General > General Technical Chat
Class 2 SMPS - Removing the Y capacitor - Good design solution wanted !
shakalnokturn:
Should it actually be called a "Y capacitor" in this case (primary to secondary on a class 2 SMPS)?
Briain:
Hi
I've always called it Y when it sits that position (as it would have to be one rated as being a Y capacitor), but it is entirely possible (possibly even probable) that I've always been entirely wrong. ;)
Anyhow, I fitted my new transformer, though as it was physically smaller (and thus lighter), I replaced the hardboard load spreading sheet (shown below the transformer in the photograph attached to my previous post) with two sheets of lead flashing material (and the added mass is very effective).
As to the performance, whereas the Triad N-68x ran at 40 deg C off-load (with the plastic case wide open) and >50 deg C with it on light load (i.e. the FY-3224s running, but not with terminations on the outputs) and with the case closed, the Bel transformer runs at just 31 deg C with the case open (and off-load) and just 38 deg C with the case closed (same load as described for the Triad experiment) so I am delighted with that result. In both cases, the temperatures were taken after > 1hour running time and ambient room temperature was 20 deg C.
With a high impedance meter (a Fluke DMM) measuring between the BNC and mains ground, the indicated Voltage was 3.2 V (actually, it only dropped by 0.2 V with the mains switched off; I added a switch -the white one in the back panel - to the primary side). As I had an ESP strap sitting right next to me (which contains a total of 4 M Ohms resistors) as an experiment I used that to link the BNC outer to mains ground and the Voltage dropped to 1.2 V, so I'd call that a great result. Pondering it further, I think I'll actually fit a 4 M Ohm resistor across the switch; that effectively still leaves the floating option, but it provides a path to prevent any chance of static build up (unlikely, but it would do no harm having it in there).
The one thing I would note about that particular variant of the transformer is that it really should have insulated guards to keep fingers separated from the dangerous bits. As can be seen in the photo, I have mitigated the risk by using heat shrink sleeves (and also stubs of small heat shrink on the exposed pins - which run at 90 deg to the main spade terminals - to which the transformer winding wires are terminated) but after ordering the above transformer, I noticed the below comment in the specification sheet:
--- Quote from: The Bel specification sheet ---Note: The VDE certified A41 series transformers with standard length and colour lead wires are readily available. See website for our AF1-L product series
--- End quote ---
If I'd spotted that the first time I'd read the specification sheet, that's the version that I'd have used (assuming that it is readily available, of course).
All the best
Briain
smile:
So can anyone actually can take a scope and measure this NOISE, ripple increase on PSU output wires, before and after Y caps removed? I would do it myself but I do not have a scope.
thank you.
T3sl4co1l:
--- Quote from: smile on September 15, 2023, 06:58:02 pm ---So can anyone actually can take a scope and measure this NOISE, ripple increase on PSU output wires, before and after Y caps removed? I would do it myself but I do not have a scope.
thank you.
--- End quote ---
Obligatory,
What PSU and what Y cap(s)? You will find the answer depends on everything.
You also need to specify measurement method. A 20MHz differential ripple measurement might not show much if anything different. The noise is largely common mode, much more pernicious, and requires a different measurement method.
Tim
smile:
Well, then can somebody take a look at this schematic https://ab57.ru/schema/LiteOn-PA-1121-04.pdf
This if for Laptop PSU from Acer. It has no double square symbol, yet is it isoloated, no Y caps.
Can this be used as reference to modify Meanwell PSU? Like the RPS-300 27v or LRS-350 48V?
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