FeelTech FY3224S Grounding Modification

I have one of those cheap FeelTech FY3224S  (FY3200S 24Mhz version) function generators. Sometimes sold under different brand names, including “Moo Hoo” and no-name at all, and sold by  “banggood” (not making that up!), amazon, and other online stores.

There is an extensive thread about these on eevblog.com that includes this post about getting a shock from the device:

I was just using my Feeltech FY3224S and felt something biting me…the culprits turned out to be electrons…I was getting a shock.  When I measured AC voltage with a multi-meter from any of the ground points on the Feeltech (e.g. the outside of the BNC connectors) to AC ground, I had around 19vrms

Here’s another blog referencing this same problem: http://www.dalbert.net/?p=322. He measured 82V peak-to-peak. On my device I measured 45V rms or so with nothing connected to the outputs, and measuring between the BNC grounds and earth ground. As all of these write-ups point out, there isn’t enough current to be dangerous; you “just” get a tingle. The problem is caused by the use of a switched-mode power supply not properly implemented for use with floating DC outputs (which this device has).

The best, but most complex, fix is to rip out the switched-mode power supply and replace it with a linear supply suited for floating-DC output configurations.

An easier solution, which many others have also done, is simply to tie the DC grounds to earth ground. In other words, don’t let the DC outputs float. I decided to do that, with a switch enabling me to go back to the original (floating) configuration if ever needed for some specific reason.

The eevblog thread is full of examples of people doing exactly what I did, so it’s not anything new. I’m just documenting it here on the assumption someone might find it useful anyway.

The original back of my generator looked like this:

You will note that the A/C input is two-prong and is not polarized.

I had a so-called “mickey-mouse” (C5/C6) power inlet that doesn’t take up much more space than the original two-prong inlet. I enlarged the opening as necessary with a rasp to accommodate the three-prong inlet. I also had a suitably-sized SPDT round rocker switch (SPST would have sufficed) and mounted it as shown below:

 

The idea of mounting it there is that the ground symbol already present serves as the label for the down-position of the switch; I wired the switch so that when it was down the BNC grounds would in fact be grounded to the earth ground. If you are wondering why the C5/C6 connector is sometimes called “mickey mouse” take a closer look at the above picture and you should be able to figure it out.

This picture shows the inside wiring:

I added the green wires going from the ground on the power inlet to one side of the SPDT switch, and from the center (pole) to the ground lugs on the back-side BNC connectors. But what about the front connectors? Well, all the DC grounds on this device are all connected together, so grounding these back here grounds them all. Obviously, the same observation leads to the conclusion that I did not need to tie both ground lugs together back here; just connecting to one or the other would have been sufficient. However, these two connectors are hooked up to the main board by two separate wire assemblies each with its own separate plug/jack, so by wiring both grounds here the grounding will still be effective even if one of those plugs works its way loose someday. But, realistically, that green connection between the two BNC ground lugs is superfluous.

In the original configuration the input power was not polarized; consequently sometimes the front panel switch was interrupting the hot A/C lead and sometimes it was interrupting the neutral A/C lead, depending on how you plugged the unit into the wall. A three-prong plug is obviously inherently polarized, and I made sure to hook up the power inlet such that the hot side went to the switch so that the input power would be fully cut off at the switch when the device is off (vs the circuit being interrupted only on the return/neutral leg).

I buzzed out the connections to make sure I knew which one was which:

This shows that the connections, when viewed from the back of the mickey-mouse connector, match up with the connections when viewing the plug face-on (the picture shows the not-connected configuration). From there I looked up which prong in an outlet was hot vs neutral. I was reasonably certain I knew this but looked it up again anyway. I carefully labeled and checked my approach 17 times to make sure I wasn’t confusing myself between the “outlet left/right” view and what I would see when soldering the back of the connector.

Obligatory safety disclaimer: don’t try any of this if you aren’t knowledgeable and skilled with 110VAC circuits. I’m not even going to tell you which one of the prongs is hot vs neutral because if you need me to tell you that, you probably shouldn’t be doing this!

Once I wired up the 110V inputs everything was ready to go back together. Here is is all buttoned up:

I used my label maker with a black-on-clear cartridge to add the FLOAT label at the top of the switch. The ground symbol already there serves adequately as a reminder for the other position. I didn’t quite get the FLOAT label lined up exactly right. I could fix it, but the switch is going to stay in the “ground” position 99.99% of the time, and all this is in the back of the unit, and only some of my overly OCD friends will notice or care. It stays as is.

With everything buttoned back up I tested the grounding:

This is showing 600 microvolts with the rear switch in the ground position. I should mention that the other multimeter lead was hooked up to a convenient ground elsewhere in my lab set up and that ground was coming from a different wall outlet. Many of my circuits are “home run” back to the panel so there might in fact have been a hundred feet (or more) of romex between these two ground connections. So a non-zero ground potential difference doesn’t surprise me, if we consider “600 microvolts” to be “non-zero” (and not a meter artifact either).

In the original, floating, configuration we get 48 volts:

That will tingle! Obviously the switch will usually be left in the grounded position and if I need a floating function generator I’ll just have to be careful, or spring for a “real” piece of kit instead of this $60 cheap, but rather useful, hack piece of equipment.

One last point, as mentioned in the eevblog threads and elsewhere. The USB port on this device is not ground-isolated. So if you want to float the device (the original configuration), AND you have a computer plugged into the USB port, AND if your computer is grounded (which it won’t be if you have a laptop running off a battery), then the USB ground will de-float the generator output. I suppose it’s also possible that if your computer is floating then the 45 volt “tingle” might make it to your laptop? Ick. In any case, it’s something to be aware of.

I did buy a USB isolator board from Adafruit. This can be used externally if I ever need to float this generator AND have the USB hooked up; alternatively I may explore permanently installing it into the device on general principles. The primary use for the USB port is for defining custom waveforms, which is something I don’t have any immediate need to do. So for now the USB isolation goes onto the “to-do” list and in the meantime I’ve got a function generator that will no longer “tingle” me unless I want it to.

Update: FY2300H

There is another version of the generator out now, the FY2300H (model numbers go backwards? lol!). A 60MHz version is more expensive than my 24MHz version: $330 vs around $90 for mine, but also obviously can provide faster waveforms. Here’s one at Amazon: https://www.amazon.com/Kuman-Generator-Arbitrary-Intuitive-Interface/dp/B06ZY2958F/

They seem to be available at varying prices for other speeds at AliExpress. The cheapest is $80 for a 6MHz version, and I found links for $130 for a 25MHz version though they were out of stock as I write this.

Note this interesting broken-english description:

With the new design of power supply, the utility model eliminates the disadvantage of small amplitude signal interference of the power supply of the hand-held instrument. (10mV small signal still has the perfect signal feature)

and as you can see from the pictures, it has an external wall-wart power supply. Presumably they provide one that is implemented properly and thus fixes the AC mains leakage problem of the power supply built into the FY3200 series. If I didn’t already have my other unit I’d probably buy one of these, even at the higher price (which will likely come down over time if you wait) rather than perform the modifications shown here, especially since that would give me a generator that could DC-float without AC mains leakage whereas the grounding modification only fixes the leakage when you aren’t floating the generator outputs.

Leave a Reply

Your email address will not be published. Required fields are marked *