@dave-in-nj, I was able to connect the heater directly to the battery an make it heat up really well. Just not when using the FET. The project will be battery powered, so I'm avoiding using the USB's 5v as an option.
@WattsThat, For the measurements, I measured both, directly on the ESP32 pin's (on the actual pin/soldering), and Right on the MOSFET's gate pin. As for the battery, I see the same exact results when using a beefy Samsung 18650 battery.
@TomGeorge, GND to SOURCE voltage (directly on the pin) is 0.4v when om, 0.0v when off. With the BATT + () connected to the MOSFET gate and resistor removed, I'm getting 1.0V
@Blackfin:
- voltage at the drain (centre pin) with respect to battery ground = 1.45v PIN HIGH - 3.65v PIN LOW
- voltage at the source (right-most pin) with respect to battery ground = 0.85v PIN HIGH - 0v PIN LOW
- and/or verify continuity between source of MOSFET and battery ground = CONFIRMED
- verify: you are driving that digital IO on the ESP to "HIGH" and not PWMing it = HIGH, NOT PWM
- can you disconnect the resistor from the ESP and connect the gate directly to battery +ve; verify heater works = Works, but with lower voltage 1.0V.
Thanks again everybody for all of the support.
Hi,
@TomGeorge, GND to SOURCE voltage (directly on the pin) is 0.4v when om, 0.0v when off. With the BATT + () connected to the MOSFET gate and resistor removed, I'm getting 1.0V
Your source to gnd wire is faulty or protoboard connection not good, the drop should be zero in both cases, because source is connected to gnd.
This will take 0.4V of gate level away from your MOSFET.
Tom.. 
TomGeorge:
Hi,
Your source to gnd wire is faulty or protoboard connection not good, the drop should be zero in both cases, because source is connected to gnd.
This will take 0.4V of gate level away from your MOSFET.
Tom..
I agree. This is suppported by "voltage at the source (right-most pin) with respect to battery ground = 0.85v PIN HIGH - 0v PIN LOW". The source should simply be connected to ground all the time and the voltage drop should be minimal.
OP, IIRC the pins on a TO220 have a rectangular cross-section, wider when viewed from the front or rear than when viewed from the side. When you insert that into the protoboard you may be spreading the contacts into a 'V' shape resulting in holes adjacent making poor or intermittent contact. Consider moving the black wires for the drain and source to the "other end" of the slot -- the last holes, furthest away from the TO220, where the 'V' would be narrowest.
You might also re-orient the TO220 so the pins go in with the narrow axis spreading the contact; this would mean bending and staggering the pins so they each have their own row. The contact-spreading problem would be lessed. Maybe it's time to move to plated-hole perf-board for the power-section of your prototype...
Sorry for the delay in getting back to everybody, I've been busy with work and family events.
But good news, I took Blackfin's advice and moved this onto a small protoboard to avoid the breadboard and its now working as expected. The heater heats up well (really hot actually), and now I'll try using PWM to control the heat.
I just want to thank everybody who took time to help me identify my problem and offer solutions that helped me out. My project would have been stuck without you guys.
