### atrumblood

Joined May 13, 2012
59
Crap!, I goofed It did not say I needed an audio transformer just a normal one. Oh well, this still does the job at least as a proof of concept experiment.

I assume any transformer with a 120:6 ratio will work for this?

Also with a step down or a step up transformer. Does it just depend on how you wire it that determines if it is step down or step up.

for example

if I wire a 12v input to the 120:6 it would be step down, but if I wire the 12v so that it is 6:120 it would be a step up right ?

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#### #12

Joined Nov 30, 2010
18,210
I'm just now figuring out you're talking about a power transformer that does 120 VAC to 6 VAC. (USE THE LABELS!) No, you can't plug an audio transformer into a wall outlet. Yes, you can run them backwards but you can't put more than 6 volts into the 6 volt winding.

Be sure to have somebody nearby in case you need to be re-started!

#### atrumblood

Joined May 13, 2012
59
Lol thanks for the information. I guess I was confused by the audio and power transformer bit.

#### MrChips

Joined Oct 2, 2009
21,663
It depends on where you plan to use this project.
It is called a "Sensitive Voltage Detector" and there is no mention of connecting this to live AC mains circuit.

The transformer required is an impedance matching transformer, i.e. one that will match a high input impedance to the low 8Ω impedance of the headphones.

So in this case an audio transformer or a power transformer with the appropriate turns ratio would work.

So we are back to the original question: How to determine the turns ratio?

The simple way is to inject an AC signal into one winding and measure the AC voltage output on the other winding. The ratio of the two voltages will give you the turns ratio.

#### MrChips

Joined Oct 2, 2009
21,663
The "audio" transformer you bought is probably a 70V Line Matching transformer that is used in public address systems.
The turns ratio would depend on the delivered wattage of the transformer.
For example, a 10W transformer will be 500Ω to 8Ω giving a turns ratio of 62.5.

http://www.ticcorp.com/25v_70v_100v_systems.htm

If you posted the part number of the transformer we would know for sure.

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#### atrumblood

Joined May 13, 2012
59
Sorry about all the confusion guys.

I wasn't able to get part numbers because transformers from radio shack apparently don't have them. .

I went out and got a 120vAC:6vAC and wired it up in place of the audio transformer.

Again it works but because I don't have some cupped ear phones I can't hear very small signals such as the galvanic effect.

I have another question about the Sensitive Voltage Detector.
I don't exactly understand why I am connecting the input to the 120vAC side of the transformer.
If the device is supposed to being able to detect very small AC signals wouldn't I need to step up the voltage to do that?.

In other words connect the Input to the 6V side.
I understand that obviously since it works as I was instructed to build it but I need to understand more about it.

#### wayneh

Joined Sep 9, 2010
16,399
I don't exactly understand why I am connecting the input to the 120vAC side of the transformer.
Your headphones require current moving through them to make a magnetic field, which moves the cone to produce sounds. Connecting them directly to the source would "short" all the current available there - which is very little - and produce little or no sound. Reversing the transformer would increase the voltage and reduce the current still further.

The transformer is used to match the load to the source (their impedances), just like shifting gears on a bicycle. Some power is lost in the process, but that's a small price to pay for getting the match-up.

#### atrumblood

Joined May 13, 2012
59
Thanks for the clarification on that wayneh.

#### atrumblood

Joined May 13, 2012
59
Well it's done. All soldered up and tested.

here are some pics.

Now to get this into a project box.

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