Just my 2 cents on the ET232 and 312BT
Historically, there are 2 main types of estim units:
- Vintage/Classic - these are based on a transformer (or inductor for the really cheap ones). The low voltage winding is controlled using transistor switching and a Mosfet for current control. The high-voltage winding goes to the genitals.
Examples: ET312 (both old and new), StereoStim, plenty of older gen medical tens.
Disadvantages include: the energy stored in the transformer magnetic core is what drives the output. If the transformer is too small, the result is disappointing (it saturates). Also, there are non-linearities in the transfer function. Limitations for complex waveforms. Also pretty heavy and power-hungry.
Advantages: Robust; if well-made and well programmed, it is a memorable experience.
- New Style/modern - there is a variable high-voltage supply which can raise voltage to 100-120V. This feeds the channel switchers that then go to the genitals
Disadvantages: basically none; but there is a hitch - see below
Advantages: it can deliver precisely measured current; can easily cope with complex waveforms. Both Voltage and Current are controlled. However, it requires more detailed programming because it's so flexible. Much more efficient and very light construction.
Example: Most of the new TENS units.
Usually, the new style runs on 3v to 6v (2-4) batteries.
The 312 was cloned/open-sourced by the community and schematics, boms and pcbs are available. You can either buy completed units or build your own. The Big thing about the 312 is its programming - it is outstanding and to this day the gold standard.
Below is a review of the ET232 including pulse oscilloscope captures.
https://www.cpony.com/home/reviews/erostek-et232.html
I would guess by the graphs that the ET232 is new-style however the hi-voltage supply is sagging under load (possibly due to limited capacity of 9v battery)... The pulse is reminiscent of transformer output.
You can also see that stimulation voltage varies between circa 25V to full 100V.