FeelTech FY3200 PSU upgrade

[ledtester]

I have a question about the original power calculations used to size the transformer...

The output power needed for the 5V supply is 5V * 250 ma = 1.25 W. Assuming an 80% efficient buck converter means that about 1.6W are needed from the transformer. However, this power is only coming from one of the windings in the 15-0-15 transformer, so when sizing it we need to double this amount even though that power from the other winding won't ever be used.

The requirements for the +/- 12V supplies has been computed to be another 1.6 VA each. So the total VA required from the transformer is 4*1.6 = 6.4 VA.

Is this correct?

[Bob Sava]

The point of using a transformer is to be able to use linear regulators and avoid EMI and ripple from  SMPS.  Adding a buck converter for 5V output seems to defeat that goal.

[DSE_Kid]

Hey guys, I got my FY3224 almost a year ago and after stumbling on this post I decided I should test it to see if it had this leakage problem. I found that I was getting approx 3.4vac mains earth to output ground channel A, which is thankfully well within limits, so I opened the unit up and discovered that the power supply board was a different design. My real concern however came about when I took a voltage reading between the USB signal ground and computer and found a significant 85 VDC. I ended up ordering a cheap USB Cat5 extender which I'm hoping will have some sort of optocoupler built in to provide that isolation, otherwise I'll have to pony up the cash for a proper USB isolator.

[RovDan]

Hi everybody,

I Just bought a FeelTech FY3224S. I read many messages about the leaking risks because there's no wire to ground.

I used a DVM between BNC sockets and ground. I read 1V AC. So my question : do you think the manufacter modified the supply to avoid any link between the power (230V here), and the sockets ?

In fact, if really necessary, there's 2 ways to fix it : doing a DIY power unit +/-12V +5V  or use a separating transformer. I think the transformer would be better.

Can a guy explain me what is the use of the DC mode I see nothing on my scope in this mode !

Many thanks for your advices.

RovDan

[tom66]

I have a question about the original power calculations used to size the transformer...

The output power needed for the 5V supply is 5V * 250 ma = 1.25 W. Assuming an 80% efficient buck converter means that about 1.6W are needed from the transformer. However, this power is only coming from one of the windings in the 15-0-15 transformer, so when sizing it we need to double this amount even though that power from the other winding won't ever be used.

The requirements for the +/- 12V supplies has been computed to be another 1.6 VA each. So the total VA required from the transformer is 4*1.6 = 6.4 VA.

Is this correct?

Cross loading a transformer doesn't double the load requirement.  You might get a difference in voltage between the windings because of the crossload, but you only need to consider the per-winding rating and the total rating of thr transformer.