The AA8V Wingfoot 813 Amplifier
High Technology Of The 1950s In The 2000s
by Greg Latta, AA8V

Inrush Delay Circuit Description and Schematic Diagram

Power Supply Circuit Descriptions
Main Power Supply Schematic Page
 Plate and Plate/Screen Supply  Bias Supply
 Transformer T1 Primary Wiring  Inrush Delay Circuit
 Power Connector Wiring  

Current Inrush Circuit
Circuit Descriptions and Sub-Schematics
 Rectifier Diode  Relay
 Filter Capacitor  Inrush Limiting Resistor
 Relay Dropping Resistor  Fuse

Click On A Section of the Schematic Below for Information on That Part of the Circuit:

Inrush Circuit T1 12V Secondary Inrush Fuse Relay Relay Dropping Resistor Filter Capacitor Rectifier Diode

Click here for a high resolution schematic, more suitable for printing.

Circuit Description:
When the power supply is first turned on, the surge of current that charges the filter capacitors can damage the rectifier diodes. To limit the surge, a 10W limiting resistor is inserted in the the primary of transformer T1. This limits the maximum inrush current in the primary of T1 to about 12A. With the input power to T1 limited to 120V x 12A=1440W, the current in the 1500V secondary is limited to 1440W/ 1500V=0.96A. This is far below the maximum surge current rating of the diodes, so the diodes are well protected. After a short time of roughly 0.05s the limiting resistor is bypassed by the relay, placing full line voltage on the primary of T1. A fuse protects the limiting resistor in the unlikely event that the relay fails to close and bypass the resistor.

Click here for pictures and information on the matching Wingfoot 813 Amplifier

T1 12V Secondary Winding:
Transformer T1 has a 12V winding rated at 1A that is used to power the inrush delay circuit. The delay circuit only draws about 100mA, which is well within the capability of the winding.

T1 12V Secondary

A simple 1/2 wave rectifier circuit is used in the inrush delay circuit, since the current used to power the relay does not need to be well filtered. The diode passes current in only one direction, charging the filter capacitor.

1N4002 Diode

Filter Capacitor:
The filter capacitor is charged by direct current passing through the rectifier. The relay cannot pull in until the voltage in the filter capacitor is high enough, and this causes the needed delay between the initial turn on of the power supply and and the closing of the relay, which bypasses the inrush limiting resistor.

Inrush Filter Capacitor

Relay Dropping Resistor:
The relay needs 12V DC for proper operation, but the output of the half wave rectifier is larger than 12V, so a dropping resistor is used to bring the voltage down to the proper value.

Relay Dropping Resistor

The relay used in this circuit is the same as the relay that is used in several Ameritron amplifiers, including the AL-572 amplifier. It is Ameritron part number 408-6148. The relay is a SPST unit with contacts rated at 10A AC and a coil rated at 12V DC. The coil resistance is 127 ohms.

The relay is used to bypass the inrush limiting resistor after the voltage on the relay coil becomes high enough. Until the voltage becomes high enough and relay pulls in, current in the primary of T1 must flow through the inrush limiting resistor, which limits the current in the transformer primary, and thus the secondary. It is the delay in the activation of the relay that gives the filter capacitors in the plate power supply time to charge up, and prevents the current surge in the rectifier diodes.

In this power supply, it takes about 1/20s=0.05s for the relay to turn on, which is more than enough to protect the diodes.


Inrush Limiting Resistor:
The inrush limiting resistor is the heart of the circuit. Before the relay closes, any current flowing through the primary of T1 must flow through the inrush limiting resistor. If the primary of T1 were a short circuit, the current would be limited to a maximum of 120V/10W =12A. In reality the primary of T1 is not a short circuit. In fact, it was found by direct measurement that if the relay fails to close and bypass the resistor, the power supply, while idling, draws about 1A through the resistor, limiting the power in the resistor to 1A x 1A x 10W=10W.

The maximum peak dissipation in the resistor, if the primary of T1 behaved as a short circuit, would be 120V x 120V / 10W=1440W. However, the actual peak dissipation is much less than this, and is very brief. 10W turns out to be more than adequate for this resistor.

Inrush Limiting Resistor

Inrush Protection Fuse:
We have already shown that in the unlikely event that the relay fails to close, the inrush limiting resistor could handle the idle current requirement of the plate power supply, which is about 1A. It could also handle the initial turn on surge of up to 12A. However, if power supply is used above idle while the resistor is still in the circuit, the average current would be larger than the maximum of 1A and the resistor would overheat and eventually burn out. Before burnout, the power supply would not operate properly, since the voltage on the primary of T1 would be greatly reduced. This might not be noticed by the operator.

To avoid a burnout situation and alert the operator to a problem, a 1A fuse is placed in series with the inrush limiting resistor. The fuse can easily handle the high inrush current, since it is very brief, and it can also handle the power supply idle current, should the relay fail to close. However, if the relay fails to close and the operator attempts to use the power supply above idle, the 1A fuse will blow, completely disabling the plate power supply, which would definitely be noticed by the operator.

Inrush Fuse

meterBack to Dr. Greg Latta's Electrical Engineering and Amateur Radio Pages

Questions, Comments, and E-Mail

LetterIf you have any questions or comments, you can send E-Mail to Dr. Greg Latta at

Thanks for stopping by!