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DC
Board
Below is shown the DC output circuit.
Transistor T1(T4) and associated components are
only fitted if you are not using
the AC option for the heater control.
Whilst all three MOSFET outputs look
the same, there is a subtle difference in the way
that the humidity and over temperature control act
in respect of the heater output. The heater
control (if fitted) is a plain on off switch. The
other two outputs feature a 'soft-start' control.
They manage this by being controlled by a PWM
signal, starting with a zero duty cycle
rising to 100 % over a period of 1 second and at
switch off performing in reverse.
It was envisaged that the
humidity control especially, may be used to switch
on a fan or water fountain to bring moisture back
into the air mainly of use with air conditioning.
One of those indoor ornamental
fountains switched on permanently and a control
switched fan was found to take the dryness out of
the air in a small room.
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DC Board
Circuit |
DC Board PCB |
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As a basic unit only MOSFETS T1-3 need
be fitted, and with the transistors
specified upto 10A can be switched. More than this
would require T4-T6.
The flyback diodes D5-6,
are required if they are switching an inductive
load. If using a resistive load such as
underground heating wire or similar then the
diodes are not absolutely necessary. The 100 ohm
gate resistors are to help protect the Pic pins
against excessive current and also helps to
minimise the heat dissipation that might occur
while the MOSFET is switching due to the inherent
gate capacitance of MOSFETS.
The 10K resistors
from gate to ground are to ensure that the MOSFETS
stay off when the Pic is being powered up. The
switched voltage can be upto 24V DC and it is
recommended that any motor driven equipment is
powered from a separate power source with just the
ground line common to both circuits.
To help
prevent power dip problems caused by a
large current draw from a motor at rest, provision
has been made on the board for capacitor CX . This
will definitely be required if using the
controller supply to provide power to motors etc.
It is also
suggested to minimise interference, to fit 0.01uF
ceramic capacitors between each brush of the motor
and the motor casing. The PWM frequency is 20KHz
and with no shielding and not using a toroidal
transformer the cables from the prototype were
able to induce two very pretty vertical black
lines moving horizontally across the picture of a
television set 30' away in another room. This is
only apparent if your local television station
broadcasts in the VHF band as most still do in
Australia, however the UHF band did not suffer
from this problem and good earthing and the use of
a case and the specified transformer should reduce
these effects. With loads up to 4.5A the MOSFETS
should not require heatsinks. See panel heatsinks
for further details. The transistors specified have an RDS(on) of 45
milliOhms any MOSFET with the same
pinout and lower RDS(on) may be used, it should be
noted that the tracks of the PCB are only rated
for approx 10A. See the MOSFET table for possible
alternatives. Back to Climate index On to AC board
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SOME ALTERNATIVE
MOSFETS |
Heatsink Thermal
Ratings |
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ALL LOGIC LEVEL TYPES
PHP45N03LT
VNP35N07 protected
OMNIFET™
BUK102-50GL protected TOPFET™
BUZ100L
IRL3303
IRL2203N
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Load Amps: 4 10 15 20 C/W* - 8 2.2 0.3 FETs fitted 1 1(2) 2 2 *°C/W calculated max
internal temp 100°C
The °C/W ratings could be doubled if
using the substitute MOSFETS
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