Electric Radio Magazine - Symbolic Publishing Company

300-Watt Inrush Limiter with Voltmeter AB300M

The Inrush Current Limiter uses solid-state devices to prevent the sudden surge of current that you get when turning on cold equipment from damaging your valuable vintage radio equipment.
It is in the same style case as the lower-powered version.

If you are concerned about high inrush current damaging your older equipment, or even some newer gear, this little product will prevent such damage. They are not intended to be used as a voltage-reducing device. When cold, the inrush limiter has high resistance, and after it warms up the device has lower resistance.

This unit is small in size, measuring 2″ x 2.5″ x 4″ with a sturdy all-plastic case. The rear has the standard three-prong male 120 VAC connector and the front has the standard three-hole female connector. The front of the metered model also has a nice little AC meter to monitor the load voltage. There is a top-mounted fuse holder with the standard 1.25A, 3AG fuse. The unit has a maximum rating of 300 watts at 2.5 amperes. It provides a slow start for equipment such as a Johnson Ranger, Hallicrafters HT-44, or Collins 32S-3 transmitters. It will also protect vintage broadcast radios such as a Zenith 12-tube set.

With your equipment plugged into the front power outlet, but turned off, the voltmeter will read the line and load voltage as the same value, typically 120-VAC. This tells you that line voltage is present, and that the fuse is good.

The instant the equipment is turned on, the internal meter will show a drop to about 60 VAC, and then it will rise over the next ten seconds to a value near 100 VAC. This rise will continue for about two minutes where the load voltage will level off and hold for as long as the equipment is turned on. The drop in load voltage at turn-on from 120 volts to 60 volts represents substantial inrush current limiting. This slow built-up to operating voltage is what protects your equipment from possible damage. For example, power supply filter capacitors in a power supply are present nearly a short circuit to load current when they are discharged. Vacuum tube filaments will last a lot longer if they are not subject to full voltage when they are cold.

After the initial warm-up period, the limiter will have a constant voltage drop of about 6 VAC. This voltage drop means your vintage equipment will operate at the line voltage it was originally designed for, which was about 115 VAC.

This is a rugged little package with a long life, and no internal mechanical parts to wear out. The 6-volt drop across the unit may seem a waste of power, but old gear designed for the lower line voltages will reap the benefit.

5 or more limiters are shipped free!


Available on backorder

Additional information

Weight14 oz
Dimensions12 × 8 × 6 in