"The voltage across the resistor was 0v when off and 4.65v when on "

is what I expect between the FET gate & source
when the CHP 01 is switched off and on



"With the relay and resistor connected the voltage was about 1v when on and 14.7 when off (between op1 and -24v)"

implies the FET is leaky and not switching off completely

when working correctly

the FET's ON resistance should be 155 miliohms

as the relay coil resistance is 650 ohms

at 24V DC
the relay coil and 27 ohm resistance should pass around 35.5 mA

the volt drop across the FET when switched on will be

0.155 ohm x 0.0355 A = 0.0055 V = virtually zero on a basic multimeter

either the FET is not switched hard on so the on resistance is higher
or your measurement includes the volt drop across your wiring and contact resistance of the connections



"14.7 when off (between op1 and -24v)"

when the FET is off no current should flow through the 27 ohm resistor and 650 ohm relay coil !

to measure 14.7V at the output
24- 14.7 must be dropped across the total 677 ohms of the relay coil and resistor

the leakage current must be 9.3/677 =0.137 A

so instead of the FET looking like its open circuit when off
its more like a 107 ohm resistor !!


"With the relay disconnected it was 0v both on and off"

is correct
the FET acts as a switch between the -24V/common
and the CHP O1

without the relay being connected
there is nothing between the +24V and CHP O1 to pull the output up to the +24V supply


John