Hello folks,

This is a very basic question, and I apologise in advance.

For my own personal use, I am experimenting with Pulsed Electomagnetic Field Therapy. I have my reasons, I have spasticity.

There is plenty of peer-reviewed science out there. I won't go into it right now.

Generating a waveform from 0 - 3 Hz is well within my capabilities, but I want to amplify, say, a bi-phasic square wave to 10W.

An op amp is the way to go, but which type? I am reading up at the moment, can someone point me in the right direction of a schematic?

Thanks a lot, I shall post up my finished design on here.

Talk to you all later.

 

You need a power amplifier, not an opamp.
Most audio amplifier ICs can produce the very low frequencies that you want if the coupling capacitors are huge and the supply is well regulated. To produce a 10W squarewave then the amplifier IC will not heat much.

But you don't say how much voltage and you don't say how much current. 10W can be made with 200V peak-to-peak into 1000 ohms with a peak current of 100mA. But 10W can also be made with a voltage of 20V peak-to-peak into 10 ohms with a peak current of 1A.

 

Thank you very much for the answer.

Voltage and current.

These are good questions. I don't know is the quick answer.

https://www.micro-pulse.com/products/replacement-coils-1

I shall be using these coils. Note, no information about impedance.

I would welcome suggestions as regards voltage and current.

I'd guess a maximum current of 1A. At worst, I'll just burn out the coil. Hardly going to damage myself

 

Hardly going to damage myself

Are you sure?
Looks like the coils in that link are connected up using a standard 3.5mm jack. I would be surprised if they are intended to pass more than 1A and doubtful that they can handle 10W safely for any prolonged period. At 5W per coil they could get very hot.

 

Hello,

You say there are pulses.
How long are those pulses?
The repetition rate is between 0 and 3 Hz as I read it.
For that low rate, you will need a DC coupled amplifier.

Bertus

 

Apex and TI make power opamps that will do what you want, but they are relatively expensive and will need a heatsink and possibly a small fan. But since you are using a simple square wave, there probably are much less expensive and more efficient circuits to produce what you want. Please post your circuit.

Also, you do *not* need to replace the coils every few months. Other than from physical abuse, a coil of wire does not "wear out".

ak

 

Also, you do *not* need to replace the coils every few months.

..... as for buying the advertised coil tester, well .......

 

Quick google search found this commercial unit. Monkeybus are these specs similar to what you are trying to build?
SG

The Combo LG-TEC consists of 2 great units in one!

• LG-TEC TENS/Muscle Stimulator Combo Unit
• Lead Wires
• 4 Self-Adhesive Reusable Pre-gelled Electrodes
• 9V Battery
• 5 modes of TENS, 2 modes of EMS, 7 total mode

Detailed Description:
Channels: Dual Channel, isolated between channels

• Pulse Amplitude: Adjustable, 0-100 mA peak into 500 ohm load each channel.
• Voltage: 0 - 50V (Load: 500 ohm)
• Pulse Rate: Adjustable, from 2 to 150 Hz, 1 Hz/step
• Pulse Width: Adjustable, from 50 to 300 microseconds, 10uS/step
• Wave Form: Asymmetrical Bi-Phasic Square Pulse
• On Time: Adjustable, 2~90 seconds , 1 Sec./ step
• Off Time: Adjustable, 2~90 seconds , 1 Sec./ step
• Ramp Time: Adjustable, 1~8 seconds, 1 Sec./ step, The "On" time will increase and decrease in the setting value.
• 3 EMS Modes: Constant ,Synchronous, Alternate

 

Probably not. TENS is pulsed DC through muscles by direct contact, not electromagnetic fields.

ak

 

You do not need a linear amplifier to amplify a square wave. A simple transistor, bipolar or MOSFET, configured as a switch will do.

Bob

 

You do not need a linear amplifier to amplify a square wave. A simple transistor, bipolar or MOSFET, configured as a switch will do.

Maybe.

My experience in this field is that most people who produce and promote these kinds of devices do not have a firm grasp of how they function, or which electronic characteristics are important to gaining the desired effect. In driving a coil, for example, an open drain, an open drain with a suppression diode, and a true totem-pole output stage produce magnetic fields with three very different shapes over time. Finding someone who knows that is rare; finding someone who knows why one is preferred over the others is ... rare-er.

ak

 

Hello folks,

This is a very basic question, and I apologise in advance.

For my own personal use, I am experimenting with Pulsed Electomagnetic Field Therapy. I have my reasons, I have spasticity.

There is plenty of peer-reviewed science out there. I won't go into it right now.

Generating a waveform from 0 - 3 Hz is well within my capabilities, but I want to amplify, say, a bi-phasic square wave to 10W.

An op amp is the way to go, but which type? I am reading up at the moment, can someone point me in the right direction of a schematic?

Thanks a lot, I shall post up my finished design on here.

Talk to you all later.

NS used to do various high power buffer op-amps in multi pin TO3 style cases - but I doubt if any are still available. The Old NS appnotes include examples of buffering outputs on regular op-amps.

Some very old RCA appnotes describe how to buffer op-amps with 4007 uncommitted MOSFET arrays.

 

Hello,

Perhaps the following articles might help:
https://www.allaboutcircuits.com/te...r-an-op-amp-output-for-higher-current-part-1/
https://www.allaboutcircuits.com/te...r-an-op-amp-output-for-higher-current-part-2/
https://www.allaboutcircuits.com/te...r-an-op-amp-output-for-higher-current-part-3/
https://www.allaboutcircuits.com/te...r-an-op-amp-output-for-higher-current-part-4/

Bertus