DIAC question

[gunterh...@gmail.com]

It has been a few minutes since I was "active" (like ~8152380 minutes, but who's counting)

I am working on putting together a simple AC motor speed controller for a blower on a coal forge (blacksmithing). I've found a few circuits that may work. 

A couple of questions. DIACs. I'm not familiar with them. The circuits that I've found just say "DB3 DIAC" is there more to spec out? 

I'm looking at something like this

https://drive.google.com/file/d/1Q6AmzJralQQyKOZZ-AdcMVTbjqM0PKkL/view

The other question is who do you get parts from now a days? I've ordered from mouser in the past, but much prefer if I can get local. Surplus Gizmos doesn't have everything that I "need" so am thinking I need to order, but unsure who from anymore.

Thanks.

Dan.

[Jerry Biehler]

Ahh. DIACs. First ran into a schematic that needed one in the 80s when I was kid. Very uncommon even back then. 

I’d personally look at a single phase output VFD. They make them for fractional HP single phase motors. 

DigiKey or mouser is where I order from. About the same service between them. 

-Jerry

On Nov 6, 2025, at 12:19 PM, gunterh...@gmail.com wrote:

It has been a few minutes since I was "active" (like ~8152380 minutes, but who's counting)

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[Jerry Biehler]

Just looked at the circuit.  That’s… something. What kind of blower do you have?

-Jerry

On Nov 6, 2025, at 4:38 PM, Jerry Biehler <jerry....@gmail.com> wrote:

Ahh. DIACs. First ran into a schematic that needed one in the 80s when I was kid. Very uncommon even back then. 

[Doug Ausmus]

I have a supply of diacs... Look at this curve:

It's a spec'd trigger device. Neon bulbs used to exist that were tightly spec'd and were used as switching devices way back when. Good luck finding stable/spec's neon bulbs now. It is useful when you need this kind of controlled breakdown.

Doug

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[gunterh...@gmail.com]

I have a couple of blowers that I want to control. The simplest way that they are often controlled is by a blast gate, which often works well for this application. I'd like to try the variable speed route (for reasons). One of my portable forges is built from scrap, random parts, car wheel, old pipe fittings, etc . The blower is from an old vacuum cleaner. It was originally controlled by a ceiling fan speed controller but that seems to be less "controller" than it once was. I didn't build this, a friend did and I inherited it when he passed. There are commercial versions of speed controllers that I could get for not too much, likely less that it would cost me to build, but buying things is for suckers. The other not-so-portable forge has a small squirrel cage blower. Looking at similar online they spec at 1/70 HP, so not much.

I'd like to say that I'm better with DC low voltage/digital stuff, but I do have more experience failing with those than I do with AC mains.

Thanks. 

[Jerry Biehler]

The vacuum cleaner motor is a universal wound motor and can be controlled using a router speed controller from

Harbor freight. 

The other one can be controller through a bathroom fan speed controller or something like that. 

Different motors will need different speed control solutions. 

-Jerry

On Nov 19, 2025, at 4:07 PM, gunterh...@gmail.com wrote:

I have a couple of blowers that I want to control. The simplest way that they are often controlled is by a blast gate, which often works well for this application. I'd like to try the variable speed route (for reasons). One of my portable forges is built from scrap, random parts, car wheel, old pipe fittings, etc . The blower is from an old vacuum cleaner. It was originally controlled by a ceiling fan speed controller but that seems to be less "controller" than it once was. I didn't build this, a friend did and I inherited it when he passed. There are commercial versions of speed controllers that I could get for not too much, likely less that it would cost me to build, but buying things is for suckers. The other not-so-portable forge has a small squirrel cage blower. Looking at similar online they spec at 1/70 HP, so not much.

To view this discussion visit https://groups.google.com/d/msgid/dorkbotpdx-blabber/ebb0669a-b071-4cb0-ad06-c4b683e90e08n%40googlegroups.com.