How to get the fuse rating you need for a DC power supply

[George317]

building a DC supply with a load voltage of 3.3V and load current of 3.3mA.

all the books and schematics I'm seeing is that they're putting the fuse in the primary of the transformer, why?

if i were to fuse the primary as well how do i get the rating of the fuse that i'll need? its said in my book that Ifuse=Pload/Vrmspri

but if i do that then I = 3.3mA(3.3V) / 220 = 49.6uA(in my country we use 220Vrms), isn't that quite small of a current rating for the fuse considering I'm going to put it in the primary of the transformer which has super high voltage.

also side question what is the ampere rating that i see in the secondary of my step down transformer? in my transfoer there's 12V -0-12V then above it there's 1 Ampere written. does that mean my transformer produces 1A in the secondary?

tnx

 

[.Scott]

You put a fuse on the power input so that if your device shorts or catches fire, it will not draw too much power.
So select a fuse that will keep the current low enough to avoid drawing more power than you can safely dissipate as heat. I would be thinking about a 100mA - but you know more about your device.
You can also fuse or current-limit the output.

 

[Averagesupernova]

Little story here to share. I was involved with an amateur radio club some years ago. Well known to most hams are the Astron linear power supplies. They are pretty basic. Power transformer, rectifier bridge, large filter capacitors and a regulator that is the well known 723 IC drives the pass transistor(s). This club had several of them and someone had replaced the primary side fuse on one of them with something considerably larger than it should have been.
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Also on these supplies is an SCR across the output that fires and shorts the output in the event of a regulator failure (overvoltage on the output). This will do one of two things. Either the supply will go into current limit and go along safely, or it will draw enough current to blow the primary side fuse. Guess what? For whatever reason this SCR fired. I have seen this countless times and it is often caused by a transient or something of this nature. The fix is to simply power cycle the supply. Well in this case, for whatever reason, the current limit did not work and the fuse was sized too large to blow. If we were talking about an ideal transformer I'm sure the fuse would have blown but this was not the case. The transformer was simply not capable of passing the amount of power required to blow the over sized fuse. Damaged components were: Rectifiers, SCR, regulator, pass transistors (4), and I can't remember for sure but I think the driver transistor as well. And I am pretty sure that the transformer got really really hot. Chances are that the regulator failed as a proper operating supply would have gone into current limit, but this one did not and its just lucky nothing got hot enough to start a fire as this was used at a remote site unmanned and probably cooked this way for a couple of days.

 

[Asymptotic]

also side question what is the ampere rating that i see in the secondary of my step down transformer? in my transfoer there's 12V -0-12V then above it there's 1 Ampere written. does that mean my transformer produces 1A in the secondary?

How many secondary wires? (12-0-12) sounds like a center-tapped 24V transformer.Full load rated secondary current would be 1 amp.

Each 12V coil would then be rated 12VA (12V * 1A), and 24 VA (24V * 1A) for total secondary power rating. Figure primary fuse size for the 24 VA power rating. I=P/V = 24/220 = 0.11 amps.

Select a slow blow fuse rated near to the 0.11A value. Inrush current will clear a closely rated fast blow fuse.

If the transformer won't intentionally be operated near the power limit then a 0.1A fuse is a good choice, else use the next available higher rating, which may be 1/8th amp (0.125A). In the first case, the fuse is rated at 91% of transformer capacity (0.10/0.11 amp), and 0.125/0.11 (114%) in the second case.

all the books and schematics I'm seeing is that they're putting the fuse in the primary of the transformer, why?

Overloads and short circuits originating on the secondary side of the transformer can be protected against by fusing the secondary. However, what happens if the problem is on the primary side such as a turn-to-turn short, or short to case? Lots of heat, smoke, and possibly fire without circuit protection, and why fusing is typically located in the primary side. It isn't unusual in industrial settings to see both primary and secondary separately fused with primary fuse sized to protect upstream wiring and primary coil full load rating, and secondary fuse sized to more closely protect the downstream load.