# How to amplify the current of an AC signal?

## Main Question or Discussion Point

Hello,

This is my first post; so first of all, let me thank all of you for starting and running such a good forum that provides lot of help and insight into various engineering problems.

As to my brief introduction, I am an IT professional with educational background in (Mechanical) Engineering.
I have this practical problem about Electrical Engineering regarding an amateur project I'm doing @ home.

The question is:
I have an electrical power input AC signal of ~80 V and 0.8 mA.
So the electrical power available is = 80V x 0.0008A = 0.064W.
Ignoring conversion/transformation losses for now, is there a way to convert this into electrical output that has - say - 8V and 8 mA?

I can connect more power sources in series to collect more power, but essentially I need to strengthen the signal current (of course at the cost of a proportionate voltage drop). So I'm looking at current amplification.

Based on my reading, it looks like a (step-down) transformer should do the job of reducing the output voltage and proportionately increasing the output current ( V[in]/V[out] = I[out]/I[in] = a (turns ratio: N[in]/N[out]).

Q1. Is this understanding correct?
Q2. If yes,
Q2.1 What should be the specification of the transformer? Are there any available off-the-shelf in marker that I can use here?
Q2.2 Also, eventually I need to convert the signal to DC. So what would be the right thing to do? Should I rectify the voltage first and then use a DC transformer (does one exist)? Or I need to rectify the output of the transformer?
Q3. If transformer is not the solution to my problem, then what is the solution? I also read about some OpAmp and other amplifier IC chips for current amplification. Is that also a viable option for me?

I'm discounting the possibility of connecting my power sources in parallel in order to add current, because that would require a large number of power sources (in parallel). Please let me know if that's incorrect too.

Thanks.
Shaktidaas.

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berkeman
Mentor
Hello,

This is my first post; so first of all, let me thank all of you for starting and running such a good forum that provides lot of help and insight into various engineering problems.

As to my brief introduction, I am an IT professional with educational background in (Mechanical) Engineering.
I have this practical problem about Electrical Engineering regarding an amateur project I'm doing @ home.

The question is:
I have an electrical power input AC signal of ~80 V and 0.8 mA.
So the electrical power available is = 80V x 0.0008A = 0.064W.
Ignoring conversion/transformation losses for now, is there a way to convert this into electrical output that has - say - 8V and 8 mA?

I can connect more power sources in series to collect more power, but essentially I need to strengthen the signal current (of course at the cost of a proportionate voltage drop). So I'm looking at current amplification.

Based on my reading, it looks like a (step-down) transformer should do the job of reducing the output voltage and proportionately increasing the output current ( V[in]/V[out] = I[out]/I[in] = a (turns ratio: N[in]/N[out]).

Q1. Is this understanding correct?
Q2. If yes,
Q2.1 What should be the specification of the transformer? Are there any available off-the-shelf in marker that I can use here?
Q2.2 Also, eventually I need to convert the signal to DC. So what would be the right thing to do? Should I rectify the voltage first and then use a DC transformer (does one exist)? Or I need to rectify the output of the transformer?
Q3. If transformer is not the solution to my problem, then what is the solution? I also read about some OpAmp and other amplifier IC chips for current amplification. Is that also a viable option for me?

I'm discounting the possibility of connecting my power sources in parallel in order to add current, because that would require a large number of power sources (in parallel). Please let me know if that's incorrect too.

Thanks.
Shaktidaas.
Welcome to the PF.

What frequency range do you get from the power source? Can you say what the power source is (like maybe a small windmill?)? Initially, a step-down transformer sounds like the right approach, depending on the frequency range of the signal.

If you are working 60 Hz, a simple 10 to 1 ratio transformer will do that. You understand that you have not increased the total energy available but just upped the current at the expense of voltage, right? If you need more total energy you would need an active amplifier like an op amp, which is pretty simple in itself.

@Berkeman:
The power source is a small generator (a Tacho Generator).
Unfortunately my old DMM does not have a way to measure frequency. I'll need to get a new DMM and check it out.
Till then, can you comment on the range of frequency for which the standard step-down transformer should work?

Also, another question related to the frequency:
If I need to couple of multiple generators' output in series, then theoretically their outputs need to have the same frequency. Right? That may be a bit tough to achieve. That's why I had asked about the feasibility of the option of rectifying the output of individual generators, then adding them up in series and then feeding the total output to a DC transformer. Is that a technically feasible solution? Do we have DC transformers commercially available? for the small range of voltages that I'm dealing with?

Thanks.

@litup:
Thanks. I'll check the freq. of the output.
Yes, I understand that I cannot create energy :). Hence I'm merely looking at bumping up the current at the cost of voltage.

However, the option of OpAmp sounds interesting!
How trivial it is to get an OpAmp and connect one in my circuit in order to get a real increase in the output? Are they in IC form or in component form?
Can you point me to any old threads on this forum or resources elsewhere, where I can get an overview of OpAmp and also some working knowledge of the ones readily available in market?
Based on my input power conditions and output requirements, how do I decide the specs of the OpAmp that I need?

Thanks.

meBigGuy
Gold Member
Since it looks you are trying to harvest energy, an opamp is not the way to go. You want a transformer.
An opamp needs a power supply capable of supplying its output current, which is exactly what you don't have.

The other possibility is a switching converter that will convert it to DC in an energy efficient way, but that is nontrivial. Efficiently connecting those in parallel is also nontrivial.

I don't know much about inter connecting multiple AC generators. There might be fancy transformer designs to do that. [EDIT] Doesn't look like it. http://www.basler.com/downloads/VR_parallel.pdf

There is no DC transformer. Just DC to DC converters (switch mode power supplies)

Last edited:
Thanks.

From your response, it looks like interconnecting AC generators would be complex. OpAmps are not really relevant to my use case. Also there are no DC transformers.
So it seems the only valid option for me is:
1) Step down each generator's AC output separately to the desired current and voltage.
2) Rectify each generator's stepped down output separately.
3) Add each generator's rectified output (in series) to get the desired voltage at the required amperage.

I still need to confirm the frequency of the AC output; but I guess it would be in the 40 - 100Hz range.

The problem with this approach is that the number of transformers and rectifiers grow linearly. So for 2 or 3 generators, I'll need to spend more on the extra number of transformers and rectifiers. And it would make the assembly that much heavy.

berkeman
Mentor
@Berkeman:
The power source is a small generator (a Tacho Generator).
Unfortunately my old DMM does not have a way to measure frequency. I'll need to get a new DMM and check it out.
Till then, can you comment on the range of frequency for which the standard step-down transformer should work?

Also, another question related to the frequency:
If I need to couple of multiple generators' output in series, then theoretically their outputs need to have the same frequency. Right? That may be a bit tough to achieve. That's why I had asked about the feasibility of the option of rectifying the output of individual generators, then adding them up in series and then feeding the total output to a DC transformer. Is that a technically feasible solution? Do we have DC transformers commercially available? for the small range of voltages that I'm dealing with?

Thanks.
Could you please say more about what a Tacho generator is? I did a quick search on it, but the characteristics were not obvious. A big factor is the synchronization of the output AC power waveforms. Taking windmills as an example, it is generally more efficient to synchronize all of the windmills' AC generator outputs than it is to rectify them all and combine their DC outputs. But without knowing more about your generators, it's hard to comment further...

Could you please say more about what a Tacho generator is? I did a quick search on it, but the characteristics were not obvious. A big factor is the synchronization of the output AC power waveforms. Taking windmills as an example, it is generally more efficient to synchronize all of the windmills' AC generator outputs than it is to rectify them all and combine their DC outputs. But without knowing more about your generators, it's hard to comment further...
I think what he has is a tachometer generator. I have one on a motor that pulls a platen back and forth in an argon plasma atmosphere, through ferrofluidic vacuum seals but the motor has a tachomter tacked on the end of the shaft so the electronics can make the physical scan speed as accurate as possible, in terms of cm per minute, slow speed. The motor is cranking at around 1000 rpm however. This makes a voltage dependent on the rpm which is fed to a feedback circuit to keep the drive motor at a constant RPM. In this case the tach is generating DC through a rectifier I think, not sure exactly of the internal workings but it does a dc output fed to op amps and such.

His generator might be running somewhere between 40 and 1000 Hz. So I would suggest an audio ten to one transformer, the actual power levels are moderate, something like what you would feed to a pair of earphones so an audio transformer would work well.

Could you please say more about what a Tacho generator is? I did a quick search on it, but the characteristics were not obvious. A big factor is the synchronization of the output AC power waveforms. Taking windmills as an example, it is generally more efficient to synchronize all of the windmills' AC generator outputs than it is to rectify them all and combine their DC outputs. But without knowing more about your generators, it's hard to comment further...
I think what he has is a tachometer generator. I have one on a motor that pulls a platen back and forth in an argon plasma atmosphere, through ferrofluidic vacuum seals but the motor has a tachomter tacked on the end of the shaft so the electronics can make the physical scan speed as accurate as possible, in terms of cm per minute, slow speed. The motor is cranking at around 1000 rpm however. This makes a voltage dependent on the rpm which is fed to a feedback circuit to keep the drive motor at a constant RPM. In this case the tach is generating DC through a rectifier I think, not sure exactly of the internal workings but it does a dc output fed to op amps and such.

His generator might be running somewhere between 40 and 1000 Hz. So I would suggest an audio ten to one transformer, the actual power levels are moderate, something like what you would feed to a pair of earphones so an audio transformer would work well.

Shak, did you ever try hooking up a pair of earphones where you can hear the voltage being produced? You could use your own ears as a judge of the frequency then.

@Berkeman:
Yes, As litup mentioned, it's a Techometric Generator. I don't know a whole lot of details about it myself; but it's essentially a generator (AC or DC) that produces output voltage that is proportional to the speed. So the speed v/s voltage curve is pretty linear ( at least up to the max rated speed).
These couple of links should give you more details:
http://en.wikipedia.org/wiki/Electric_generator#Tachogenerator

I agree with you that the approach taken by the commercial windmills - of syncing the generators' output - would be the most efficient one. But I'm just doubtful about how much of that I'll be able to put together myself - considering that this is just an individual's amateur project.

@litup:
I guess you are right - frequency in my case would be in the range of 40 - 500 Hz.
So do you suggest I try an audio transformer? What's the difference between a standard electrical transformer v/s an audio transformer?
Can you suggest any specific model?

I'll also try the "ear" test you mentioned and post my "hearings" about the frequency :)

Thanks!

meBigGuy
Gold Member
You can download free fft apps for android or iphone that will tell you the frequency (if you drive a speaker).

Thanks! That's a good idea..I'll try that.

If you want to sync several of these puppies together you might just have a big gear say the size of a bicycle wheel and matching gears on each tach generator, then drive the whole assembly with a windmill or some such. I assume this is for a wind generator project, right? I'll try to suss out some audio transformers and come back with a model. You might check out the electronics hobby rags, there are several, on nice on from the UK. Volt I think is the name of one. You will drool at the ads there:)

Here is one I found, it sounds like it matches your voltage source nicely, 70 volt in and then lower voltage out with several taps available.