SMPS and O-SCOPES

[loudthud]

In the past, SMPS's made for sale in the USA used a Voltage doubler on the incoming AC in the 120VAC mode. Since one side of the incoming AC was grounded at the AC Breaker panel, that created the +/- 170V DC. The inverter transistor(s) of the SMPS pulled current from +170 to -170 . By a clever arrangement, a full wave bridge and a SPST switch were used such that in "120V" mode, the circuit formed a Voltage doubler, in "240V" mode, the full wave bridge was used normally and the incoming 240VAC was rectified to about 340VDC. The inverter circuit ran off of essentially the same DC Voltage in both modes.

Many modern day SMPS's don't have the switch to change from 120VAC to 240VAC. All you need to do is connect the right line cord. The inverter has been designed to run off of a wider range of DC Voltages internally, at high efficiency. The exception may be high power designs.

[loudthud]

The SMPS really became popular when IBM PC's (and clones) started to proliferate, and there were lots of dead power supplies laying around. They were cheap enough that people just replaced them, possibly with an upgrade to the power level. I ran into a guy who had a simple method that could fix 90% of them. All you did was watch the fan when you switched the supply on. If the fan didn't move, the fuse was probably blown and you replaced the inverter transistors. If the (12V) fan kicked off and then stopped, you replaced the rectifiers on the 5V supply.

[Enzo]

There are many "universal" SMPS designs around now that run on anything from 90vAC to 250vAC. NO switching, jumpering, reconfiguring. Like LT says, just use the right power cord.

[GainFreak]

I'm not into SMPS but I was wondering how safe will be to use one of those USB isolated PC scopes like this one for example:

https://www.amazon.com/Owon-VDS1022I.../dp/B00HC4UP52

[Enzo]

USB isn't isolation, there is still a ground connection from the computer. You really need an isolation transformer for the unit under test.

[GainFreak]

The USB 1.1 interface also allows the USB isolation, which is performed by an ADUM3160. USB power is isolated by a TI DCP020505 2W isolated DC-DC converter.

Isn't this USB isolation from PC power and ground?

Schematic:

https://www.eevblog.com/forum/testge...h=900212;image

[Enzo]

Unless I misread the schematic, I see input jacks with grounded shields, and I see USB port with the same ground for power.

[GainFreak]

There are two versions of this PC scope. One is isolated (VDS1022I) the other is not (VDS1022). From what I see the schematic shows both version. For the isolated one USB port goes to the ADUM3160 which isolates the USB +/- and ground.

https://www.analog.com/en/products/adum3160.html

DCP020505 isolates the positive supply:

https://www.ti.com/product/DCP020505

That means no direct connection with the USB port.

[loudthud]

If you dig into the specs on the TI part is says:" Continuous voltage applied across isolation barrier: 60 VDC / 42.5 VAC "

Not something you should trust in a lethal hazard situation.

[Enzo]

That isolates the positive supply, but doesn't the ground though connection remain?

[GainFreak]

Not something you should trust in a lethal hazard situation.

The scope has a 400Vpp rating (1:10) so I guess it was tested to work fine in that range.

That isolates the positive supply, but doesn't the ground though connection remain?

No. I have two cheap PC scopes and this is one of them. When I touch an amp chassis with the unisolated one it creates ground loop and hum.When I touch it with the isolated one no loops, no hum.
This was also checked with a DMM - no continuity between probes' ground and USB cable/PC chassis and or AC line ground.