[Boatanchors] Re: Station Set up follow up questions

[Drew Papanek]

Eugene wrote:

>When the TR Relay (a dow 115vac relay) is in transmit mode (coil 
>energized),
>there is a distinct 60Hz hum coming from it. I am sure it is amplified by 
>the
>wooden board it is mounted on and the wall that the board is in turn, 
>mounted
>on.  I was wondering if I could reduce that hum by putting a rectifier in 
>place

A good solution is some form of acoustic isolation between board and relay.  
You could use a flexible mounting to damp the sound-causing vibration.

I have had AC relays that hummed even when equipped with a shading ring.  
Operating the AC relay on DC would stop the hum.  The DC voltage rating for 
an AC relay coil will be different than its normal AC rating.  I have 
operated 110 VAC relays/solenoids on 24 VDC with no trouble.

The relay coil can be operated from a full wave bridge rectifier and series 
resistor right off the AC line. No filtering capacitance is needed.  The  
resistor is needed to keep current to a reasonable level.  The exact value 
is not critical; one can gauge reasonable coil current by the temperature of 
the coil.  The resistor value would likely lie between 500-2000 ohms.

If the coil's AC power consumption is known, it can be used in conjunction 
with its DC resistance to calculate an approximate resistor value.   
Calculate coil current as sqrt(p/r).  Calculate resistor as 
(120/current)-coil resistance.  The value will not be exact because the 
ripple component of the  rectifier's output  will not behave as DC when 
applied to the inductive coil.  But it will be close enough to work.

If you don't feel like tinkering with resistors, you could dispense with the 
resistor/rectifier and just use a DC wall wart of appropriate voltage.

Drew

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