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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Inducktion wrote ...
Has anyone ever tried to make a relay based buck/voltage regulator? It seems to work pretty well in simulations, anyway.
It certainly simplifies driving techniques!
Switching a mechanical relay at 88 Hz does not qualify as simplifying driving techniques ;)
At my first year in school as control and regulation electrician, we put 50 relays in series to switch on the next, turn off the prior relay, plugged it into the wall socket and witnessed a alarming noise from the 50Hz switching of mechanical relays.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
I think the biggest problem is the inductive kickback that can open a spark at the relay contacts, thing that doesnt happen at semiconductors because *I think* they switch much faster than relays, and that peak can go through the right way, not on relay contacts.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
This circuit isn't actually a buck converter, it could perhaps be called a switching linear regulator.
By starting with a 22kHz square wave voltage source, which would ordinarily be provided by the switching transistor, you're already kind of cheating No ordinary or practical relay could sustain that switching speed. A transistor could do so quite easily.
Another very important part of the buck converter is the inductor in the output filter. An inductor can provide (in the ideal case) lossless filtering when connected in the appropriate way with a capacitor. The RC filter here will never provide lossless filtering. The lack of an inductor means that to filter your square wave (two square waves really) you need that 1 ohm resistor there, otherwise you'll have a 22kHz square wave at your output. Since the full different between the output and input voltage is being dropped across that resistor, with a certain amount of current, power is being dissipated in it. A inductor can have voltage and current on it without dissipating power because all of the power being absorbed is building up as magnetic energy which will be released as current later, so an LC filter is (in the ideal case) lossless.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546
Relays have life ratings. My GE MICRO PLC rates the relays at 2E7 operations dry and 1E5 operations under rated load. It is interesting that telephone companies managed hundreds of millions of relays without connecting any external dv/dt suppression shunting the coils. A diode across, or even in series with a DC relay can change the release timing.
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Relay based buck converter?
No ordinary or practical relay could sustain that switching speed. A transistor could do so quite easily.