Step-down Isolating Transformer with Soft-Starter and DC Blocker

I purchased this “2000W” isolating transformer in Hong Kong in about 2003.  It has a large encased Japanese EMI/RF filter and a very heavy 2kVA British toroidal isolating transformer and was set up 1:1 (220V in – 220V out) for the local market.  It was basically unsafe as anything connected to it is not protected by the safety switch at the meter box.  It was subsequently modified to a safe autotransformer using a smaller toroidal, but I will leave the page here anyway.

The photos show it with a few interim modifications:

It has a chassis fuse on the neutral but relied on a UK-style plug fuse for the active, so I added an internal fuse (seen above – middle/bottom) when changing the plug for an Australian type.

This is the back with an array of US-style output sockets:

The toroidal transformer has numerous taps for various primary and secondary voltages and straps could be added or removed from the long terminal block to give it a 240V output on 220V mains if desired.  By swapping a few connectors it could be “back-fed” to give a 220V output on 240V mains and this would be very useful for powering a pair of 220V C-J Premier 8 mono blocks in Australia.  Past experience told me that some 220V equipment does not like 240V mains voltage and my house voltage actually measures 250!

It worked, but I found that every other time it was switched on, the 20 Amp circuit breaker in my meter box tripped.  I then learned about a nasty thing that large toroidal transformers do and that’s draw high “inrush current” on initial power-up.  There were several “soft start” devices on the market to address this and the best option for me was an ESP project.  Most of the others at the time could not be adapted to accommodate a 2 kVA toroidal as they were limited to around 500 VA.  Here is the soft start and its dedicated 9V transformer which powers its relay triggering circuit.  A bank of ballast resistors is at the left of the two relays:

The recommended second relay trigger timing was only 100 milliseconds which was too quick to stop the circuit breaker from tripping, so I increased the timing to about a second by changing a capacitor.  I also doubled the recommended number of ballast resistors so that they didn’t blow up or overheat during this extended lag time and also soldered strips of wire over the relevant tracks of the PCB (under the resistors) to handle the high surge current.  It has worked perfectly now for over 5 years.

Just for fun and learning, I installed a “DC trap” in front of the toroidal so that any DC on the mains could not get in and cause the toroidal to vibrate in protest.  I found a circuit on a Swedish web site and tried it.  It comprised a couple of large electrolytic capacitors back-to-back and lots of diodes.  It worked for about 5 years before I noticed some transformer vibration and noise when it was under load.  I put that down to old age and maybe the transformer was just getting a bit tired, but no.  I tried mounting it on thick neoprene padding and even installed rubber washers under the metal ones at the mounting bolt, but further investigation found that the diodes in the DC trap had started to fail causing the trap to inject its own asymmetry (i.e. DC) straight into the transformer causing the core to saturate.  I can’t blame the circuit design.  It was my own fault for thinking that 6A diodes (which are already pretty big) were sufficient in an implementation requiring something more robust for continuous reliability.  Anyway that lead me back to the ESP web site where it was recommended that a high current bridge rectifier be used for the diodes.  So I installed a 50 Amp version shown here on the right:

Silence returned to the transformer.

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