Deleting speed control on my Random Orbital Sander

[Stu_dledoo]

Hi all, i hope you can help me.

My random orbital sander went haywire yesterday, and as it suddenly went on to full speed, i'm guessing the speed control unit shook it soldered joints apart.

I've ripped the "speed knob" apart, and it's now beyond repair.
But before i threw away the entire thing, i thought "i only ever used it on it's lowest speed setting, so surely i could just solder in a resistor of the right rating, and i'm good to go."

am i right? i can supply a diagram, (or pictures) of the existing circuit, but i don't know
a) if i'm right, or
b) how to work out exactly what i need.

its 230V, i've always been happy following circuit diagrams(to a degree) but i haven't the first clue about the theory behing all this wizardy, so please keep any replies simple;-)

Thanks for reading,

Stu

 

[Hero999]

Yes a picture of the original speed controler would be helpful.

My guess is that it uses a phase control circuit and the TRIAC has gone permanently closed circuit which is a common failure mode. The chances are, it can be fixed by simply replacing the TRIAC.

 

[Stu_dledoo]

Here's whats left of it, there were some copper strips and springs in there too.
It was a moulded unit, and has to be smashed apart to see inside.. sorry

it had a copper "slider" which slid along the two black strips on the circuit board.

**broken link removed**

as i said, i only need it on its slowest setting.

 

[Stu_dledoo]

oh, the black bit with the three prongs has the following stamped in to it:

BT136
600E
PHm0046 B 7

and the green thing with the two prongs says:

630V2227

 

[Hero999]

Yes, it's a TRIAC and it's mostly likely failed.

Replacing it with a TRIAC with a high enough voltage and current rating will fix it.

The green thing is a capacitor, 2.2nF 630V.

 

[Stu_dledoo]

Thanks for that,
and please pardon my ignorance,
but am i right in thinking that :
1 leg of the triac is "common"
1 leg would give the maximum current flow,
and 1 leg would give the minimum flow,
and the circuit board just controls all the settings inbetween?

meaning i could solder the 2 correct legs to my 2 loose wires in my sander and hey presto?

or am i completely wrong? and i need the capacitor and circuit boart aswell?

as i said, i dont need it adjustable, i just need it on it's slowest speed.

Thanks for your help Hero.

 

[Hero999]

Sorry, you've got that al wrong, it's much more complicated than that.

The TRIAC is basically a type of switch. There are three terminals, anode 1, anode 1 and the gate. The TRIAC is to be connected in series with a load, which in this case is a motor. When the power is initially applied the TRIAC is off so no current flows. When a short pulse of electricity is applied to the gate, the TRIAC turns on allowing current to flow even if the gate signal is removed. To turn off, the current through the TRIAC needs to drop below the holding current. As this is an AC circuit, the current drops to zero every half cycle.

To control the speed of the motor, the TRIAC is fired at different points of the AC cycle, the earlier the faster the motor speed, the later the slower.

Google for TRIAc and phase control will give you more information.