I'm in the process of building the power supply for my amp project. I am
having problems getting the correct delay in the step start circuit. I
decided to use the same circuit as the amp project by G. Daughters, K6GT,
that appears in the most recent ARRL handbook.
I am using a 24 V DC relay that has a coil resistance of approx. 280 ohms
(Rcoil) and I want to get about a 3-4 second delay in the RC circuit. When
I got all the parts together I knew the circuit wouldn't work just looking
at the values, so I mocked it up and it indeed didn't work.
The circuit consists of a diode (D5) and 3.9k resistor (R13) in series
connected to the coil of the 24 VDC relay with a 600 uf capacitor (C11)
connected across the coil of the relay. 120 volts AC is connected to this
circuit. This circuit is basically a half-wave rectifier and R13 and the
Rcoil act as both a voltage divider and resistance for the RC time
constant. I am only getting about 4 volts across the coil where this relay
needs a minimum of about 14-15 volts to kick in. This makes sense looking
at the circuit equations:
Vcoil = Vin * Rcoil / ( R13 + Rcoil ) ~= 55 * 280 / (3900+280) ~= 4.0 V
where Vin was measured and is about 0.45 * 120 volts which makes sense for
a half-wave rectifier circuit.
Am I doing something really stupid and missing something?
73s
Bill, W6QD
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