Unregulated Power PCB

[luvdunhill]

I have a few uses for a simple PCB that supports a number of different ~25VA PCB mount transformers with a bit of filtering on each side of the transformer. I'm trying to nail down the schematic a bit and wanted to see if there was any advice out there Improvements perhaps?

[luvdunhill]

ECO #1 is a properly connected bridge on the bottom half

[JimT]

I would allow for a thermistor like a CL60 on the input of the transformer.

Jim

[luvdunhill]

Originally Posted by JimT

I would allow for a thermistor like a CL60 on the input of the transformer.

Jim

Jim, I could do that, but we're dealing with probably a max of 25VA here. What value would be appropriate in this case? Seems like it wouldn't be much of an issue?

[Sch3mat1c]

Please wire the transformer as CT'd and use one FWB.

[AndrewT]

Hi,
How about adding lots of MOVs or similar across the L to N to limit the voltage spikes from the mains?
Allow for R+C across the diodes. some diodes do not need snubbers, other do need snubbers. Some misbehave badly when a single cap is paralleled across the diode.

Nothing wrong with dual rectifier. Rectifying each secondary and connecting them in series after the first stage smoothing is recommended by many.

Move the Safety Earth/Chassis connection from the output end of the PSU to the common point between the first stage smoothing caps. Join this zero volts point to the PSU output.
Send the PSU output to the main audio ground.

[luvdunhill]

Thanks for the help, first of all!

A few comments:

I've never used a MOV, but the theory seems the same as a thermistor? Would it be possible to incorporate two of the suggestions and put one CL60 thermistor between the secondaries for 240V operation and use two for 120V? Around 10 ohms I'd assume would be fine? If I've assumed incorrectly, and a MOV behaves differently, any recommended part?

I understand moving the ground loop breaker to the first stage smoothing cap, good catch. However, if I connect this zero volt point to the output, I've effectively shorted one of the chokes, and ruining the idea of using a common mode device. Comments?

Finally, in hunting for these common mode devices, some don't list inductance at all in the datasheets, but impedance at a given frequency, DCR and a rated current. Some investigation says not much inductance is really required. I'm looking at this device at the moment and was wondering what you guys think:

http://www.steward.com/web_info/CADP...71B-10-C-2.pdf

It's one of the rare, relatively compact through hole devices... my question is, perhaps I should be looking at a higher impedance device, in the order of 4-5 times larger?

[AndrewT]

MOV metal oxide varistor.
Varistor - Wikipedia, the free encyclopedia
It avalanches when the voltage exceeds it's rating, passing enormous current for a very short time.
Quite different from a Thermistor.

[luvdunhill]

yup, I see that now. I think this will be suitable:

V250LA2P Littelfuse Varistors - MOV

[AndrewT]

Hi,
almost, but the wrong voltage rating.
In the UK we use 275Vac, chosen to exceed our maximum of 254Vac on our 240Vac nominal supply.

I would expect there to be 130/135/140Vac suitable for your lower voltage supply.

John Curl recommends a 10uF cap across the L&N at the distribution board to help with this spike and harmonic attenuation.