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Topic: creating a controllable tens machine (Read 12346 times) previous topic - next topic

dc42

#15
Apr 11, 2013, 08:13 am Last Edit: Apr 11, 2013, 08:16 am by dc42 Reason: 1

Can I use this one Newark #67R8826 as ferrite-cored as sample to order custom made transformer?


Looks like that transformer uses a RM14 core. I designed mine around a RM10 core (smaller and lighter). The target specs of my transformer were:

Primary inductance 100mH min.
Primary resistance 2.7 ohms max.
Turns ratio between 1:6 and 1:10 (depending on required output current & voltage, which depends on the electrode size)
Secondary resistance 120 ohms (1:6) or 340 ohms (1:10) max

Ideally, the primary inductance would be higher (say 300mH) and the resistances a little lower (say half of the values above). This might be achievable using an RM14 core. A transformer-winding shop should be able to tell you what is achievable.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

sonnyyu

Quote
Turns ratio between 1:6 and 1:10 (depending on required output current & voltage, which depends on the electrode size)

What is ratio for 1.5 Inch x 1.5 Inch Electrode Pads?

Quote
Ideally, the primary inductance would be higher (say 300mH)


I use ferrite cores formula to get turns of Primary.

turns=1000*sqrt(L/Al)
L:mH
Al:mH/1000turns

L=300 mH

Al=6*1000=6000=6 mH/1000 turns

turns=1000*sqrt(300/6)=7071

primary turns is 7071?  something is wrong and very wrong.  I use wrong formula?







dc42


Quote
Turns ratio between 1:6 and 1:10 (depending on required output current & voltage, which depends on the electrode size)

What is ratio for 1.5 Inch x 1.5 Inch Electrode Pads?


My guess is that with pads that large, it would be nearer the 1:6 end, perhaps 1:7. That would give you up to about 70mA @ up to 50V.


Quote
Ideally, the primary inductance would be higher (say 300mH)


I use ferrite cores formula to get turns of Primary.

turns=1000*sqrt(L/Al)
L:mH
Al:mH/1000turns

L=300 mH

Al=6*1000=6000=6 mH/1000 turns

turns=1000*sqrt(300/6)=7071

primary turns is 7071?  something is wrong and very wrong.  I use wrong formula?


L = Al * N^2 where N is the number of turns

For that core, Al = 6uh for 1 turn

So N = sqrt(300000/6) = 224 turns.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

sonnyyu

224 turns. does make sense!

I got that formula from university web site, also author use ARRL Handbook, Chapter 2, American Radio Relay League, 1992. as biased.
http://www.ece.ucsb.edu/~long/ece145b/Lab2_145b/INDUCTOR_w02.pdf
might be I did something wrong?



dc42

The formula on that page is right, however Al is usually quoted in nH or uH for a single turn these days, not for 1000 turns.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

sonnyyu



Quote
Turns ratio between 1:6 and 1:10 (depending on required output current & voltage, which depends on the electrode size)

What is ratio for 1.5 Inch x 1.5 Inch Electrode Pads?

My guess is that with pads that large, it would be nearer the 1:6 end, perhaps 1:7. That would give you up to about 70mA @ up to 50V.


I think I got it,

7/10=(l^2)/(1.5^2)

l^2=7*1.5*1.5/10

l=1.25  Inch

50*10/7=71V

ratio is 1:10 for 1.25 Inch x 1.25 Inch Electrode Pads, That would give me up to about 70mA @ up to 71V.





dc42

The maximum current that can be switched by the chip I used is  500mA. Dividing this by the turns ratio will give you the maximum electrode current. Similarly, the maximum voltage that the chip can supply is about 7V, assuming the supply is a 9V battery that isn't quite fresh. Multiplying 7V by the turns ratio will give you the maximum electrode voltage. That's why I said that 1:7 will give you around 7mA and 50V maximum.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

sonnyyu

#22
Apr 12, 2013, 04:21 am Last Edit: Apr 12, 2013, 04:29 am by sonnyyu Reason: 1
I get ready to contact transformer-winding houses, my friend help me get a petty long list of them.  

Quote

Primary resistance 2.7 ohms max.
Secondary resistance 120 ohms (1:6) or 340 ohms (1:10) max


one of last thing is to meet Primary resistance 2.7 ohms max and  Secondary resistance 120 ohms (1:6) or 340 ohms (1:10) max

I have one of last (hopeful never needed) plan Z;-
Silver bullet wire plan.
Silver   : 6.30×10^7     ? (S/m) at 20 °C
Copper: 5.96×10^7   ? (S/m) at 20 °C
Silver will out perform Copper by 5.7 %

if we are just missing that much, the Silver bullet wire plan is the way to go.

Here is the reason I put it at last and hopeful never needed plan;-

Today Commodities price 4/11/2013
   
Silver price
$27.70/troy ounce

Copper price
$3.43/pound

http://money.cnn.com/data/commodities/

1 troy ounce = 31.1034768 grams
1 pound= 453.592 grams

Silver is outprice Copper by 117.77 time  base by weight.

Here is density table.

Copper density  8.94 g·cm?3
Silver density 10.49 g·cm?3

by spent 11777% to gain 5.7%  might not be smart move.

I was testing one time with silver speaker wire and can not make difference at all but every one else make big one, I guess my ears is NG.

"No Silver Bullet" is not correct, It should be "No Cheap Silver Bullet"

sonnyyu

#23
Apr 12, 2013, 07:24 am Last Edit: Apr 12, 2013, 07:29 am by sonnyyu Reason: 1
Retroplayer, many thanks for your hard work and kindness sharing, I already put your idea for plan B. However since you are concerned liability, I make work-a round by download your incomplete diagram on local machine then upload to Google images search. I got complete diagram from Google, and liability shift to Google. Same reason I will not put my upload Google image search link here. Anyone need it do it yourself.

Dave, You are the man! nothing else I could say.

dc42

btw I'm fairly sure that commercial TENS machines don't have transformer-coupled outputs, because there probably wouldn't be room. I used transformers because it was an easy way of achieving isolation and constant current output. The only machine I got to hook up to an oscilloscope produced pulses of both polarities, and by trying various load resistors I established that the output resistance was around 2K - so neither constant current nor constant voltage when typical loads are considered. The output pulses had noticeable droop under load, suggesting that a capacitor was discharging. So I think that unit was using a flyback converter to charge a capacitor to the required voltage (depending on the intensity setting), and connecting each pair of electrodes via an H-bridge and series resistor(s). This arrangement wouldn't give full isolation between the 2 output channels, but as long as only one channel is pulsed at a time, there wouldn't be any current flow between pairs of electrodes anyway.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Retroplayer

That's the main reason for my hesitation about posting up a complete design for one of these. So many things could go wrong, especially with an experimenter's unit where people can muck with software and do things like turning both electrodes on at the same time. Of course, that type of stuff could be fool-proofed in hardware, maybe even enough to make me feel comfortable posting something, but then the design would get pretty complex. An easy solution is to use resistors in series with the electrodes to limit current, but then that can reduce its overall effectiveness. I bet the cheapy versions of these handle it that way.

The transformer idea is interesting and not one I have messed with before, so I have been sitting by just watching the thread. :)

Lubna

Hello

You mentioned that we can output 80V from the Arduino using PWM. Can you please explain how this can be done ?

oric_dan

OP, good thing you're a doctor, and you're also experimenting on yourself, ;-).
I actually have degrees in electrical and biomedical engineering, but haven't looked
at TENS specifically, but I always step in to warn people off applying large voltages
to their bodies. In your case, you're certified to proceed [!!!].

First, I doubt that Arduinos are anywheres near close to being NIH approved for
use in medical devices, but you would know a lot about the approval process.

Secondly, the orginal ckt that Retroplayer showed indicated it could output up to
80V/150 ohms = 0.533 Amps. That sounds pretty darn deadly to me. I think I would
limit the current a lot more.

Thirdly, 100V and 30mA also sounds very high. Certainly enough to stop a heart,
but at least the current is limited to a safer range - assuming the ckt is designed
properly.


btw I'm fairly sure that commercial TENS machines don't have transformer-coupled outputs, because there probably wouldn't be room. I used transformers because it was an easy way of achieving isolation and constant current output. The only machine I got to hook up to an oscilloscope produced pulses of both polarities, and by trying various load resistors I established that the output resistance was around 2K - so neither constant current nor constant voltage when typical loads are considered. The output pulses had noticeable droop under load, suggesting that a capacitor was discharging. So I think that unit was using a flyback converter to charge a capacitor to the required voltage (depending on the intensity setting), and connecting each pair of electrodes via an H-bridge and series resistor(s). This arrangement wouldn't give full isolation between the 2 output channels, but as long as only one channel is pulsed at a time, there wouldn't be any current flow between pairs of electrodes anyway.

I do not have any experience with flyback converters.  Could you describe how that circuit would work?  Thanks!

dc42

I am not an expert in flyback converters, and I didn't use a flyback converter in the unit I built. You will find lots of information about flyback converters on the web.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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