Yah I know im being a pain in the ass and yall want a perfect schematic, but the reality is the problem (if there is one) is probably not in a theoretical drawing, but some error in fabrication that would not show up on a schematic.
Ermm, the question you were asked was do the MOSFETs get hot....
mis understand the question, no they do not get hot...i have used these on many projects, in the early days I did not understand to drive them correctly & they did tend to get hot. They are also way over rated for the application driving 30ohm coils @ even 60v is only 2 amps. If someone could confirm that driving those gates @10.7v is within a reasonable range.
You never said what the resistance of the heater elements are. Assume you have a 48VAC transformer, what is the VA rating (Volt Amps)?
No idea, bought at surplus, rewound the secondary & added taps for several different voltage outputs. Currently running on the 45vac tap. Ask about VA rating suggests you think I might be overloading (saturating?) the transformer...I can't prove thats not the case, but the thing weighs about 10lbs and is wound with ~10 gauge enamel wire. Also have not seen at temps beyond 38c.
he OP said his DC rectified and filtered voltage was was 60, 60 / 1.414 = 42.4V, Full wave rectified DC = 0.9 VRMS, VRMS = 42.4 / 0.9 = 47.15. Thus I assumed the standard value of 240 / 5, 48.
Not sure what the notation "240 / 5, 48." means, but the primary gets 120vac 50hz and the secondary is as described above. The math you showed is constant with what I found theoretically and by measurement.
I would not drive heaters with high frequency* PWM. Heaters respond slowly, generally a few seconds to minutes or hours, and the things they heat respond even more slowly, think of how long it takes to heat a kettle or your living room. I use PWM with a cycle time of 720 seconds for my central heating, I suggest you use something similar. Once you do that the relays are OK, no need for a rectifier or diodes or capacitors. The other sensible way to drive them is with a triac from AC using burst control, so some number of cycles on, some number of cycles off. For example 1 cycle on 3 cycles off gives 1/4 power.
this is a cast iron heated print bed, not central heating. my PWM frequency is like 500 hz, I think one is linked to timer0, so its more like 900hz.
Someone asked about the coils, they are custom wound lengths (~30 ohms) of kanthal wire (sorry I said nichrome earlier)
Is this with ALL your MOSFETS turned ON?
yes and no -- with fewer, say just 1, mosfet running, the supply voltage measured at the bulk/filter cap is higher (~50vdc), but there is still a 10-15v less across the coil itself.
What is the DC voltage across the drain - source of the MOSFET.
that is the voltage in question and it shows up 10-15v less than the supply voltage.
Switch the DMM to AC mode and measure the DC power supply with and without load.
I have tried that in the past, as I recall it did not pick up anything, I took that to mean the ripple was sufficiently smoothed. This was a while ago, I will re-run that test this evening and confirm the findings.
A full schematic including your power supply and transfomer will help?
transformer --> full bridge rect --> cap (270 uF). I don't have any more information than that.
Can you please tell us your electronics, programming, arduino, hardware experience?
I would say I am a well versed amateur. Significantly better with on the programing side than hardware. Probably have 10-15 arduino projects running simple things like my soldering iron, to the most complex system I built for extruding 3d printing filament. And I am always looking to learn, so feel free to talk over my head, if I dont understand I will ask.
Wire one coil direct -test
by direct, you mean ps straight to the coil? That's a good idea, I will try it tonight.
Rewire with mosfet then drive mosfrt full on hard. That is max powersupply voltage to the base
are asking me to drive the mosfet directly? That I have not done, but when I drive a pin high (255), I see 5V at the opto & 10.7v at the gate of the mosfet. At that point the filter cap reads ~50vdc and the coil reads ~40vdc. Does that not provide the same information as the test you purposed? If not, it sounds like you want me to hook 12v straight to the base, not sure how that's going to be different, but I will give it a try.
Rewire base from power supply to have 12v power direct drive base as a switch full on 12v to the base- test
not sure I understand the difference between this test and the prev one.
I see the incremental approach you are taking to troubleshooting, but doesn't the test I outlined above confirm those gates are driven properly?
Also are you asking me to drive the mosfet with 12v & switch 12v (as apposed to the 40-60v supply) ? why? to ensure too much current doesn't hit the coil during the test?
what component do you need a data sheet for? If its the heaters, they are hand wound lengths of kanthal, ~30 ohms each. That's all I have to give you.
--worth mentioning: I have thoroughly looked for shorts between coils or the frame and found nothing.
looks like people want more specific question, understandable, so let me ask this:
is the 10.7v @ the mosfet gate(base) an appropriate voltage to turn the mosfet all the way on while maintaining low resistance (prevent mosfet from heating up).