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Henry Radio 5K Classic Amplifier
New Step-Start Inrush Limiter
Completed in the summer of 2003



The most difficult aspect of adding a step-start system to this amplifier was the total lack of room to mount the components.  I initially planned on using a mercury contactor but just couldn't squeeze one in.  After talking with WD7S about the available space problem, I decided to reconsider using solid-state relays because of their smaller size and the ability to control them with a low current DC supply.  After making several mounting bracket mock-ups out of paper (they were drawn full size in AutoCAD and then printed), it became obvious where the best location for the relays would be.  Placing them on a right angle bracket above the HV transformer meant I could use the existing wiring without making any changes.  Makes it nice if I ever want (or need) to return it to the original factory condition.

The mounting bracket was made out of 1/8" aluminum by a friend of mine.  I drilled and tapped the holes in the larger 3/16" plate material that makes up the lower portion of the power supply chassis.

The relays are Crydom D2475s -- 75-amp 240vac with a control voltage requirement between 3 and 32 vdc.  I put a 1100 ohm resistor in series with the control lead to each SS relay as a current limiting/voltage dropping device to protect the relay from the higher voltage of the relay supply.

The airflow switch mounted on the blower turns on the amps relay control supply (30 vdc) after the blower comes up to speed.  I use the time it takes for the airflow switch to close - 5 to 10 seconds on average - as the time delay mechanism instead of a separate time delay relay.  This allows the HV capacitor to charge slowly through the 50-watt 25-ohm Dale aluminum clad resistors before the SS relays are energized and then bypass the 25-ohm resistors.  50-watt resistors might sound a bit light until you read the specs.  These resistors have a 5-second load rating of 250-watts!

Both relays were purchased on Ebay for prices far less then you'd pay at a supplier like Mouser.  I did purchase the plastic relay covers from Mouser to ensure that the filter screen on the back of the amp won't be able to short things out if it were pushed in by accident.  The aluminum mounting bracket is solidly secured to the amps aluminum sub-chassis assembly to facilitate heat sinking.

After further experimentation, I determined that only one of the two relays was really needed for the step-start system.  I subsequently removed one of them from the circuit.

It's so nice not having the amp practically jumping off the floor when it's turned on.

Schematic of the wiring changes I made to the 5K Classic.  The lower left corner shows the Crydom SS relays connected to the HV transformer primary leads.  The SS relay for this step-start system is powered by the amps existing 30 vdc relay supply.  This schematic was drawn with Express PCB and Schematic software -- It's FreeWare.  CLICK HERE to download my actual ExpressSCH drawing file.


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