40 V Op Amp for 200mks pulses

Discussion in 'The Projects Forum' started by alexv, Mar 27, 2011.

Jul 7, 2009
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Hi,
I need to amplify biphasic 100-200 mks pulses modulated at 5-40Hz generated by sound card (V=0.5-1). I need to get out up to 100mA into 500 ohm @40V. I currently use 200W power amp and it works but Im looking for more intelligent, cheap and compact solution. Can you recommend me an Op Amp that can do this job?
Thank you for help.
Alex

2. SgtWookie Expert

Jul 17, 2007
22,183
1,728
So, you have two outputs. Stereo? Meaning you're using both the left and right channels?

What do you mean by "mks"? Microseconds, aka uS?
Are these basically square waves?

How modulated? In frequency, pulse repetition time, amplitude?
OK, you're using the line level output(s).
The math doesn't work. It would take 50V to cause a 100mA current through a 500 Ohm load. E=IR, so 50v = 100mA * 500 Ohms.

It's not going to be a single opamp, nor a single audio amp.

Meanwhile, it would be helpful for you to show an O-scope plot of your sound card output(s).

Jul 7, 2009
14
0
So, you have two outputs. Stereo? Meaning you're using both the left and right channels?

one channel

What do you mean by "mks"? Microseconds, aka uS?
Are these basically square waves?

uS,
bipolar square waves

How modulated? In frequency, pulse repetition time, amplitude?

modulated in pulse repetition time

OK, you're using the line level output(s).

The math doesn't work. It would take 50V to cause a 100mA current through a 500 Ohm load. E=IR, so 50v = 100mA * 500 Ohms.

500ohm is a max value, the load is the skin - resistive (non -linear decrease of R with voltage increase) and capacitive in series

It's not going to be a single opamp, nor a single audio amp.

Meanwhile, it would be helpful for you to show an O-scope plot of your sound card output(s).

Sorry, I dont have O-scope, but typical signal with constant amplitude and frequency

on cycle: 100us pulses@10Hz repetition 1s, off cycle 2s, duration 5min

then

200us pulses@20Hz repetition 5s, off cycle 5s, duration 10min

in some cases repetition time is modulated 1-10S

Thanks

4. SgtWookie Expert

Jul 17, 2007
22,183
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What exactly do you mean by the bolded portion?
You say "the skin" - is that human skin, on a persons' body?

Is this some kind of Rife device?

5. beenthere Retired Moderator

Apr 20, 2004
15,815
283

Under what circumstances do you apply stimulation to these skin electrodes? Have you any means of limiting current to medically safe levels? Is the power amp isolated such that an internal failure cannot send lethal current through the subject? Are you aware that 40 volts is way more than enough to drive lethal current in the body? 100 ma is far above the level of current that can stop the heart.

What is the purpose of this? Is this done under any supervision? Do the subjects know they can easily be killed?

Jul 7, 2009
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human skin, it's a kind of neuro muscular stimulator

7. SgtWookie Expert

Jul 17, 2007
22,183
1,728
That makes it a medical-type device, with dire safety consequences, and unfortunately places it out of the scope of the forums. Safety is the #1 rule here.

Jul 7, 2009
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40V and 80ma @100 ohm are typical characteristics of TENS and EMS devices that you can buy for home use. keep in mind that pulses are very short.

in my isetup current is limited to 100ma. Subject is myself and postion of electrodes makes it impossible to get killed.

Dec 26, 2010
2,147
300
Tens devices are usually operated from self-contained batteries, or otherwise are galvanically isolated from other circuits, including mains ground.

Unless you can guarantee that this will be the case with your set-up, a casual contact between ground and some body part outside the intended circuit could have serious consequences.

10. beenthere Retired Moderator

Apr 20, 2004
15,815
283
There appears to be no attempt to follow medical practice to prevent harm to the subject under all conditions.

Without an oscilloscope, how are you able to determine you are placing 40 volts and 100 ma to the electrodes? Are you certain that, when applying stimulus to one arm/leg, contacting a grounded wire or case with the other arm won't allow current through your heart?

Jul 7, 2009
14
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I actually hoped to get an advice here on op amp that I can use for my application and that can run from 9V battery.

Dec 26, 2010
2,147
300
If the amplifier works from a 9V battery, it will need either to employ some kind of step-up converter to produce higher DC voltages, or else use a step-up output transformer to boost the output voltage.

Either way, the battery life is likely to be extremely short, as by my reckoning the output power is about 5 Watts. A PP3 style battery won't do at all, you would need something like half a dozen AA cells, or even bigger.

And how were you planning to couple the input signal? Note that this would need to be isolated, unless you use an isolating transformer for the output.

Jul 7, 2009
14
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I realize it would need an output transformer. From my experience 9V battery in TENS device normally lasts for more that 6 months. I dont think I need to isolate the input since I use laptop which is run by 12V battery, but since I will use step up transformet it will be isolated anyway.