Quote:
Originally Posted by alistairsam
Hi Brendan,
The design looks great but had a few questions with the basics.
With PWM, aren't you basically raising one terminal of the dew strap to +12 and then to zero with the FET? Not sure how the outer sheath can become +ve if the core is the terminal being raised high and outer sheath is connected to ground, won't that stay at ground?
FET's and transistors aren't my strong suit so pls pardon the ignorance.
Secondly, can't a single microcontroller be used if all that's needed is a TTL/CMOS high/low at the gate of the FET?
with an mcu, you can easily get 4 or more independent channels without actual pwm by just changing the interval between high and low pulses with the pot using ADC.
hikerbob's thread had a simple circuit with an 08m picaxe driving an FET. so can't that be extended to 4 or more channels?
what is the max voltage to be applied to the FET?
thanks
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The reason I used a high side switch was because when the switching device
(with a low side switch.....as most designs are) is set to a low (turned down) output (with a low side sw cct)..or just on...the voltage at the collector (we'll pretend we are using a transistor switch here although same same with FET) will be almost at +12V...
see here.
No matter if you swap the inner or outer core wires...the shield or outside of the RCA connector will be close to +12V.
Touch this against your mount....you'll have +12V applied to ground...or thru expensive electronics.
With a high side switch and earth connected to the shield...the shield
will always be earth.
The problem mainly lies with the pseudo adoption of RCA as connectors for
Dew heaters. But be aware most..if not all commercial ccts use low side sw devices (N channel's)! Why...they are cheaper than P channel FETs! Although now days you can use a N channel FET as high side sw if you use a charge pump gate
control.
Max voltage allowed thru a FET is determined by the FET specifications...most have +15V max on gate some have TTL or +5V gate drive.
I'm playing with PIC micro controllers ATM and yes it would be easier than individual 555's. However micro controllers, PICs/Arduino etc are generally not good at multitasking.
This cct was for those not greatly skilled in electronics for them to cheaply & easily understand and make a multi-channel device which is IMHO better and safer than the commercial devices (wrt high side switching) and can be easily repaired for it's life.
In reality I've kinda gone off
PWM dew heating
control idea mainly because of switching noise it can create. However, that said it is still the the easiest method of
control to prevent
dew.
I do not know why you cannot unzip the files? I can. They were done in winrar and the (.rar) and re-named to ".zip" but they download and open for me...might be because I'm using winrar??
Do you have eagle PCB free installed and winrar?
