# Firing angle in a rectifier

1. Dec 5, 2012

### daveo91

Hello

I have recently been covering firing angles in rectifiers at uni. I didn't really understand it in the lecture, and the tutorial questions/one to one with the lecturer didn't help either.

It would also seem that the internet isn't of much help either from a quick Google search and as we now have a coursework question on this subject all the relevant textbooks are out in the library.

So I was wondering if anyone has any good links to resources on rectifiers, in particular on calculating the firing angle.

From what I understand it "can't" be calculated?

2. Dec 5, 2012

### jim hardy

what you need is GE's "SCR MAnual", an exhaustive text on that field. They literally wrote the book that trained generations of engineers. Look for fourth or third edition if you buy one. They're usually on EBAY..

Meantime -have you tried searching on phrase "Phase control thyristor" ?

old jim

3. Dec 6, 2012

### Staff: Mentor

Such a sweeping statement, it's bound to be wrong for some cases, at least. What are the factors that have led you to conclude that it can't be calculated?

4. Dec 6, 2012

I am assuming you mean what should the firing angle be for a given load? This is actually pretty difficult to calculate - they are almost always dependent on a (real time)control feedback loop - for ether constant current output or constant voltage. Light dimmers are the biggest users - but this is not a precise application and you do not set by phase angle - you adjust by light.

There are many factors affecting the energy throughput a Thyristor controller: The Source, The thyristors themselves change with temperature, and the load - all vary over time.

For Example if you build a basic DC supply with thyristors ( controlled rectifier), and have the firing angle set with something like a potentiometer ( e.g. no realtime control) - you set the phase angle to get a voltage under no load, then you use the DC supply - and the DC voltage is no longer at the set point.

I would guess - you could theoretically define ALL of the circuit, and calculate a necessary phase angle - but IMO this would just be an exercise - and I can not think of any pracatical application.

And yes the GE SCR Manual is the Bible - Looking through this now (Chapter nine) it has mostly reference graphs for relating Load Voltage to Phase control ( based on source voltage) - but also with notes concerning the type of load. And they reference 15 different typologies. Note- reference to my comments above the section discussing calcuating phase angle being just an exercise - The Manual has 9 pages out of a 600 page book - discussing these calculations....