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paul · Joined: 27 Jan 2003 Posts: 1 Location: suffolk uk

:? hi
can you explain how transformer rectifiers work in respect to thyristor controlled rectifiers???

ChrisR · Posted: Sat Feb 22, 2003 12:47 am Post subject:

In what context exactly? Basics - a transformer converts an AC voltage to another AC Voltage. The rectifier used with a transformer is normally a ring of 4 silicon diodes- 2 legs each- which produces DC voltage. It isn't smooth DC, but a series of pulses at twice the frequency of the original AC. Big capacitors can be used to smooth out the voltage, but for things like lamps it isn't necessary, because the filament doesn't cool appreciably between the pulses, and it's getting a bit quick for a human eye to see anyway.

Thyristor controlled rectifier - I can't remember coming across that combination of the words as such. But a Silicon Controlled Rectifier, = SCR, = Thyristor, is a 3 legged thing a bit like a diode (which conducts DC one way only) but has an input (Gate) which can be used to make it conduct, or not. There may be an application where SCR's are used in a rectifier arrangement (4 in a ring as above) but also using the Gate to turn the output voltage on & off.
The other device you may come across is a Triac, which is like an SCR, or rather two SCR's together, making a thing which conducts AC, and has an input which can be used to turn it on & off.

Hope that helps. For cathodic protection all you need is a steady DC voltage, so a transformer and a standard rectifier, best with a bit of smoothing, is all you'd need.
(yes I am the moderator from the plumbing section - long story though, I did degrees in Metallurgy & Materials Science, and Electronics & Computing)