Not clear. He wants it within 18 ohms. He's already lost 1 or 2 with leads. My picky bran sees that 240+18 and 240-18 as not being 18. Some part of that 18 is uncertain based on meter leads. 17-19 maybe? Generally you would use a 4-terminal ohmmeter, but his +-18 has more uncertainties. 18 ohms you might.
True.
I bought a pack of 50 carbon resistors. I tested them all with my digital meter. The value is suppose to be 240 ohms but the meter says, none are 240 ohms. Most of the resistors are on the LOW end and HIGH end but not many in the middle near 240 ohms. Low end is 216 ohms and high end is 264 ohms. There is a very large range of values so I soldered a 261 ohm is parallel with a 222 ohms this gives me 120 ohms. I soldered 259 in parallel with a 224 this gives me 120 ohms. I soldered both of those 120s is series it should be about 240 ohms and the meter says 24o for this group of resistors and I have 241 ohms on another group or resistors.
1/4 watt resistors seem to be fine for pin 1 I just thought I would play it safe and use 1/2 watt resistors.
I am turning the mosfet ON/OFF manually with 2 power supplies and a load on the work bench just to test and see what is going on. Voltage to pin 1 is 11.8 VDC. Voltage to pin 2 is 21.0 VDC. These are no load voltage readings.
The original circuit drawing calls for a STP40NF10 using a 220 ohm gate resistor but the mosfet over heat and go up in smoke pretty quick. A larger 240 ohm resistor solves the mosfet smoke problem. Mosfets still get very hot and can not be ON more than 30 seconds or they burn up.
Now I am experimenting with a P55NF06L. I was told Mosfet resistance is lower so it wont get hot as quick as the other mosfet and that turns out to be true. I am still using the 240 ohm gate resistor.
I have several items for a mosfet load, DC motors, light bulbs, hot water heating elements. If I replace the fixed resistor a variable resistor it makes a nice variable speed control for the motor and light dimmer.
The mosfits do not seem to be turning completely off with a resistor less than 240 ohms.
At my age 65 my brain does not soak this up into memory like it did when I was young tooking electronics in college. I have to make good notes to refresh my memory next week about the experiment's that I did today, that is very frustrating.