I purchased a Dremel tool from the United States and using it in India. As always, the trouble is with the power inputs. A step down converter is simply too expensive for those power ratings (>2 Amps output), and hence I am trying to mode the circuitry which is driving the Dremel motor.

Here is the circuit diagram which I have been able to trace from the driver board!


I have searched online and could not find a way to control the firing of the Triac.The BR is a bridge rectifier and connects directly to the power plug.There is a diac in series with the 23ohm resistor.The space was so tight, I could not read the values of the capacitors.

My understanding is that as the cycle progresses, the voltage across the last branch on the left will trigger the triac into conduction.The variable potentiometer is what controls the speed of the DC motor which is in series to the bridge rectifier.

Which part of this circuit will need modifying so that it works with 230v input.The bridge rectifier is capable of handling 400 volts, the lowest voltage on the motor is 33 volts and the highest is 100 volts. Component values not in the Diagram Bridge Rectifier- KBPC604 Full wave Bridge Rectifier with 400V input limit BTB06- 6A Triac Capacitor series with 100 ohm resistor-2E104K 0.1uF 10%

  • \$\begingroup\$ TRIAC - 600V 6A = OK, 400V is marginal for bridge but PROBABLY OK (You say 700V and 400V?). See end of answer re cap \$\endgroup\$ – Russell McMahon Oct 26 '11 at 14:48
  • \$\begingroup\$ Is it rally 2A? 2A x 110 VAC = 220W !+ 0.3 HP. That's a owerful motor for such a wee beastie. May be right. \$\endgroup\$ – Russell McMahon Oct 26 '11 at 17:25
  • \$\begingroup\$ I might be opening my Dremel on the weekend to fix a problem with it running rough. It's a 230v model so I could take a look at the driver board and post the differences. \$\endgroup\$ – snoopen Oct 27 '11 at 1:21
  • \$\begingroup\$ Changed the questions and errors in values! I will post an image of the information plate on the dremel. \$\endgroup\$ – Rupin Oct 27 '11 at 7:23

I assume AC symbol top right = motor?

Doable but some (not too great) risk. - see below.

A small external 2:1 stepdown transformer is safer.
What is Wattage on rating plate?


  • C in series with 100R is a suppression capacitor. If it is not 230 VAC working rated it may tell you so loudly.

    • If the TRIAc is not rated for 230 VAC ... as aboive. It should have a part number on it.

    • The bridge rectifier suggests that the circuit is working with full wave rectified AC. The TRIAC may actually be an SCR.

    • The timing capacitor should never see full mains voltage but being 230 VAC rated would "be wise".

Your diagram is hard on the brain because it is "upside down". The TRIAC control voltage is referenced to the TOP of your circuit (usually = terminal VT1) and not the bottom. It is std practice to draw this reference connection at the bottom as "ground of sorts" :-).
As shown the implication is that the voltage that turrns on the TRIAC is between gate and the bottom of the circuit as shown. This is not the case.

Here is a more conventionally drawn circuit. Capacitor C is charged by R until it reaches a voltage high enough to trigger the DIAC and feed an energy pulse to the gate. The speed is slowed down by taking longer per cycle to charge the capacitor to DIAC firing level. This is achieved by increasing the minimum and maximum values of R. Rmin sets fastest charging time so earliest time in main cycle to trigger so maximum speed. Rmax sets longest time to charge so slowest speed.

enter image description here

Methods / approaches:

The aim throughout is to slow down the time taken to charge the timing cap to DIAC trigger voltage.

(1) It is possible that your Dremel is "designed" to be either 110 VAC or 230VAC and that the 50 ohm resistor across the pot reduces Rmax and Rmin enough so that it operates on 110 VAC. By removing that resistor you MAY increase Rmax enough to slow the motor down enough. But you won't affect Rmin.

(2) A simple method that will have a reasonable chance of working is to double all resistors. It MAY be enough to about double the 620 ohm to 1200 ohm - or bit more or a bit less.

(3) If you have room them doubling the value of the timing cap should work well. Or connect an indentical one electrically in parallel with it.

This circuit may look familiar. Compare to diagram and text above. Ask questions.

enter image description here

Timing cap size:

As noted, doubling the timing cap should APPROXIMATELY halve the effective motor voltages. Very approximate because of the waveform shape / timing issues.

You can try putting misc caps in parallel with existing cap and seeing what happens.

How big is the existing cap?
Cap must be charged to DIAC voltage in a half cycle worst case. Any longer is ineffective.
t = RC.
R seems to be about 620 R + (50R in // with pot)
Those values are sustpect.
Measuring 620R WITH POWER OFF AND PLUG OUT may be useful.

Say Rmax = 1k. T = 1/2 cycle at 60 Hz say = 1/120h second ~= 8 mS.
T = RC.
C = T/R = 0.008/1k = 0.000008 F = 8 uF. Is it an electrolytic? Seems too high but a few experiments with mains rated caps there will show. eg 0.1 uF in parallel should have minimal effect IF C = ~~ 8 uF
If 0.1 uF swamps existing cap and makes Dremel run really slow then try 0.01 and 0.001 etc.


Iwould be tempted to

  • Read rating plate for max power.

    • Use a small isolating transformer.

An auto transformer may do.

Apply mains to a centre tapped 240 VAC winding. Connect Dremel across one half.


Take two identical transformers rated at 110:X or 240:X
Connect primaries in series = mains in.
Connect secondaries in parallel with correct phase = inter unit coupling.
Connect Dremel across one primary winding.

Here X V winding can be any voltage as long as transformers are identical.

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  • \$\begingroup\$ Thanks a ton Russel for the tips! I will dig deeper and see what the component values are. \$\endgroup\$ – Rupin Oct 26 '11 at 13:16
  • \$\begingroup\$ I edited my question with the component values which I could read from the board, the value of the capacitor series with the 620ohm resistor could not be read as it was covered with a lot of sticky glue. \$\endgroup\$ – Rupin Oct 26 '11 at 13:53

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