So Ive never made anything before and and am considering trying to construct a Tx/Rx relay circuit.
My radio's PTT output is an open drain, its limitations are 48 VDC, 500 mA max. My understanding of this type of circuit is that it presents a high impedance on the output when PTT is not set, and when it is set, it presents a ground to the output?
They are giving the maximum open voltage with no current, and maximum current with no voltage.
This means you have two limits:
48 volts (probably positive) with relay off.
500 mA with near zero volts with relay active.
So, if I were to wire up one side of a 12vdc/160mA coax relay to +48vdc, and then use a 225 ohm resistor in between the other side of the relay and my radio, would this be a good way to handle the circuit, assuming my relay power source and the radio share the same ground? This would have the relay dropping 12v and the resistor 36v/5.76watts. Possibly could also use a .5Amp fuse inline as well?
You are switching an inductive load with a relay. Current starts slow and then reaches the dc level, but when the current is interrupted the inductance tries to keep current flowing. This means the relay coil starts adding to the voltage in an attempt to maintain current. The relay could add 50-100 volts or more back in series with the original 48 volts, so you could have over 150 volts back-pulse. Other than that, your calculations are good.
Without a back-pulse diode on the relay coil, you run a risk of back-pulse damage. Also, you are too close to the rating anyway. I'd use a lower voltage source.
By the way, many relay companies now have relays made in China as we close factories and send jobs overseas. The relays used to be very consistent in current when domestically manufactured, but now we are seeing relays vary over 50% in current or coil resistance. When the correct the current on a rejected sample, the screw up the pull-in voltage. You have to be careful with relays from the big C even from formerly reputable relay vendors, because they might not be what the ratings say.