Not sure if I'm posting this in the right place.
Anyway, I came across a thread by joecool85 where he wanted to use a relay for switching channels on his amp. To cut a long story short, it was necessary to lower the voltage needed for the relay.
J M Fahey suggested the following:
Quote from: J M Fahey on November 23, 2010, 10:12:58 AMThe series resistor is needed to bring those 19V down to needed 12V.
Value?: measure the DC resistance of the coil, and multiply it by 0.58 .
Example, if your relay coil has 1200 ohms, you'll need a (1200 * 0.58)=696 ohms.
My question is, why 0.58? At first I thought it was because it's the ratio of needed voltage to available voltage, but
12 / 19 = 0.62
Pretty close, and most probably good enough, but not exactly that.
So, what am I getting wrong?
I have no real idea but it might be because a coil has impedance that acts different then a straight resistor ?
Just like a speaker that is rated at 8 ohm Impedance measures lower in DC ohms with a multi-meter.
Not sure how he got that number, magic? Or maybe I'm just not on top of my game right now. ;)
But in any case, pick an example number, solve for current, and the added resistor will drop about 7V, leaving 12V on the relay coil.
Quote from: g1 on October 18, 2024, 05:04:33 PMNot sure how he got that number, magic? Or maybe I'm just not on top of my game right now. ;)
But in any case, pick an example number, solve for current, and the added resistor will drop about 7V, leaving 12V on the relay coil.
Yeah, that's what I've done a few times as an exercise, use Ohm's law to find the value needed. The ratios I got were pretty close. I mean, using Mr. Fahey's number the calculated voltage values were good for the relays I had on hand, so perhaps I'm splitting hairs on this one?
First off 12/19 is closer to .63 than .62 ;) .
Then, that 63% is of the total, the relay resistance plus the series resistor.
So that's why it can't be used applied directly to the relay resistance.
Quote from: g1 on October 19, 2024, 03:30:47 PMFirst off 12/19 is closer to .63 than .62 ;) .
Whaaaaa? How did I get that one wrong? :duh You're right ofc.