Joe Sommer wrote:
>
> On Jan 20, 3:54 pm, "Don Shankin" <[email protected]> wrote:
>> You could get some cheap standard 24v (marine?) relays, and turn them on/off
>> from your RC receiver. Don't connect the receiver output directly to the
>> relay; use a BJT or a MOSFET between them. Nice and cheap
>
> Driving a MOSFET directly from a radio Rx will
> produce 50 Hz pulses with 2% to 4% duty cycle
> which will not drive a relay.
Based on the website diagram, he's tapping the power lines running to
the motor inside of the servo, which has a much higher duty cycle, but
it's still not a good choice. The power lines are still PWM, so if you
hook a MOSFET to them, the MOSFET will be turning on and off rapidly
whenever the relay is actuated. Furthermore, since the gate voltage
will be lower than the load, the MOSFET will not be switching
efficiently. In short, the MOSFET will get hot if the relay is actuated
for extended periods.
Even if those problems were overcome, you would still want to isolate
the relay coil circuit from the servo control circuit to prevent damage
to the control logic. So, think "opto-coupler" instead of MOSFET.
Of course, by the time you've added an opto-coupler to the circuit, the
"battle-tested" Anvilus R/C Switch is looking mighty cheap indeed.
Frank P.
PS: The goal is not just "cheap", but "cheap and reliable". The
cheapest systems are those that work forever.
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