# Step-Up Current Via Transformer

• John Raines
I'm a mechanical engineer by trade, but I am working on developing a testing apparatus that oscillates a neodymium magnet between two solenoids offset by by a 180 degree phase shift. Basically, one solenoid pushes, the other pulls. To get this, current is essential. I need each solenoid to get a minimum of 750mA. The problem is, the old function generator has a safety feature to shut off current if it exceeds around 500mA.

To get around this, I've set up an inverting op-amp circuit using a 741 op-amp. This kills my current output, but let's me stack energy in voltage. My plan is to push this voltage through a step-down transformer to convert my volts to amps. My understanding is that it's akin to gear ratios: low speed/high torque gets converted to high speed/low torque on a step-down. So I'm wanting high voltage/low current to step down to low voltage/high current. I've crunched the numbers, and my step doesn't seem unreasonable: 70:1. However, when I run the simulations in PSPICE, it shows my circuit only outputting a total of 400mA to my solenoids. Is there something fundamental that I am missing? All I am trying to do is to put a 10Hz AC signal with a current magnitude of 1.5A sent to 2 parallel solenoids.

Also, PSPICE is predicting that this set up leads to a large current coming out of the op-amp. I thought this was impossible? I thought op-amps bump voltage, but kill current? Was I told wrongly on this? Because it would be delightfully simple to just run the output of the op-amp to the solenoids.

Thanks for the help!
-John

Vitro
Do you have an audio amplifier that you don't mind frying by accident? Hook up the waveform generator to the input and power the solenoids from the output. If it's stereo you can use both channels, one per solenoid. It also allows you to adjust the output with the volume knob.

phinds
That sounds perfect, thank you.

Gold Member
Dearly Missed
Also, PSPICE is predicting that this set up leads to a large current coming out of the op-amp. I thought this was impossible? I thought op-amps bump voltage, but kill current? Was I told wrongly on this?
741 is capable of only milliamps, hence milliwatts.

There exist high power op-amps but they tend to be expensive. Last LM12's i bought were over forty bucks apiece but they'll output 6 amps.

You might search on 'power op amp'
but vitro has the better idea - look for a subwoofer amplifier. Audio sites have them aplenty.

Gold Member
Dearly Missed
@John Raines
While replying to another thread i stumbled across this audio amplifier and thought "That just might drive his electromagnets" .
You'd have to try one out and see if it'll handle the inductance. A couple ohms of resistance will help, perhaps your electromagnet's windings will be enough. ..

http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00001048.pdf
(be aware their drawings are inconsistent as to capacitor numbering . Input capacitors are the 0.22 μf's )

There's a number of kit audio amplifiers available on Amazon for under ten bucks that'd save you from having to lay out a circuit board.
Increasing value of the input capacitors will allow use at lower frequencies.

That one is a linear circuit and straightforward.

There are also "Class D" amplifiers based on digital technology . They're more complex but better suited to high power at low frequency .
Several are based on a very capable TI chip the TPA3116 that is good for two amps of current and can be set for low frequency cutoff as low as 2 hz.. Below that frequency the intelligent amplifier is likely to think there's something wrong and shut down to protect its usual load, a loudspeaker.

Search on those IC numbers with words "amplifier kit"

I get a lot of Amazon hits for pre-assembled kits but can't get any details out of Amazon. Usually those cheap kits just copy the circuit suggested in the IC manufacturer's literature.
The IC manufacturer's datasheet tell you which capacitors determine low frequency cutoff. Better kit manufacturers give you their schematic and board layout in their user manual.

Good Luck - you have a practical and interesting project there.
.
old jim

John Raines
Gold Member
2 parallel solenoids.

You could also try putting them in series and use twice the voltage at half the current. You will get a bit of distortion depending on the position of the magnet since it will change the impedance of the solenoids.

BoB

As a matter of conclusion and to help anyone with a similar issue, I figured I should post my results:

I purchased a MAX9744 audio amp from Digi-key. The hardest part was waiting for it to be delivered. I hooked it all up and my oscillator was churning the first time I threw the switch, and the volume nob is especially useful so that I can adjust amperage on the fly. Thanks so much for the advice!

-John

Tom.G and jim hardy