> There is a 240/12V transformer to provide power to the controller itself.
> Here is where I now become puzzled.
>=20
> 1. The transformer is centre tapped and they have shorted it to the lower=
 leg
> of the centre tap, but still used a full wave discrete bridge.

Are you saying the secondary centre tap is connected to one end of the wind=
ing, creating a shorted section of winding ???=20

>=20
> 2. More strangely (IMO) is they have connected the 7805 regulator gnd to =
AC
> active.The MT2 connection of the triac is also connected to this 'gnd'/ l=
ive
> point. MT1 wanders off to neutral.

Huh? The TRIAC is connected across the mains terminals?

>=20
> There is a label stating that the ELV supply is at mains potential on the=
 heater
> and schematic.
>=20
> 3. The heater case is grounded.
>=20
> The only two reasons I can see for using a transformer and then connectin=
g
> the 'ground' leg to active is:

The 'ground' side of the low voltage electronics may well be connected to o=
ne side of the mains voltage to make driving a TRIAC easy without having to=
 use isolated drive.=20

I remember once looking at a controller for a sleeping blanket where the te=
rminal of the TRIAC that the gate is referenced to was connected to AC hot =
side, and then the control IC (an LM393 IIRC) was 'hung below' that by havi=
ng its +ve side connected to the AC hot side, and all the voltage dropping =
from mains voltage being on the -ve side. This allowed the open collector o=
f the IC to nicely drive the gate of the TRIAC.

>=20
> A) This makes the circuit 'dangerous' enough to require a warning label s=
o
> that most people won't delve into the electronics.
>=20
> B) A capacitive dropper would not provide enough current to drive the uC
> and a two digit 7-segment display. A straight zener type arrangement woul=
d
> waste too much power.
>=20
> What think you all?

A transformer may be cheaper than an X2 type capacitor that they would have=
 to use in such a circuit, while still meeting the safety requirements. If =
a capacitor dropper is used they may have to add other components to discha=
rge it when the power disappears so there is no possibility of voltage bein=
g present on the pins of the power plug when unplugged while switched on.

> The display is supposed to (I haven't plugged it in yet) show the room
> temperature, but I see no temperature sensing device anywhere and th euC
> doesn't have an onboard one.
>=20
> The mystery chip has ports labeled as K, REM (probably remote as it drive=
s
> the transistor that earths the IR diode) , SEL, OSC0/1 , LMP (not connect=
ed
> and CCS also not connected.

The mystery chip may have a temperature sensor inside it as well. Does it l=
ook like it is positioned physically to be in the input airflow side of thi=
ngs?


--=20
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--=20
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