[CreativeLab] bought a cheap arbitrary waveform generator and noted that it only had a two-pin power cord. That has its ups and downs. We feel certain the intent was to isolate the internal switching power supply to prevent ground loops through the scope probes or the USB connector. However, it is nice to have all your equipment referencing the same ground. [CreativeLab] agrees, so he decided to do something about it.
Opening the box revealed that there was hardly anything inside. The main board was behind the front panel. There was also the power supply and a USB board. Plus lots of empty space. Some argue the case is made too large to be deceptive, but we prefer to think it was to give you a generous front panel to use. Maybe.
It was a simple matter to ground everything to a new three-pin connector, but that left the problem of the USB port. Luckily, since it was already out on its own board, it was easy to wire in an isolator.
Honestly? We’d have hesitated to do this unless we had made absolutely sure it didn’t pose some safety hazard to “jump over” the switching power supply. They are often isolated for some reason. However, the likelihood is that it is just fine. What do you think? Let us know in the comments.
A similar unit had a reverse engineering project featured on Hackaday many years ago. While these used to be exotic gear, if you don’t mind some limitations, it is very easy to roll your own these days.
Huh. So… how much current flows into the earth pin?
Sometimes, too much. Which is why your microwave has an earth pin. On this world (signal gens, scopes, and other test gear), ideally none at DC, and controlled at signal frequencies.
EArthing the signal ground is a many edged sword. You can float an unearthed output or input, but an unearthed output or input can float, which can reduce noise immunity.
An earthed signal ground can lead to shorts to earth, bu with appropriate protection (GFCI), this can protect equipment from some typed of shorts.
Many other edges here, including loading at frequency, max allowable ground float, and so on..
First of all using blue and brown cables for earth wires is not a good idea, better to use a yellow/green wire to see that is an earth conductor. Now If it’s necessary yo have a ground reference, it’s better to have a ground post and ise it to have a ground reference.
If the SMPS has a capacitive coupling between mains and the rest of the circuit and tis is a concern, maybe adding an insulting trasformer wit a electrostatic shiel between primary and secondary it’s a better idea.
First, I consider this a quite bad video. @01:19 you get a glimpse of the power supply and there seems to be an RF suppression / filtering cap in the lower left corner of the power supply (Next to the transformer, there is even an isolation cutout under (or near) it.
Hackaday’s concern about a “safety hazard to jump over the switching power supply is groundless . :) The Protective Earth terminal is meant to do exactly this: connect the metal parts of a case to GND.
You don’t get a high quality power supply in a function generator in this price range, but apart from the overall quality of this thing, the leakage is normal. It’s not a safety concern for humans, as the current is very low, a handful of uA. This power supply does have input filtering and some electronics on the secondary side, I’ve seen much worse then this, but there is not enough info to judge whether that power supply is of decent enough quality to be safe. Chinese standards are not very high in this regard (Or they are not checked very thoroughly).
When I sit on my chair and touch the tip of my scope probe, I can also see 100V on my Siglent scope. It’s just picked up from the mains wiring all over the house. This is all perfectly normal and harmless to humans, but I do agree that this leakage can damage electronics. You get a similar situation if the PE (GND) pin of your scope is not connected properly (Some of the mains sockets here in the EU accept plugs with PE, but the wall sockets do not have the PE). In that case you can feel a tingling when you touch the metal PC case. It can even be strong enough to hurt a bit, but as long as there are no real faults in the power supply, then it’s still just earth leakage from the input EMI filter and harmless (for humans).
It’s also quite common to modify these function generators and put in a low frequency mains transformer with linear power supply. This has much lower leakage. Usually good enough to keep the whole box floating without an PE connection, and that has it’s advantages too. Always having your function generator grounded is not really ideal.
I will stick with my 45 year old HP…..
Plastic… YUCH!!