I HIGHLY recommend sticking with the exact 3.9 ohm resistance specified in the midistim plans. Those resistors wired up in series on the primary side of the transformer are where most of the magic happens. When their resistance value and the transformer turns ratio are correctly matched, you end up with something close to a constant current output to the electrodes. This means that as the electrical resistance of your body to the stim signal changes, the current (and felt power) remains nearly the same. This is important because your body's resistance can change significantly based on its response to changes in the stim waveform/frequency, as well as things like fluids flowing out of your sensitive bits. This constant current output is exactly what you need for a wonderfully smooth and safe e-stim experience!memma wrote: ↑Wed Jul 15, 2020 12:25 pm I've been scouring the interwebs for the bits I need. Ordering various bits from aliexpress, etc.
Now I've got to the resistors and I noticed the suggested resistors are 10W:or 10/5W in the hand-drawn schematic (OK, so quoting imgaes doesn't work, but they're marked 5.6Ohm 10W and 15Ohm 5W):BoundSquirrel wrote: ↑Thu Jul 02, 2020 3:55 pm Midistim box:
I've put the schematic for the Midistim box here:
Midistim schematic
This is pulled right from the smartstim post and, when followed correctly, works great.but BoundSquirrel's suggested resistors are 25W:I'm guessing 25W was used to reduce heat? Or what's the reason? Is there anything I should bear in mind when selecting my resistor values and wattages? How do they affect the feeling (if such a thing is describable), and do different values relate to anything like body build/fat, or anything like that?BoundSquirrel wrote: ↑Thu Jul 02, 2020 3:55 pm Resistors:
https://www.amazon.com/gp/product/B00NQ ... UTF8&psc=1
These are listed as unavailable, but there are a bunch of alternatives that should work. Just type, "25 watt 3.9ohm resistor" and you'll find plenty of options.
Thanks all!
The one minor drawback of this setup is that those resistors end up eating most of the power your amp puts out, so they tend to get hot. On smartstim, the midistim schematic shows 10 watt 3.6 ohm resistors, the build instructions picture a 3.9 Ohm 25 watt resistor, and the more recently updated recommended parts list shows a 3.9 ohm 15 watt resistor. 25 watt would be preferred IMO. The only potential downside to the higher wattage rating I can think of is that they might be larger and more difficult to work with.
Regardless of resistor watt rating, they will need a heatsink of some sort. Smartstim recommends mounting the resistors to a metal plate is at least ~10" square. I ended up using an old CPU heatsink I had laying around instead. I used a hose clamp to attach the resistors to the heatsink cold plate, and then I used zip ties through the fin stack to hold down the transformers and wiring terminal blocks.
Don't think for second that the resistors "eating the power" results in a weak e-stim signal though; my midistim feels A LOT stronger than my 2B, and the 2B is considered one of if not the most powerful of the commercial e-stim boxes. I have no problem maxing out the 2B on high power setting with the AC adapter plugged in, but I have only made it to max power on my midistim a couple times, and even then it was only because I worked up slowly and was getting desensitized.
