# My take on Miter Saw dust collection



## EEngineer (Jul 4, 2008)

This all started when the company I work for threw away 3 nice tabletops from some electronic benches. They were perfectly flat, 1 1/4" thick and laminated with a round bull-nose on the front edge. One of them made a real nice workstation for the miter saw. I'll blog about that after I finish the wings and drawers. In the meantime…

Nothing makes sawdust like the miter saw. OK, maybe the table saw or the router table, but I already have dust collection for those. So I decided some dust collection was in order for the miter saw, too. I started with a dust collection funnel:









Framed it into a box and cut a coupla access holes…









Epoxied a blast gate and extension tube into the finished funnel:









And mounted it to the wall between two studs where the miter saw table sits:









Here's the connection beneath the table to the vacuum:









And here is the connection to the top of the saw (instead of the stupid little bag they give you for dust collection):









I designed my own vacuum switch (check the handle!) to turn on the vac when the saw runs:









And attached a "buckethead" vacuum. The vacuum and bucket costs ~$20. I defy you to find cheaper dust collection than that!










All in all, it works pretty well. I would have to say that it collects about 65% of the saw dust generated. I may have to upgrade to a larger vacuum or surround the miter saw to do any better.


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## toolie (Mar 16, 2011)

a DC, replacing that shop vac, would probably make for better dust collection from the funnel. what was used as a current sensor to automatically activate the shop vac when the miter saw is activated? and is that "override" switch a 3 way switch that toggles the shop ac between automatic and manual operation?


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## PurpLev (May 30, 2008)

slick!

I like it.


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## redSLED (Mar 21, 2013)

Well done. Just great. Your system is giving me all kinds of ideas now that will cause me to take on more projects. It just never ends.


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## vikingcape (Jan 3, 2013)

I may steal some of your idea. Great post


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## EEngineer (Jul 4, 2008)

I used a current transformer to detect when the saw was powered. It drives a solid state relay to turn on the vacuum. The override switch is a simple one-way connected right across the SSR that turns the vac on without powering the saw. Since it still leaves a lot of sawdust on the table behind the saw, I use the override switch to turn on the vacuum and use a brush to sweep all the leftover sawdust into the funnel.

Here's a quick series of shots of the vacuum switch build…

Testing:









Circuit board:









Built into an electrical box:









Switch and outlet wiring:









Completed:









This really isn't woodworking so I didn't want to post it as a project.


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## toolie (Mar 16, 2011)

neatly done. but while you identify two components (the current transformer and the solid state relay), the first pic seems to display three components in addition to the sj wire. any chance for info that identifies the components (make, model, source, etc) for those who might want to replicate it?


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## hobby1 (Feb 10, 2012)

Just taking a guess at it,
I think the round component is a bridge rectifier, to convert the ac to a rectified dc, then a filter capacitor, with a diode across the output that is to eventually go across the input of the relay coil, to keep from inducticve kickback , then finally this output is a strong dc signal across the coil of a relay, to energize it?

It seems to be a miniature dc power supply, for an AC signal current.

Just a fun guess at it.

I wonder how close I got it. hehe


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## EEngineer (Jul 4, 2008)

OK, toolie, you asked for it! History says, as you well know, this doesn't end well!

Hobby1 - give the man a Gold Star… you were close. Here is the schematic:










But as far as the diode on the output - no!
1. the relay being driven is a solid-state relay - no inductive kickback at all.
2. the SSR has a constant current limit. The current transformer will increase the voltage until its current limit is met. In practical terms, this means that the max voltage at the input of the SSR will be exceeded unless we provide a place to dump the excess current. That 6.8V zener does exactly that. If the voltage exceeds 6.8V, the SSR takes whatever current it demands (measured values: 8.7 to 9.1 mA) and the remaining current from the current transformer flows through the zener diode. Notice thast is a 5W zener - I always overbuild things.

Did that make any sense? If not, ignore this conversation. That is part of why this never ends well.

Now, here is the problem with this statement (and another part of why this never ends well):
any chance for info that identifies the components (make, model, source, etc) for those who might want to replicate it?

All of these parts came out of my junkbox. You know, that box where all the spare parts from 30+ years of designing electronics ends up. I don't even know what the part numbers for some of these parts are - they were in my junk box, they worked!

Now, you wanna do this… look at the sources I did:
Gadget Freak 98
pdf
Note the way this circuit limits the current into the SSR input. This is required if you use a SSR other than that spec'd above. If the SSR just has a photodiode in series with a resistor, then you must use his circuit to limit diode current input to avoid destroying the SSR emitter.

Instructables - DIY Shop Vac Dust Collector
pdf

DIYwiki current activated switch

Fine Woodworking book that shows how to hook up a relay in your subpanel

Current Sensing Slave Power Switch

and, finally, here are the part numbers that I know about in the above circuit:
Bridge Rectifier - Digikey part# W01GDI-ND
SSR - BG Micro Electronics - BG Part Number: REL1114

The only part missing in the above list that is critical is the current transformer. Many current transformers will only output mV - the core saturates and limits voltage output. To drive the >1.4V needed to overcome the bridge diodes' threshold you have to get a current transformer with a large core area.

The one I pulled out of my junkbox is 1000:1 ratio. That implies 1 mA of current in the secondary for every 1 A in the primary. The primary is usually just 1 turn through the center of the current transformer. When I hooked this up to test, I found about 7mA with one turn. Since I had already measured the SSR and found 8.7 to 9.1mA draw on 4 of the SSR's, I knew I would need more current. Doubling the primary (2 turns through the center of the current transformer rather than 1) I read 14 mA through the secondary.

When I hooked it up through the bridge, it provided in excess of the 6.8V zener that I used to limit the voltage excursion. Thus the final schematic you see above.

I think (but I cannot guarantee it) an equivalent part number from Digikey is:
Digi-Key Part Number: TE2274-ND

I take no responsibility for this being the wrong choice!

Everything else is non-critical: any electrolytic capacitor will work. Value may range from 2200 to 470 uF It is just not that critical. The zener voltage isn't critical either. The SSR spec'd above current regulates from about 1V up to 42V (max spec on input) - anything from 5V to 15V should work fine.

Good luck! Mine works so I know it is possible.


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## hobby1 (Feb 10, 2012)

Hi EEngineer,

You guys kept on mentioning solid state relay, and it never registered with me, I kept on thinking coil relay, but if my understanding is correct, the zener, acts as a voltage regulator?

Since around 1980's my first hobby was electronic circuit design, using all discreet components.

I remember building a magnetic flux sensing circuit for an alarm box that would tell me when my alarm clock would lose power, it had no battery backup.

But back then I didn't have access to a current transformer, so I used a small radioshack audio transformer, then coupled it into some very sensitive transitor amp stages, finally to a buzzer output.
(However I don't know if a current sensing transformer would have worked in this aplication, seeing I needed to detect the magnetic field, through an enclosure.)

It works beautiful, I would set the box near the power transformer, of the alrm clock, then unplug the clock and the buzzer would sound.

I built a similar one without a buzzer, to locate live wires in the wall, with a led indicator.
For that I used a small power transformer to sense the magnetic field.

They were fun projects.

Seeing your design of your circuit, makes me think about designing one of my own, to have a vacume switch.
Only I would probably use my High current coil relays, since I have so many in my junk box.
It would be an opurtunity to get back in the mood for designing basic transistor circuits again.

Thanks for taking the time to write out the details on your last post.


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## RussellAP (Feb 21, 2012)

If you have the room, this would be an awsome addition. I use it and it keeps the dust down really well.


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## EEngineer (Jul 4, 2008)

hobby1 -

The zener is actually more of a voltage limiter. The current transformer wants to deliver a set amount of current and will drive the voltage to whatever is required to reach that current. The literature is filled with warnings about not running large current transformer (big power applications) without a burden resistor - the voltage can reach kilovolts! The SSR will fail if more than 42V is applied.

Take a harder look at the Gadget Freak article I linked above. He actually used a small power transformer as the current sense. If I hadn't had a current transformer then I would have done this. And power transformers have large, not easily saturable cores. Note that he also drove a large relay but he used a SSR to switch that.

Because of the low level currents involved with a current sensing transformer, an SSR is almost required. When you are talking relays with any kind of current handling, then you need 100's of mA to drive the coil and you simply won't get that out of a current transformer.

When I was playing with this circuit I tried to see just how much current I could get out of the current transformer. I had a 200:1 current transformer that should have given me 50X as much current on the secondary. That's when I started to learn about core saturation and voltage limiting on current transformers.

I am currently (every pun intended) working on another version of this circuit for my TV. The intent is to sense when the TV is turned on and powerup the audio amp and subwoofer. That circuit uses a more traditional burden resistor on the output of the current transformer and only develops about 10 mV. I have an opamp circuit that then amplifies and rectifies (find the circuit here) the signal to drive the same SSR that I called out above (they were cheap so I bought 4 of them - see how that junkbox gets filled?). The opamp circuit is powered by a +-12 supply that is always ON. You could certainly do something similar to this and drive a relay directly.


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## EEngineer (Jul 4, 2008)

Russell -

I simply do not want to build a tent around my miter saw. What I have captures most of the dust and I can still see my miter saw blade and table. How in the hell do you use yours when it is all covered up like that?


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## hobby1 (Feb 10, 2012)

EEngineer

Thanks for the good description, I have been away from this hobby quite along time, I would have to get my books out again, to get back up to speed on this stuff.

However when you guys start talking opamps, I bow out, because for some reason, I always had a facination to design circuits from the very basics (discreet components), I always liked the challenge of trying to take the integrated circuit functions, and reverse engineer there functions to design a transitor circuit to emulate the same results.

Thanks again for taking the time to explain your circuit.


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## toolie (Mar 16, 2011)

thanks for the feedback. good write up and i thoroughly get the parts identification issue. i'm a parts "pack rat myself". i especially liked the DIY shop vac dust collector PDF. that write up used the same strategy i employed in creating a relay so my 220v tools would activate my 220v DC via my 110v ivac switches ( i am too cheap to upgrade from my 110v ivac switches to their pro line of products (which looks to be really impressive)).

a current sensor within the 220v outlet enclosure has one leg of the 220v line passing through it. as it's a very mninimal output sensor, it activates a relay which energizes a 60W light bulb which is powered via the power tool outlet of the first of my ivac switches. when the power tool outlet of the first ivac energizes the light bulb, it also activates the slave outlet on the ivac (which is 110v) which powers a 110/220v relay which energizes the 220v DC. a little rube goldberg but this way, either 110v or 220v tools activate my 110v shop vac and 220v DC simultaneously through my daisy chained ivac switches.










and here's a short video of how it works:






EEngineer….your solution is far simpler and more elegant than mine. thanks again for sharing. with theory and wiring diagram in hand, as well as several indications of specifications and capacities, replication of your accessory should be feasible for others. thanks again for taking the time to document it so thoroughly.


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## joeyinsouthaustin (Sep 22, 2012)

The old box behind the saw…. a very poplar and effective solution. I like your effective use of the wall and studs to help make up the top section.

*Russellap* Get some smores and invite the saw over!! That thing looks effective, never seen something like that, so thanks for showing it off. I don't think I could work with one of those on a daily basis, but we have been doing a ton of high rise condo installs, and work lately… and one of those would fit in great up there, even just to smooth the client over. That reminds me… much props to the city carpenters out there.. dock schedules, parking, freight elevators, all the extra measuring just to make sure stuff goes in.. my gawd!!!


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## RussellAP (Feb 21, 2012)

Joey, I think that was the original intent, for contractors saws in area's where dust could be an issue. After using one nearly a year, I love it. It doesn't get in the way of any cuts, in fact you don't even pay attention to it after a while. If you were to hang a hose from your DC up in there, you'd be about as close as you can get to dust free.


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## joeyinsouthaustin (Sep 22, 2012)

A quick search turned up a mess of stuff about stolen cars, and hipster Chinese restaurants. Any chance you would post a link?


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## EEngineer (Jul 4, 2008)

hobby1 -

However when you guys start talking opamps, I bow out, because for some reason, I always had a facination to design circuits from the very basics (discreet components)

I find that a strange attitude. If it can be done with a coupla discretes, I'll do it. But there are whole classes of problems out there that only opamps are suitable for. In fact, years ago, I was forced to do a discrete design (voltage and current requirements) that was really suited to an opamp. The first step in the design was to create an opamp from discrete transistors. They aren't that difficult.

I notice that most of the comments here center around the electronics aspect of this. That isn't really woodworking and I wasn't going to post it as a project. Maybe I should create a project under "tools" or such just for the vacuum switch?

In fact, since this circuit worked out so well for the miter saw, I am thinking about something similar for the table saw and router table. Time permitting, I'll put that up as I go.


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## hobby1 (Feb 10, 2012)

Hi EEngineer,

Electronics for me is a fun hobby to see what circuits I could design using all discreet components.
That's why all my projects were basic systems, I wanted to be able to design my own little mobile robot projects.

I figured if I could get some good results without the comercial IC's I'm ahead of the game, because I would read on those robot building forums, how people used all those "stamp processors" for IR detection, and motor controle, ect…

And I would think, how would I design for IR detection, and motor control, directional steering, ect…

Then go to work on the design and builds.
A lot of fun.

Well that explains a little bit about, my comment about using "opamps"

That may be a good Idea to put a thread out about your electronic designs for woodworking power tools, looks like a lot of people are interested in building these projects for themselves.

Have fun in the shop..


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