Universal Step-Start System

Universal Step-Start Board

W8JI 12/21/2021

Amplifiers and power supplies can have very large initial contact closure currents. This starting current, also called “inrush”, primarily damages line switches and powerline relays or contactors. Severe inrush can falsely trip line breakers or open fuses. Otherwise, inrush is generally harmless to typical components used in Amateur Radio amplifiers. It generally does not reduce life in tubes, transformers, or filter capacitors. It often does cause an annoying “thud” when an amplifier or transmitter is powered up.

There are two main causes of power switch and power relay contact damage; transformer residual magnetism and filter capacitor charging currents.

Residual magnetism parks the transformer out in random places on the B-H magnetic curve. If this happens to hit wrong, even if the switch starts from zero crossing, the first fractional cycle transformer primary impedance can be close to zero. When this happens, transformer primary dc resistance alone limits transformer primary impedance. Extreme residual magnetism inrush cases are somewhat common in large closed-core transformers such as toroidal and Hypersil style transformers. This effect occurs with or without secondary loading.

The capacitor charge current is spread over the first few cycles. This current is limited by capacitor ESR, secondary system resistances, transformer flux leakage, and the total joules absorbed and stored in the capacitor system. Transformer primary system resistance aids in limiting this longer duration current. Switching supplies also have inrush, primarily from energy storage capacitor charging currents.

No matter what cause, the bulk of contact damage occurs during the first few milliseconds while power switch or relay contacts are bouncing or settling.

Properly designed step-starts, more recently called “soft starts”, can be an effective solution to false breaker or fuse openings, as well as switch and relay damage. They reduce starting current enough to prevent false breaker or fuse opening, simultaneously limiting closure currents to reasonable levels during contact and switch bounce periods.

I designed dozens of commercially produced amplifiers for multiple companies. I included step-starts or “soft-starts” as necessary, the need primarily determined by the transformer style or other inrush determinants. It is important to mention there have been no determinable tube life change in amateur radio power level amplifiers. Step-starts or soft-starts primarily improve switch and contactor life, as well as reducing annoying false breaker openings or fuse fatigue. Step-starts also reduce or eliminate unimportant starting noises, such as cabinet “thuds”.

How Soft Starts and Step Starts Work
Step start systems insert a series resistance in the primary power source path. This start resistance can be inserted in either line lead, or in both line leads. Maximum possible inrush is found with Ohm’s law, E/R = I with E being the peak supply voltage and R being the total effective start resistance.

This start resistance is removed three ways:
1.) By a fixed time-delay system

2.) At a targeted primary winding voltage

3.) At a targeted secondary winding voltage

Each method above has advantages and disadvantages.

Systems In General  
All systems require a high overload capable start resistor. Without protection, the start resistor may go incandescent or melt during prolonged fault states. The start resistor should be fused with a fuse that opens before major damage. The resistor, fused or not, should be kept clear of all other easily damaged objects such as wiring harnesses or circuit boards. It will always be a balance between resistor fuse life and resistor damage.

Timer Based Systems Timer-based systems close the step-start (sometimes called a “soft-start”) at a preordained fixed time regardless of inrush or starting load current. Time based systems must always add some unnecessary start delay to ensure they do not close prematurely. Time based systems also will close into a dead short, unless something detects the short and disables the timer. These systems will commonly start even if the inrush is not handled, such as when the start resistor has failed. The operator may think the inrush is working when it has failed.

Voltage Triggered Systems   The simplest and most common systems are voltage triggered systems. Voltage triggered systems start at a predetermined trigger voltage range.

When a power supply starts the initial impedance across the power line is quite low, this is of course what causes the inrush. By adding resistance in series with the powerline, inrush current is limited. Voltage dropped across the inrush start resistor prevents activation of a shorting relay until equipment load impedance is well past the point of inrush. When supply impedance rises to safe values, trigger voltage becomes high enough to close a resistor shorting relay. This always insures the fastest, smoothest, start without excessive inrush. The time delay adjusts to time necessary to prevent excessive current. The time will always be well beyond contact bounce periods.

As with the timer system, limiting resistors can be damaged by a power supply defect. Voltage triggered systems will not start.

W8JI / CTR Engineering Step-start Board
My step-start design uses a commonly available heavy-duty high-current cube relay. The relay is specifically intended for powerline switching applications. More important, the relay is a very common hole pattern and readily available in 5 to 110-volt coil voltages. My board readily fits a wide range of applications. By repositioning two plug-in shunts, starting resistances of 11, 22, and 44 ohms are selected.

As determined by relay coil and board component population, trigger voltages of 5, 12, 24, 48, and 110 Vdc and 6, 12, 24, 48, and 120 Vac are available. Additionally, two 2-watt series-connected resistors allowed trigger threshold customizing to other trigger voltages.

This board accepts European style wire terminated or removable plug blocks.  Terminals can also be eliminated and connections directly soldered to the board.

My board requires minimal changes in existing wiring, requiring only two small low-current trigger wires and interrupting of only one high current power line wire. This makes installation simple, fast, and clean-appearing. This board requires 1-1/2 inches or less vertical space and about 3 inches of length and width.

Additionally, we include a drill template and suitable hardware. All you need are common hand tools.

Minimal cost and complexity dc triggered board, UNSS12Vdc:

Universal Step Start basic DC system

Universal Step Start configured for 12Vdc systems.

Basic board cost  $35.00

Universal Step Start Option Deluxe configured for 120Vac operation:


UNSSD120AC fully loaded.


Example of a Universal Step-Start 12Vdc unit installed in LK500Z amplifier 

Original unit had complex wiring, no fusing, and was glued in place:


Original Soft Start in LK500

Replacement Universal Step Start activates from 12Vdc bus. Requires only two short small wires for trigger and interrupting of only one 240Vac lead! This makes a clean simple installation:

UNSS12dc installation

UNSS12dc installation Amp Supply Amplifier