1
\$\begingroup\$

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?

\$\endgroup\$
5
  • \$\begingroup\$ polarity is wrong on this schematics I don't see that: What makes you assume so? \$\endgroup\$ Commented Jan 28, 2023 at 22:29
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 28, 2023 at 22:45
  • \$\begingroup\$ @JimmyFalcon What do you imagine R4 and C2 are doing? Also, redraw the schematic. It usually helps to clear up some clutter. \$\endgroup\$ Commented Jan 28, 2023 at 22:48
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 29, 2023 at 0:30
  • 1
    \$\begingroup\$ What is "thermometer"? CTN ... \$\endgroup\$ Commented Jan 29, 2023 at 12:31

2 Answers 2

Reset to default
1
\$\begingroup\$

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.

\$\endgroup\$
9
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 29, 2023 at 0:29
  • 1
    \$\begingroup\$ Thing is, UJTs are often used to control SCRs. Here is an example mi.mun.ca/users/rmatchim/assign/e1100/triggers.pdf \$\endgroup\$ Commented Jan 29, 2023 at 8:28
  • 1
    \$\begingroup\$ 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? \$\endgroup\$ Commented Jan 29, 2023 at 8:30
  • 1
    \$\begingroup\$ @JimmyFalcon Diacs are relatively "higher" voltages (>20 V ?). \$\endgroup\$ Commented Jan 29, 2023 at 12:25
  • 1
    \$\begingroup\$ If all values were known ... would be "easier" ... \$\endgroup\$ Commented Jan 29, 2023 at 15:20
0
\$\begingroup\$

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

\$\endgroup\$
5
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 29, 2023 at 15:44
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 29, 2023 at 16:23
  • \$\begingroup\$ 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? \$\endgroup\$ Commented Jan 29, 2023 at 17:11
  • \$\begingroup\$ But V1 should be off at that moment, how does that work then? \$\endgroup\$ Commented Jan 29, 2023 at 17:18
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 29, 2023 at 17:37

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.