Quote:
Originally Posted by Herald1360
Quote:
Originally Posted by Argus25
One trick, if you want to get fancy, is to have a timer with a relay contact that shorts the NTC out a few seconds after switch on. You can make a timer just from a diode, resistor, capacitor and a relay, no semiconductors needed.
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Only if the diode's thermionic!
And if it is indirectly heated, it'll do a delay on characteristic all on its own |
I don't think that is correct. Consider this:
A 33V DC supply is made from a half wave silicon diode rectified 24V rms transformer output and a 10,000uF filter cap. This can be used to power a 24V DC relay, say just picking one from the RS book with a 20A contact rating and a 1440R coil.
Simply about a 600R resistor is placed in series with the relay coil so that the relay's operating voltage is about 24V.
A 4700uF capacitor is connected across the relay coil. The 4700uF capacitor delays the rise in voltage across the coil at power up so that it has risen to the relay's pull in voltage only after about 2 seconds. (the Thevenin resistance that the capacitor is charging from is the parallel equivalent value of the 600R and the 1440R, or about 420 Ohms) that with the 4700uF cap makes the time constant about 2 seconds, and the relay gets close to its pull in voltage maybe around 16V at that time.
So when the circuit is powered, it takes about 2 seconds for the relay to close. Then a set of contacts can be used for some application, like say shorting out an NTC in a surge limiter system.
When higher voltage DC relays were once available, it could be done quite easily by just half wave rectified mains, and a stepdown to 24V wasn't needed as in the example above.
Then with this circuit, one set of spare relay contacts that were NC, could be placed in series with the power to the circuit, then after the relay closed (say switching on a mains lamp with the other set of contacts) and then the power feed was interrupted by the NC contact opening, the capacitor would discharge into the coil for the next half of a timing cycle until the relay dropped out and it would make a mains light bulb flasher with no thermionics and no bimetallic strips, the hysteresis being provided by the different pull in and drop out voltages of the relay. I had one of these as a teenager flashing a mains lamp in my room using a vintage post office style high voltage DC relay, a resistor a silicon diode & a capacitor. It worked a treat.
A similar idea was used in early automotive lamp flashers that replaced the bimetallic strip ones.
It is amazing what you can do when you combine capacitors & relays to create delays, they might not be precision delays, but sometimes that is not needed.