Basic electrical changes - taliesin/T-962-improvements GitHub Wiki

The schematics

T962_schematic for the T962A (which has no onboard Triac)

There are (still) some errors in the schematic:

  • The simple switcher LM2575 is drawn wrong (output on the swichting node).
  • Vref (pin 63) of the uP is connected to 3.3V (directly)

Improve the main earth connection

See video in 'Mechanical Changes'

Make the system cooling fan switchable

The system cooling fan is rather loud. So make it switchable by software. The pin used is GPIO0.25 connected to a via AD0 left to the microcontroller, best using a FET (like the Si2302 and a 10k pulldown resistor on its gate). See 'Improve power board' below for details.

Fix floating inverters and increase opto-coupler drive current

The 74hc04 below the bridge rectifier has floating pins, so Pin 1 and 3 would need pulldups to 3.3V (nearby large via), pin5 should be connected to 4, but ...

  • it interfaces 3.3V to 5V and drives the opto couplers, but the thresholds are not safe at all
  • the output stage of the 74hc04 is not specified even for the current it originally drives:
    • about 3.5mA for the MOC3021
    • about 8-10mA for the LED

To improve the driving current (using the original MOC3021):

  • completely remove the inverter
  • replace R6 and R8 by 220 Ohm resistors (delivering about 15mA)
  • place 2 N-channel MOSFets (e.g. Si2302), like so:

Use FETs

Using a MOC3083 (zero-crossing) instead, a driving resistor of 680Ohm will do. This will reduce EMI.

TAKE CARE: Using FETs instead of the inverter needs a SW-change!

PMW drive is currently inverted, change this to non-inverted, this needs to be done in io.c (Set_Heater(), Set_Fan()).

Improve power board

The heaters draw up to 600W (max.) which is approx. 2.6 Arms, the triac used is a BT139, with a Vdrop typ. 1.2V, this leads to approx. 3.1W of dissipated power, the heat sink (sized 2cm x 3cm) should have about 10K/W + 1.7K/W (half cycle Rth-j-mb) ~ 12K/W, which leads to a temperature rise of 40K! With Tjmax=125°C this ok for Tamb < 85°C. Additionally the triac has little thermal bonding material to the sink and is located far off the cooling fan (which is on the other side of the housing).

Take care: The heater triac is used in half-wave mode, so peak-currents are about 12A! This is probably due to the usage of 115V infrared lamps (US-model).

There is an official modification which inserts a FET into the GND line of the internal fan (which is simply connected to +12V and GND and therefore always runs). With the mod it controls the temperature (using the cold junction sensor). This might further degrade the heatsink performance!

Additionally the plug for the internal fan is located dangerously near the 230V lines on the power side of the board!

  • Find a larger heatsink for the heater triac (make sure isolation to the housing and the heat insulation is intact!). A 5K/W will fit (about 30x40mm²) and improve temperature rise to about 20K instead of 40K. Note that the heatsink is at mains potential!
  • Place the FET (and resistor) on the backside of the PCB directly under the fan connector (cut the ground line that runs there and wire a thin insulated wire to the specified pin of the uP (AD0, Testpoint), add an additional 10k pulldown to the gate.
  • Remove the series resistor R16 (2.4k .. 2.7k), it only delays turn on after zero crossing.
  • Cut away part of the pad on the bottom or R13 it's dangerously near to the 5V line!
  • After testing put qualified varnish on the bottom of the PCB to reduce the likelyhood of leaking currents (the trace safety distances are NOT observed!

If MOC3083 is used, replace the opto couplers (put them on sockets), put some varnish on the topside of the PCB to make sure there a no leaking currents from the mains side (under the opto couplers!) Note: The original MOC3021 has 400V max off state voltage which is rather tight!

See 'Fix floating inverters and increase opto-coupler drive current' above!

Testing

To validate the power drive, it's easier to use a say 36V transformer and a load of about 10 Ohms to test the output stage. This way no harmful voltages need to be applied and output power is a lot lower while driving the same currents. For peak currents of 12A use 3 of these loads in parallel (and a strong enough transformer!). Using car bulbs with 12V (3 of them in series make pulsing even visible).

Note: this only works if the gate current thresholds can be driven with these voltages, i.e. with 300 Ohms and 20mA, the turn on voltage will be about 6V+Vgate+Vthrough(driver). The fan driver has 3kOhms which will turn on very late in the cycle (36Vrms → 50Vp), but this should be removed anyway! This also provides galvanic separation (useful when using the oscilloscope).

To turn on the power stage, simply short the drain of the driving MOSFET to ground using a returning switch, or go to bake mode (in the shell).