tDCS – Does Ripple Voltage Matter?

For those seeking general information on tDCS, see my other posts below. This one is for the geeks :-)

I’ve been tinkering with tDCS for several months now and have had opportunity to try home-built and commercial tDCS devices. In recent weeks there has been a lot of chatter on the tDCS blogs and sites about using the ActivaDose II as a tDCS device. Intended for iontophoresis, it’s ability to deliver very low current for a fixed amount of time seems to fit the bill for tDCS. It can be purchased on the web for less than $300, making it one of the lowest cost ways to get a tDCS capable device.

I had an opportunity to try an ActivaDose II and noticed it seemed to cause far more phosphenes than devices I have built using the LM334 regulator. The phosphenes were subtle (sometimes visible with eyes closed only), but in some cases occurred several times per second.

I wondered what this was happening and if it matters for overall treatment effectiveness. I decided to ‘scope the output of the ActivaDose II and my homebuilt LM334 device to see what that might reveal.

I set both devices to the 2ma level and attached scope probes to the electrodes. The images below show two important things:

1. Overall ripple on the DC voltage supplied to the electrodes
2. The wave-form generated by pressing the electrodes together and releasing them – causing the voltage regulator to adjust voltage for the changed resistance.

20121229-211214.jpg
(Output from my home-built LM 334 device. Approx 50mv overall ripple, smooth response to need to decrease then increase voltage.)

20121229-211223.jpg
(Output from ActivaDose II. Approx 250mv overall ripple, more ragged response to need to decrease then increase voltage.)

The level of ripple from the ActivaDose II surprised me. I’d love someone else to ‘scope the output of some tDCS devices to check my observations and to provide additional comment.

I think these questions are worth asking:

1. Does ripple on the DC voltage applied to the scalp matter?
2. How much ripple is ok?
3. What about ripple frequency?
4. Are harsh (fast) changes in voltage of concern?
5. Do harsh changes or ripple affect variability in treatment (if different devices are used?)
6. Should tDCS device manufacturers specify ripple (or other noise) voltage in their specs?

Again, I dug into this out of curiosity about all the phosphenes I saw with the ActivaDose II. I don’t think they are caused by ripple, but rather the harsher, faster changes in voltage as the units electronics responds to scalp resistance changes (as compared to the LM334). Maybe that is a concern?

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5 comments on “tDCS – Does Ripple Voltage Matter?

  1. I’m wondering what the best low-cost equipment to be able to do these tests would be.

    An MIT EE friend suggested the NI myDAQ which was used in one of his courses.

    SparkFun also seems to carry something called the USB Oscilloscope – MSO-19 that looks like it might work.

    Both are around $250.

    My friend said there are ones more around $99 that are “crap”, but I’d just be buying to do tDCS device testing, so “crap” would be fine as long as it would work for that purpose.

    • Any good oscilloscope should be able to display the “quality” of the DC delivered as part of a tDCS session. Also, there are several USB scopes that use a PC as the display device now available. Again, any moderately priced unit should easily have the bandwidth and features required.

      Brent

  2. I believe you should be able to smooth any DC signal to whatever degree you like by applying the correct capacitor.

    http://www.kennethkuhn.com/students/ee351/power_supplies_filter_capacitor.pdf << shows how to calculate correct capacitor.

    http://www.bcae1.com/capacitr.htm << scroll to "with / without capacitor" diagrams. shows simply wired in parallel.

    Its common practice in car audio, to smooth out ruff DC signal produced by the alternator… You can hear how well it works by starting your car, tuning to your favorite AM station, then disconnect the battery.. if you then rev the engine you will be able to hear the noise… plug the battery(capacitor) back in and it goes away.

    Is there some reason this is not applied to these devices ? Perhaps the cost of the correct cap is high ?

    • Maybe because putting a capacitor across the output poses an obvious hazard… i.e. the capacitor becoming charged prior to you connecting to the electrodes, then discharging through your head. That’ll get your attention ;-)

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