What is the importance of R1 connected in this circuit?

Question

What is the importance of R1 in this circuit? What would happen if I removed R₁ and kept VR₁?

think about what VR1 is ... think about its effect on the circuit when it is adjusted to either extreme — jsotola, Sep 27 '22 at 23:45
are you able to read the component values in the schematic that you posted? — jsotola, Sep 27 '22 at 23:47
@PStechPaul: Schematic is sufficiently readable to answer the question. I don't understand the down votes. — RussellH, Sep 28 '22 at 02:52
Yes, I can figure out what is being shown, but it's not that hard to post a clear schematic. I'll give the OP an upvote if they redo the schematic. — PStechPaul, Sep 28 '22 at 03:41
When an image ends up like that it's because there's some transparency junk in the image. To deal with that, take a screen shot of the image and crop, or use Windows Snipping Tool. — DKNguyen, Sep 28 '22 at 17:12

score 6 · Answer 1 · edited Sep 28 '22 at 05:47

6

If \$R_{1} = 0\ \Omega \$, and \$V\!R_{1}\$ is adjusted to \$0\ \Omega \$, then the current through the DIAC and the gate of the TRIAC would be too high, causing one or both to be destroyed.

edited Sep 28 '22 at 05:47

ocrdu

8,705
21
30
42

answered Sep 28 '22 at 00:02

RussellH

12,496
2
9
34

Thanks , For your answer.. But how do I calculate the value of R1? – Mahmoud Youssef Sep 29 '22 at 17:39

AnalogKid · Answer 2 · 2022-09-28T13:04:47.593

5

R₁ limits the maximum current into the TRIAC gate when VR₁ is at 0 Ω.

Because it is only about 1% of VR, it has very little effect on the brightness adjustment range.

edited Sep 28 '22 at 13:04

answered Sep 27 '22 at 23:55

AnalogKid

16,865
1
13
25

Kyle B · Answer 3 · 2022-09-28T00:28:15.990

R1 gives you finer control.

Suppose the ideal range of total resistance is 900-1k ohms. You can get that control with a single 1k variable resistor, but you only would use 10% of the pots possible travel. Suppose instead you use 900 ohm fixed resistor in series with a 100 ohm pot. In this case, you get the whole rotation of the pots travel to make your adjustments. I.e. you can set a value with 10x the precision.

@AnalogKid answer is right also. Either of these (or both really) could be the reason.

What is the importance of R1 connected in this circuit?

3 Answers3