AEL

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====== Dual Transformer PSU Setup ======

**__WARNING__: THE FOLLOWING ARTICLE DESCRIBES A CIRCUIT CONNECTED DIRECTLY TO THE MAINS. IF YOU ARE NOT CONFIDENT, OR KNOW EXACTLY WHAT YOU'RE DOING WITH MAINS-POWERED EQUIPMENT - DO NOT ATTEMPT IT.**

If you're using two power transformers in a project, such as a power amplifier (one transformer per amplifier), then it is **not** a good idea to use a single line fuse for both in parallel. Each transformer should have its own line fuse. The job of the fuse is to protect the equipment (and your property) from catching fire if the power transformer's **secondary-side** has a fault current, such as a short-circuit. It's a common misconception that the fuse protects the primary of the transformer; although, it does that, too - but so does the circuit-breaker protecting the wall socket the unit is plugged in to.

However, as discussed later for using my proposed circuit, if the two transformers are not the same VA rating (example, we have one 50VA transformer and a 160VA transformer); if a failure was to happen on the 50VA transformer's **secondary-side**, then the shared line fuse isn't going to do anything. And could result in fire. The manufacturers of transformers state the primary-side fuse rating parameters of them for a **reason**.

===== Dual Transformers of the same VA =====

If two, say 160VA toroidal transformers that require a 3A slow-blow fuse on the primary, were wired in parallel; the in-rush current is effectively doubled, therefore the fuse rating should also be doubled. This is definitely __**not**__ a good idea to do, as it will now take longer for the fuse to blow in a catastrophic failure on the **secondary-side**. We could hope, and pray that the thermal fuse built in to the transformer would open before things catch fire! But, let's not.

The below circuit shows a simple arrangement of connecting two transformers to the line, each with their own fuse (at the correct ratings for the transformers), a power switch and snubber capacitor C1. C1's job is to shunt any "arc-over" from the switch contacts that may occur; and in an amplifier, that can manifest itself as a loud click or pop at the speakers during switch-off. The capacitor should be an X2 class, 250V mains rated device.

{{:dual_transformer:schematic.png}}

**Fig. 1: The complete schematic. "L" is short for "live", "N" is short for neutral and "X1/X2" mean transformer 1/2. "SW" is the contacts of the power switch.**

===== Dual Transformers of different VA ratings =====

This arrangement isn't just limited to two transformers of the same VA rating. We can also use this in a situation where we have a high-current transformer (for the power amplifier(s)), and a lower VA rated transformer for powering the preamplifier(s). The latter having a lower rated line fuse. Can you now see why it's not a good idea to wire two transformers in parallel sharing the same line fuse? 8-)

{{:dual_transformer:no.png}}

**Fig. 2: Example of a destroyed lower value VA transformer with a shared line fuse.**

===== Proposed PCB =====

I did design a quick PCB layout for this (shown below). Why a PCB? It just makes mounting the fuses, etc., more easier than find a way to mount and isolate them from the chassis and using terminal blocks to make connections.

{{:dual_transformer:pcb-3d.png}}

**Fig. 3: 3D render of completed PCB design; if used in a high-powered power amplifier, either 2 ounce copper or 1 ounce tinned is suggested.**