help me pick a triac to fix my old Dremel

[WastelandTek]

My trusty old Dremel failed, the triac has an internal short and a trace is burnt off the board.

I don't know a lot about triacs, and this one just says "Q954" on the case which I can't find any reference to, not surprising as the tool dates from the '80s.

I know it would probably be cheaper, certainly in terms of time, to simply buy a new one, but this is the one I built models with as a kid, and maybe I will learn something.

I did a bit of googling around and found some similar circuits, which allowed me to draw up the schematic in a way that I think makes sense.  I present that with a plea for help in choosing a replacement part.

Thanks all.

[george.b]

A Dremel is a pretty low-power tool. I'd imagine pretty much any modern TRIAC would do, like a BT139, BTA24 and such, which might be overkill, but they're cheap.

[Tomorokoshi]

A shorted triac should have only resulted in the motor being on 100%. To pull enough current to burn the trace would require a high load in the "To Rectifier" circuit. Is there a fault in that circuit?

[WastelandTek]

A shorted triac should have only resulted in the motor being on 100%. To pull enough current to burn the trace would require a high load in the "To Rectifier" circuit. Is there a fault in that circuit?

one of the commutator pads failed, that is a separate micro-machining project

[Tomorokoshi]

What package is it? Can you include a picture?

[floobydust]

Dremel DR95 Benchtop Pro uses a TO-202 triac that has no tab. ST's Z04 series Z0409MF 600V 4A 10mA. Dremel also added some parts resistor, NTC to limit torque.
If you can fix the motor's short, test using a light bulb current limiter.

[james_s]

Do check the rectifier too, the fault that blew the triac and the trace may well have damaged that too.

[WastelandTek]

What package is it? Can you include a picture?

Of course, I should have done that in the OP



The gate, or base, or whatever they call it on a triac, is the pin on the right

Thanks for all the input guys!

[Pete66]

Why do you think it is a triac?  It could be a transistor.  That doesn't look like a Q it looks like an zero (0954) to me.
Could be a B0954 transistor

https://www.datasheetq.com/datasheet-download/221880/1/Panasonic/2SB0954
2SB0954

[WastelandTek]

Why do you think it is a triac? 

2 things, the diac hooked to its gate and one side of the mains hooked directly to one pin.  I could be wrong.

I admit I was on the fence on the 0/Q thing under the microscope

[YetAnotherTechie]

You should check the potentiometer too...

[james_s]

It may well be a house numbered part, it's definitely a triac though. It's not critical, any standard triac in the right package ought to work.

[george.b]

A transistor wouldn't make sense in this application. Definitely a TRIAC.

[floobydust]

I would say it's a house numbered triac in a TO-220 no-tab package.
ST offers parts in their "IPAK" package, I would plop in a T435-600H 4A 600V snubberless medium 35mA gate current part, same pinout. Or T835-600H, BTB08-600CW 8A 600V snubberless 35mA gate current. Or BTB06-600CW 6A. Not sure of the motor current, but there's no real heatsink for the part so you wouldn't get more than 1A average before she cooks.

There is safety issue if the tab contacts the potentiometer, so I would not get too cute with substitutes that have a big tab.

[james_s]

In a pinch you can cut the tab off a standard TO-220, I've done it a few times when that's all I had on hand.

[coromonadalix]

I had this Dremel problem before,  i simply shorted out the speed controller in the dremel and added an external "triac controlled speed control" in an electic box,  lasted for years until the dremel bearings went nuts  loll

[SeanB]

Fixed many of these things, though the motor they drive is a lot bigger than the Dremel one, 250W, and the typical failure is the bridge rectifier diodes pop, and the triac blows short circuit. Replace the diodes with 1N5403 instead of the 1N4007, replace triac with BT156 ( because it is available locally and cheap), replace pot with another, because the originals are junky types with integral switch, which cooks, but is not needed as there is an external switch, and add a 275VAC MOV input and output. No noise filtering, but that is what you get with Chinese equipment, and no inrush limiter either, but at least the thing does have a 10A fuse in it.

Got in a few speed control boards off eBay, and added an external bridge rectifier to them, along with the MOV's, for some of the more prone to blow motors. Luckily there is a lot of room in the housing to fit them.

[floobydust]

I've pissed around with the cheap chinese speed controllers "4000W speed controller" from eBay. They come with a curse https://www.eevblog.com/forum/projects/current-profile-with-triac-dimmed-magnetic-transformer-and-resistive-load/msg1791326/#msg1791326
The speed control was very non-linear, had to change capacitor to fix that. The potentiometer failed after a couple months and was arcing inside. Added resistor to fix that. The 25A triac failed with a 1A load, had to replace it.

I think it's more hassle than fixing the original speed controller.

[james_s]

Yeah I don't understand why anyone would not just fix the original controller, unless proprietary parts were toast like a pot custom made to fit the housing or something. I never cared for lazy hacks.