Fuse for 300Va transformer and 6amp capable filter

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So I'm building a power supply to replace the meanwell SMPS bricks sold with each ACA Kit. I've got a board capable of delivering 24 volt 6 amp and was advised to buy a 300 VA transformer with two 24volt secondaries to supply the board and ensure it can deliver the max current required by the board. I have bought IEC sockets with built in fuse holder and switch for 230volt mains. The fuse holders are empty.

What do I need to keep it all safe? Do pick an 8 amp slow blow fuse o allow a margin or 6 amp knowing you'll rarely use that much and it will be a momentary thing only. Fuse holder is for M205 type fuse.


300VA / 230V = 1.3A full load 2A seems about the highest value that's reasonable, but due to switch on current surge it needs to be slow-blow and might need to be a bit larger if switch-on surge keeps blowing it.
The primary fuse protects the transformer, so its based on the transformer rating and primary only, nothing to do with the secondaries.
A thermal fuse buried in the transformer windings is very useful to have too, some some transformer faults (internal shorts) can lead to the transformer becoming much less efficient, and thus dissipating too much heat even when the primary current is within bounds.

A typical 300VA transformer will dissipate perhaps 25W at full load.

Secondary fuses can be added, although normally a short on a secondary will blow the primary fuse reliably if the power rating of the secondary is a sizable fraction of the total transformer rating.
Transformer is 300va Toroidy with two 24 volt secondaries. Dual output from the sellars Audio filter/rectifier get tied together to enable 6 amps max. Two 24 volt secondary is the recommendation of the filter supplier - too expensive to take guesses on. Some power supplies on eBay step up the voltage and others step it down. Doesn’t always follow that you need lower volt secondaries. I’ll get 3 and 4 amp fuses. Being class A means current pull should be fairly constant should’t It?
I see, it's a regulated power supply which drops ~5V. Have others been successful with this type of supply?

Yep see here:


I'm building two seperate supplies one for each ACA and putting them in the same chassis as the ACA with a big hole in the top for the Toroidy to show through. Toroid base plate and internal space in the chassis both 150mm so I might need to open the holes on the ACA chassis brackets by the tiniest fraction each side to get it all to fit!!! Not sure yet if i'll be able to bolt the supply transistors direct to the chassis base plate or if i'll need slightly thicker strips of Aluminium to act as a heat pipe to the sides - I can get hold of 3mm x 25mm strips at local DIY store.
The switch in your combo IEC/fuse/switch is likely to be the weak link with time. It has a 6A 250VAC rating (if it is a KC01-104), but I'll bet my bottom dollar that is for AC1 resistive load switching (which is the kindest AC load to switch). Given its a toroidal power transformer, the in-rush primary current is highly likely to be more than 10x the VA rating, and possibly up to 20x for the first peak. That puts the first current peak at least above 10A and possibly upwards of 20A.

You are probably using a substantial rectifier and capacitor filtering, which although it may not increase the initial inrush peak, may well extenuate the inrush duration.

That's not to say the switch would fail soon, but just saying you may get switch contact issues sooner rather than later.
IEC Switch is 10amp:

IEC Fuse Chassis Male Power Plug with Switch | Jaycar Electronics New Zealand

Fingers crossed this all comes together. I'm planning to bolt the power supply to the top of ACA on standoffs for some ventilation as long as it doesn't cause problems. Then the base of the ACA needs bolting to the shelf it sits on. We've not had a shake bigger than a 3 for a while in Wellington but thankfully our house is on solid conc foundation not wobbly poles!

Great tip :)

Solves the problem of how to make handed power supplies with the switches the same way up!

But yep that's a bugger the Switch says 10amp 125v and 6(2) amp at 250v not sure what what the bracket with the 2 is for. May have to find a better switch after a while then - at least I know now that it can be swapped out.

Cheers for that. Getting some nice warm weather from Oz at the moment. Enjoy the rest of your weekend and thanks.
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