brian5857 Posted March 19, 2011 Report Share Posted March 19, 2011 When I have 40 VAC present I need a contact closure for a control circuit. Does anyone know the best way to drop the 40 VAC down to 24 VAC to energize a coil on a cube relay? I was thinking of putting a resistor in series but do not know how to calculate. any other ideas? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted March 19, 2011 Report Share Posted March 19, 2011 Yes, put a resistor in series with it.Measure the current in the coil and calculate the required resistance using Ohm's law.Hint: it's the same formula as that used to calculate the series resistor for an LED (this can be found using Google) but replace the forward voltage with 24V. Quote Link to comment Share on other sites More sharing options...
brian5857 Posted March 19, 2011 Author Report Share Posted March 19, 2011 Okay, the manufacturers literature states that the 24 vac relay consumes 1.1 VA so that would equal 45.83 mA using ohms law.Using this online LED resistor calculator... http://ledz.com/?p=zz.led.resistor.calculator i came up with this...Supply voltage = 40Voltage drop across LED =24Desired LED current = 45.83 MaResistor should be:390 Ohms1.222 WattDid I do that correct? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted March 19, 2011 Report Share Posted March 19, 2011 That sounds about right to me.I calculated 349R, the nearest standard E24 value is 360R but any value between 330R and 390R will do.I = P/V = 1.1/24 = 45.83mAR = (Vin-Vout)/I = (40-24)/0.04583 = 349RYour power calculation doesn't look right to me but fortunately your result was conservative.P = I2R = 0.045832*349 = 0.73WSo use a 1W resistor, minimum. Of course, if the resistor value is slightly higher the current and therefore the power dissipation will be slightly lower. Quote Link to comment Share on other sites More sharing options...
brian5857 Posted March 19, 2011 Author Report Share Posted March 19, 2011 Thanks for the help on that. The online calculator I used came up with .73 watts also but said a 1.22 watt is a safe bet.I'll give it a try. Quote Link to comment Share on other sites More sharing options...
Hero999 Posted March 19, 2011 Report Share Posted March 19, 2011 No one makes 1.22W resistors, it's common practise to just use the next power rating up. Quote Link to comment Share on other sites More sharing options...
brian5857 Posted March 19, 2011 Author Report Share Posted March 19, 2011 Okay, Thanks. Quote Link to comment Share on other sites More sharing options...
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