I am trying to fix a rotary tool's AC powered DC motor controller, there is a resistor that has been burned out, I want to replace it for a new one, my question is how do I find the value of the resistor since I can't see clearly the color coding anymore and I don't have the schematic.

Thank you in advance for your help.

(please see the pictures)enter image description hereenter image description hereenter image description here

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    \$\begingroup\$ Also there's a reason that the resistor burned up, possibly not having anything to do with the resistor itself. So just replacing the resistor might not fix the problem and might just result in another burned resistor. \$\endgroup\$
    – John D
    Feb 6, 2021 at 0:07
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    \$\begingroup\$ @bran I can safe you that effort: that resistor being toast means something else is broken, too. Resistors like that one don't just burn for fun. Even if it wasn't the reason, the capacitor that the resistor roasted also has to be replaced. You'll also want to replace the thyristor/TRIAC, being the most likely to have failed. Great! Now you're ordering electronics worth ca 1,50€ for 5€ shipping, so you add four rectifier diodes for a total of 40ct and the other yellow capacitor to your order. \$\endgroup\$ Feb 6, 2021 at 0:56
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    \$\begingroup\$ Is that a one sided circuit board? How about a picture of the other side? \$\endgroup\$
    – K H
    Feb 6, 2021 at 3:20
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    \$\begingroup\$ Check C3 for shorted condition (out of circuit). \$\endgroup\$ Feb 6, 2021 at 3:50
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    \$\begingroup\$ Maybe you don't care but this is a DC motor controller. I propose to edit and change from "AC speed controller" to "AC powered DC motor controller". \$\endgroup\$ Feb 6, 2021 at 7:22

1 Answer 1


Based on what's visible in this video, the resistor (R1) appears to be 1k Ω ±5% (~0.5 W?). Here are some screenshots:

Front of board Front of board. A bit blurry but the best shot I could find of the resistor.

Back of board Back of board. Resistor is across the two right-most pads on the lower edge.

And here's a schematic:


simulate this circuit – Schematic created using CircuitLab

Schematic is laid out to match part locations on the board as viewed from the top. R1 is the burned-out resistor that the question is asking about. It's not very visible but the OP has verified that the part labelled D5 is a DIAC (part marked DB3).

Here's the schematic redrawn a bit more clearly:


simulate this circuit

Use of a DIAC on the gate of a TRIAC is discussed here. The datasheet for the DIAC can be found here (thanks to @PaulGhobril for finding that). The datasheet for the TRIAC can be found here; the part used on this board does not require a snubber.

My guess is that in addition to R1, at a minimum C3 is probably also toast (shorted).

  • \$\begingroup\$ In the schematic, D5 must be a DIAC. Inputs 1 and 4 are connected to the AC power supply. 2 and 3 are connected to the DC motor. \$\endgroup\$ Feb 6, 2021 at 8:05
  • \$\begingroup\$ @Alex Hajnal, thank you for your detailed explanation, it looks that C3 is good I removed it and tested it the reading was 100.8nf ESR=15Ohms, I am going to replace the resistor with a 1kohm one, I will let you know what happens. \$\endgroup\$
    – bran
    Feb 6, 2021 at 14:58
  • \$\begingroup\$ @Paul Ghobril D5 component says DB3 \$\endgroup\$
    – bran
    Feb 6, 2021 at 15:01
  • \$\begingroup\$ @alexhajnal it is a Diac indeed. st.com/resource/en/datasheet/db4.pdf. \$\endgroup\$ Feb 6, 2021 at 15:12
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    \$\begingroup\$ @bran Regarding capacitor C1 in particular, it's probably marked as being a class X capacitor. Make sure that any replacement you get is the same class and voltage. The class of capacitor (e.g. X or Y) indicates how it behaves if it fails. See here for more details. \$\endgroup\$ Feb 6, 2021 at 21:35

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