Signal Mixer
Signal Mixer
(OP)
Hello All,
I am analyzing a signal mixing circuit that belongs inside of a generator voltage regulator. The mixing circuit is part of the feedback loop for controlling generator output voltage onto the system. A part of the circuit is a bit confusing to me. It involves diodes and transistors. While I am used to working with both seperately, together I am not sure as to how they work together.
In the schematic, the output voltage is going into an inverted op-amp (type 741) with a gain of -1. The schematic information book reads as follows:
"Inputs C and D are "auctioneering" inputs, in which the most positive of the two signals is given control.
A negative signal at Input A will override any signals present at the other inputs (note: Input A is considered a "limiting" circuit)."
To make things easier, Input B is usually riding around 14 Vdc.
I guess my questions are:
1) How does Input A override all other inputs?
2) I believe that if Input B were to change, it would override. Is this true?
3) Are the transistor amplifier circuits used to prevent overloading of the diodes? They seem to always be gated on, so I'm not sure of their purpose.
I am aware of how an auctioneering circuit works, but usually just in a diode-OR function, not in parallel with "limiting" circuitry and NPN transistors.
Any help is appreciated!!
Sincerely,
Mario Capuozzo
Associate Engineer
h ttp://file s.engineer ing.com/ge tfile.aspx ?folder=ac 82bf22-896 e-483e-82d a-492ed0b1 942e&f ile=Signal _Mixer.vsd
I am analyzing a signal mixing circuit that belongs inside of a generator voltage regulator. The mixing circuit is part of the feedback loop for controlling generator output voltage onto the system. A part of the circuit is a bit confusing to me. It involves diodes and transistors. While I am used to working with both seperately, together I am not sure as to how they work together.
In the schematic, the output voltage is going into an inverted op-amp (type 741) with a gain of -1. The schematic information book reads as follows:
"Inputs C and D are "auctioneering" inputs, in which the most positive of the two signals is given control.
A negative signal at Input A will override any signals present at the other inputs (note: Input A is considered a "limiting" circuit)."
To make things easier, Input B is usually riding around 14 Vdc.
I guess my questions are:
1) How does Input A override all other inputs?
2) I believe that if Input B were to change, it would override. Is this true?
3) Are the transistor amplifier circuits used to prevent overloading of the diodes? They seem to always be gated on, so I'm not sure of their purpose.
I am aware of how an auctioneering circuit works, but usually just in a diode-OR function, not in parallel with "limiting" circuitry and NPN transistors.
Any help is appreciated!!
Sincerely,
Mario Capuozzo
Associate Engineer
h





RE: Signal Mixer
RE: Signal Mixer
Any ideas now?
RE: Signal Mixer
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Signal Mixer
Is there any chance that the schematic you've posted is wrong, or incomplete? Is it a 3-phase generator and you're missing the components to complete the diode array rectifier and filter needed to make DC from AC?
RE: Signal Mixer
This style of circuit is very common in the older Westinghouse AVR designs: I am half-wondering if this is part of the schematic from a WTA-300 series AVR. I've spent a few highly entertaining callouts trying to work out what the heck that design was doing.
The inputs are typically DC levels from various limiters and from the error amplifier output. Normally the error amp output drives the next stage, typically a gain stage and firing angle controller, but it can be overridden by supervisory inputs such as the excitation limiter and the Volt/Hz limiter for example.
I suspect the output should be taken from the collector of the 2N1711 and not from input B as shown.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Signal Mixer
No, this is not for school. I am not sure why you ask that.
To ScottyUK:
You are correct, sir. This is a WTA-300B. You are right about the inputs. I tried not to be too specific about their functions: not everyone has seen the WTA-300B voltage regulator. I thought that I would boil it down into a basic circuits question. The output signal is actually coming from an inverting op-amp whose input is Vout. If you would like, I can make a scan of the original westinghouse diagram. I was initially hesitant in doing so, as the print from westinghouse is actually an "information only" style print, and not an "as build." Therefore, some of the inputs on the drawing are actually not being used inside the power plant.
As to my initial questions, I'm not sure what those transistor circuits are for. I've been on a trouble call recently where the AC error detector card was failing. While I was troubleshooting I came across this circuit, and so I'm just trying to make sure that I understand its operation completely.
Thanks!
~Mario
RE: Signal Mixer
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Signal Mixer
RE: Signal Mixer
RE: Signal Mixer
All other outputs except for B are tied with a reverse-biased diode.
2) I believe that if Input B were to change, it would override. Is this true?
Only if the source impedance of B is lower than A's
3) Are the transistor amplifier circuits used to prevent overloading of the diodes? They seem to always be gated on, so I'm not sure of their purpose.
The diodes bias the transistors on. The emitter resistor of each transistor determines the current output of each transistor. This appears to be basic freshman EE course material fodder.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Signal Mixer
RE: Signal Mixer
RE: Signal Mixer
I don't believe that current mirrors in series with diodes is a normally covered topic in freshman EE. As I was unsure whether or not the transistors would have some sort of loading effect preventing the diode from become reverse-biased, I had to ask the question.
That being said, I don't believe it is the purpose of this forum to leave acidic comments in lieu of constructive statements (at least, the FAQ in your signature reads that way). I could point to several threads on eng-tips that are simpler in nature than my questions. I might also add that no one felt the need to jump on those threads and note how "basic" they were. As this is the first thread I've created here, I hope that I don't need to expect the same reaction to any future questions.
That's all I have to say about that.
amptramp and ScottyUK:
Thank you for that verification on the transistors behaving as current mirrors!
Sincerely,
Mario
RE: Signal Mixer
And, note that this site does not tolerate student posting, so postings that resemble homework are particularly scrutinized.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Signal Mixer
Are you still looking for an explanation of the circuit operation? Here's my take on it - but I'm unwinding with a very large whisky after driving back from Sheffield in rush hour traffic so my thought process might go awry
Assume for now that the signals on 13 & 14 are high and thus D2 and D3 are reverse biased, and whatever is on 12 is high impedance.
Current from the current source Q1 and associated components flows via D4 into current sink Q2 and associated components. The voltage at the collector of Q2 is determined by the highest of inputs 9, 10, & 11, less the forward drop of the respective diode. The voltage at 12 is equal to the voltage at Q2 collector plus one diode drop, so it is effectively the highest of the voltages on 9, 10 & 11. The current sink Q2 draws roughly double the current delivered by the Q1 current source, and the balance of current is drawn from the highest of 9, 10, & 11.
If the limiter signal on either 13 or 14 goes lower than the voltage at 12 minus one diode drop, then it becomes the dominant signal and overrides the signals at 9, 10, & 11. The function of the signal at 12 is difficult to determine without knowledge of the upstream circuit driving it.
Westinghouse made a couple of test sets for this regulator which are worth looking for - Buffalo Power bought out this part of Westinghouse during the break-up and they might be worth speaking to about them. SWPC in Orlando will charge you a fortune if you have to buy them through their aftermarket support.
VE1BLL - it just shows how much time I have spent fighting with these damned controllers that I can recognise its circuit from a sketch a couple of years after leaving the plant. I'd like to say it is a masterpiece of analogue design, but in reality it is a horrible piece of equipment comprising of large plug-in modules containing next-to-nothing and which cost a bloody fortune from Westinghouse. Some of the circuit design is kinda cute in a very old-fashioned way, but the little discrete blocks make it a pig of a system to fault-find on. On the positive side they are remarkably easy to repair in the field.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Signal Mixer
The sink (lower 'source') sucks twice the current of the source. Not sure why.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Signal Mixer
You are 100% right on the equipment. I was at the Basler factory in Illinois last week. They took over all of the ECS 2100 voltage regulator line from Cutler-Hammer. Now, they also make the DECS, along with WTA and MGR voltage regulators (pretty much cornering the market on reliable voltage regulation). I got to see their circuit fab operation, first person. It is interesting to me that they still charge $5k for a board with a few 741 op-amps and a few resistors!
Sincerely,
Mario
RE: Signal Mixer
Agree on terminology - am I guilty?
My explanation for the difference between source and sink currents is that diode D4 acts to restore the voltage at 12 to the value of the highest of the signals on 9, 10, & 11. The current in D4 is determined by the Q1 source, and the current in the other diodes is determined by the difference between the Q1 source and the Q2 sink. By making the currents roughly equal the diode drops should be quite accurately matched.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Signal Mixer
That makes sense. The current mirrors I've seen involve transistors whose gates are tied together in some way or another (like in the 741 opamp). Thanks!
RE: Signal Mixer
RE: Signal Mixer
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Signal Mixer
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...