Transformer inrush protection circuit

Introduction

A common challenge we all face when building HT supplies for our valve amplifiers is the inrush current at start up produced when the filtering capacitors charge up and blow the fuses. There are several workaround, albeit most of them are not effective. Increasing the value of the primary fuse seems like an easy solution, but is pretty dangerous. The fuse will not blow at start up, however, what is worse, it will not blow at all before any other damaged is already produced in the supply in case of a short circuit or any other issue. If we add some resistance to the secondary, this will drop volts, waste energy and increase the supply output resistance. If we add resistance in the primary, like an NTC, is a much better approach, however we want to bypass this NTC to increase efficiency and performance.

A nice solution is to bypass the NTC (or a resistor) after initial in-rush. A simple circuit is possible to implement using a timer and a relay. The same circuit is used also to apply a longer delay (e.g. 2 min) to turn on the HT supply automatically if you wish.

The circuit

Of all the timer circuit design options available I came across to one that was very simple. In fact is a variation of a HT delay circuit I used in the past on my 45-SE amplifier. The circuit can be found in Merlin Blencowe’s “Designing Power Supplies for Tube Amplifiers” which is a fantastic book full of ideas and resources for DIY audio. I suggest strongly you buy a copy of this book in case you haven’t read it yet. A must have for any DIY-er.

To avoid a transformer, the circuit uses a common approach to derive a low voltage source from mains supply via a capacitor. A properly rated capacitor for mains (class x2) can provide a good impedance without dissipating any power and limiting the current to the circuit. An additional resistor (R1) is added in series to protect the initial in-rush current due to the capacitance

This capacitor (C1) is commonly known as a “wattless dropper”.

The diode bridge D1-D4 rectifies the AC voltage and provides a 24V DC voltage via the smoothing capacitor C3 and the 24V zener diode which is represented below by a pair of 12V ones (D5 and D6). You can change the circuit to 12V or 24V or higher depending on the voltage coil rating of your relay. I adapted the circuit to the “Finder” 24V relay I had in stock.

The delay is provided by the RC formed by R2 and C2. R2 charges C2 and when the base current is high enough to turn on Q1, sufficient current will go through Q1 in saturation to turn on the relay. R3 provides a path to discharge C2 when the supply is turned off.

Q1 is a typical darlington like the BCX38C or similar. An alternative option for Q1 instead of a darlington NPN transistor is to use a MOSFET like the 2N7000. Cheap as chips and the advantage of the MOSFET here is that the high input gate impedance which will allow you to elongate the time constant or reduce the size of C2.

A 47uF C2 cap will give you one sec or so of delay. That will be enough to bypass then the primary NTC or resistor. There is a protection diode across the relay coil that is missing on the diagram below.

The relays come normally with a pair of switches like a DPDT switch. One pole can be used to turn on a NEON light and the other pole can be used to bypass the NTC/resistor to  be connected in series to the primary of our mains transformer.

 

 

 

Build

I hacked a quick PCB with my usual technique and in 30min I was soldering the components to the PCB board without any drilling. Probably you can make this smaller but I reused a piece of PCB I had at hand to avoid any additional cutting. Time is precious these days when you have a 4 month old baby and any microsecond in the workshop is gold dust!

You can build this circuit very easily. There is no secret about it. Careful testing and handling is paramount when testing with mains. I recommend you use a variac. What I did, was to build the circuit and connect my bench LT supply set to 26V and fed the diode bridge. I tested this way the delay circuit and once I confirmed that the relay turned on after 1 second then proceded with a test with the variac. I had a voltmeter connected across the zener to monitor the 24V supply generated from mains supply as shown below: