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enter image description here Trying to figure out the purpose and functionality of this circuit. Apparently, this is some sort of temperature controller. There is no information on parts values or the load type (heater, probably?). But my question is, what does the unijunction transistor do in this circuit exactly? Does it simply work as a switch that activates the SCR whenever V8 is switched on? Or, does it work as a voltage to phase converter that allows to regulate output power by shifting the SCR turn on phase vs AC sine wave phase? Or is the V9 an oscillator? Can you please provide a step-by-step explanation of how the V9 controls the SCR?

What is the purpose of the V6 Zener diode? Looks like its polarity is wrong on this schematics, am I right?

Jimmy Falcon
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  • polarity is wrong on this schematics I don't see that: What makes you assume so? – greybeard Jan 28 '23 at 22:29
  • greybeard, because the V3, R8 and V6 comprise a half-wave rectifier and V6 in this polarity will simply work as a diode. If the AC polarity is opposite, V3 will be reverse-biased and current will not flow, so the V6 won't even have a chance to work as a Zener diode. Heaters? H1 and H2 are signal bulbs, that's sort of obvious. H2 will light up when heater is active, and H1 will glow when heater is off. – Jimmy Falcon Jan 28 '23 at 22:45
  • @JimmyFalcon What do you imagine R4 and C2 are doing? Also, redraw the schematic. It usually helps to clear up some clutter. – periblepsis Jan 28 '23 at 22:48
  • V6 seems like it might be an avalanche rectifier, or mis-marked; V7 seems more likely a zener, acting with R8 to shunt regulate a local DC supply. – Tim Williams Jan 29 '23 at 00:30
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    What is "thermometer"? CTN ... – Antonio51 Jan 29 '23 at 12:31

2 Answers2

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V9 is a "Unijunction transistor relaxation oscillator". It generates an ugly-looking triangle wave. V8 controls its shape very, very roughly. It is free-running (it's not synced with the line frequency). Since it's for heating control, I guess that it uses large values of resistance and capacitance to generate a signal with a period of many seconds.

Davide Andrea
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    But it's not a PUT, at least if the schematic is as shown. A UJT becomes conductive above the threshold, dumping C2 into V1, triggering it solidly. The triangle is not used as such, but it does appear to be a free-running relaxation oscillator; V8 seems intended to gate it, but I'm not sure that this is possible (perhaps C2 doesn't deliver enough charge to trigger?). It may be frequency modulated. – Tim Williams Jan 29 '23 at 00:29
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    Thing is, UJTs are often used to control SCRs. Here is an example https://www.mi.mun.ca/users/rmatchim/assign/e1100/triggers.pdf – Jimmy Falcon Jan 29 '23 at 08:28
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    So it does not appear to be an oscillator, although it looks like one... Why do they use UJTs like that? That's the question. Instead of a diac? – Jimmy Falcon Jan 29 '23 at 08:30
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    @JimmyFalcon Diacs are relatively "higher" voltages (>20 V ?). – Antonio51 Jan 29 '23 at 12:25
  • V9 act as a synchronized ("reset" every main cycle) oscillator where the first pulse fires the SCR. V8 acts as a "variable " resistor (variable current source) adjusting the delay for the first pulse, and then the full circuit is a regulated "temperature" supply for the load or "thermostat". – Antonio51 Jan 29 '23 at 13:45
  • "reset" every main cycle How? It's powered by flat DC and it has no input. – Davide Andrea Jan 29 '23 at 14:15
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    If all values were known ... would be "easier" ... – Antonio51 Jan 29 '23 at 15:20
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    That's actually what's unclear here. No obvious synchronization with AC cycle. Also, the way I understand UJT operation, is that it's got some threshold voltage that triggers it to switch on when applied to emitter. However, in this circuit the voltage at the emitter is constant, because a fixed resistor R4 is installed. So, it looks like the V8 acts as a mere switch that enables or disables the UJT and SCR, and UJT is also fixed at some AC duty cycle set by R4-C2. I mean, the V8 does not appear to affect SCR duty cycle because V8 does not change the UJT threshold. – Jimmy Falcon Jan 29 '23 at 16:41
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    UPDATE: Oh, I think I get it. They affect threshold via base2. When an NTC is connected to R3 this will make the V8 start to switch off. So, the voltage at base2 of V9 via R2 will start to decrease. As the R4 is fixed, this will make the V9 threshold shift. Bingo! – Jimmy Falcon Jan 29 '23 at 16:52
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Here is an example of a "synchronized reset" oscillator with PUT-UJT for SCR system.

Just replace resistor R4 ... with a current source.
Note that it is the first pulse only that fires the SCR at every cycle.

enter image description here

Alternate wiring through the load.

enter image description here

And with a "very poor" current source or BJT "very poor" switching ...

enter image description here

Antonio51
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  • The major difference between your circuits and OP's circuit is that your circuit doesn't have a power-supply filter capacitor and OP's circuit does: C1. A tiny difference is that OP's circuit uses a unijunction transistor and your circuit uses a PUT. – Davide Andrea Jan 29 '23 at 15:44
  • I don't have in my library a 2N2646 or similar ... and PUT has a behavior quasi-similar to a UJT. My circuits are examples very "near" to the OP circuit. However, the value of C1 is unknown ... And I am sure (99%) that a reset occurs in that circuit via R8-V6. And V7 is not a Zener. – Antonio51 Jan 29 '23 at 16:23
  • So, the reset must then occur thru the reverse-biased V6, V5 and V1? And V7 does not allow negative voltage to occur, but allows to discharge the C1, am I right? – Jimmy Falcon Jan 29 '23 at 17:11
  • But V1 should be off at that moment, how does that work then? – Jimmy Falcon Jan 29 '23 at 17:18
  • In your schematic, the load is wired into the mains input circuit. That does not change really the behavior. Sorry, I will rewire my last schematic. – Antonio51 Jan 29 '23 at 17:37