AC (Alternating Current) trace width

Community PCB Design
1

No, clarifying AC in the subject line was not meant as an insult to your intelligence, but instead to make it easier for someone searching to find this thread, as AC is too short to work.

Anywho, I’m wondering how wide traces should be on a board that I’m designing. I’m using a triac to switch 120VAC, at something around 12.5Amps (1500 watts). I’m pretty sure that what I have on there right now isn’t going to cut it (i think like 50mils? :P).

I found an online calculator for this, and given my inputs with a trace thickness of 35um (google says this is standard thickness), the output was something between 130mils and 230mils, depending on the calculator used.

Also, I noticed that temperature also played a role. The board is a reflow toaster controller, and I plan to put it in the oven itself (not in the heating part, but in the empty space next to it behind the controls), so it will probably get hotter than if it were external. I plan on putting in a fan or two for ventilation of the area, but this is only if it is needed.

Also, there will be DC components on the board, how much of a clearance would be needed around the AC components to keep things working?

-Nate

2

reklipz:
I’m wondering how wide traces should be on a board that I’m designing. I’m using a triac to switch 120VAC, at something around 12.5Amps (1500 watts). I’m pretty sure that what I have on there right now isn’t going to cut it (i think like 50mils?

Greetings Nate,

I use [this calculator.

reklipz:
I found an online calculator for this, and given my inputs with a trace thickness of 35um (google says this is standard thickness), the output was something between 130mils and 230mils, depending on the calculator used.

The result I get for 2oz external copper at 13.5A is 366mil (9.3mm).

Assuming a 40degC ambient and allowing a 20degC rise.

The short answer is make the high current traces as wide

as practical. Perhaps a polygon fill is better? Also I noticed

while reverse engineering other projects that PCBs discolour

due to high temps more at the junctions than in the middle

of a trace run. High current circuits often have problems

at soldered joints and connectors. If the traces must go

from one side to the other use multiple vias with large drills.

reklipz:
Also, there will be DC components on the board, how much of a clearance would be needed around the AC components to keep things working?

The required clearance from any primary (ac mains) traces

to other curcuits or to ground is approximately 6mm (236mil)

for agency approvals (VDE 0631, UL, etc.) Here’s a [good

summary.

As a practical matter you can reduce this as soldermask is

an insulator - you wont get approval but you can have peace

of mind.

Some other tricks are to cut slots in the PCB or cover the

high voltage points with a conformal coating or RTV.

I think the US Mil standard is 3mil per volt (so 510mil for

120Vac 170V peak). Commercial PCBs (for TV and CRT

monitors) were designed to 1mil per volt (back to 170mil

for ac mains).

Another approach is to export the high current connections

to a second PCB set up just for this function.

Comments Welcome!](ZETTLER electronics BV – relais & displays)](The CircuitCalculator.com Blog » PCB Trace Width Calculator)

3

Personally, I’d be inclined to use point-to-point wiring for high current/high voltage parts (if they are reasonably straightforward) rather than on a PCB, keeping all that stuff separate. That’s just a gut feel, though (indeed, would experienced people recommend that approach?)

4

Well, this is how things look currently:

[

I have yet to add a transformer, so its more than likely to change. Grid is 100mils, each of the main traces are 300mils wide.

Comments?](ImageShack - Best place for all of your image hosting and image sharing needs)

5

the one issue I’d be concerned about is the contact area on the pin going to the heater connector. it looks like it’s about 25 mil X 100 mil. I don’t know if that’s an issue or not - if the chip is rated > 12.5A then it must be ok.

By the way, if you tin the high current traces, you will increase the thickness by a significant amount and thus increase the carrying capacity of the trace. I have not idea how to calculate that, though.

6

Philba:
if the chip is rated > 12.5A then it must be ok.

The chip is rated for 35Amps, so yeah, lets hope that'll work.

Philba:
By the way, if you tin the high current traces, you will increase the thickness by a significant amount and thus increase the carrying capacity of the trace. I have not idea how to calculate that, though.

Sounds like a plan, I just have to make sure to leave the solder mask off of the traces?
7

Another thing I’ve done, in addition to making the traces as wide as possible on all four layers, is to leave the solder mask off the bottom layer, and solder in the wire trimmed from resistor leads.

I’ll put two of those legs covered in solder on each trace. Then I’ll cover all of it with a conformal coating spray.

8

reklipz:
Well, this is how things look currently:

…

I have yet to add a transformer, so its more than likely to change. Grid is 100mils, each of the main traces are 300mils wide.

Greetings Nate,

Bad News. Your design will not work at all. Here’s why:

(1) The Alternistor is a trade name for a Triac. When conducting

the forward voltage drop is approximately 1.2V. At 13.5Amps

you need to dissipate 26Watts of energy. A heat sink is needed!

(2) The footprint looks like a D2PAK, certainly a SMT. The only

thermal contact is through the pins and the tab. There is not

enough area to dump the heat.

(3) The gate signal is milliamps and low voltage, so it does not

require a fat trace (although having one there help with

power dissipation).

(4) The driver for the Alternistor is an opto-coupler and

resistor. The resistor should have a 1/2W rating, you seem

to have a tiny SMT part, possibly an 0805.

(5) The clearance around the opto-coupler needs to be

6mm from low to high side. An SMT part with 150mil

body can’t do that. Take a look at the DIP06 package

and don’t run any traces under the body. (There are

modified TH packages that have 6mm clearance - the

leadframe extends out from the body).

(6) You do not need four terminals for a two node circuit.

Unless you need the common terminal for another circuit

(not shown) you can use a two terminal part.

Here’s a project I did with multiple Triacs and opto-

couplers:

http://www.stonard.com/SFE/Controller_cropped_2.jpg

http://www.stonard.com/SFE/IO_cropped_2.jpg

Notice that the power devices are bolted to a metal bracket,

which is bolted to the metal cabinet. This eight channel

IO card is rated to 5A, so only 10W (approx) is dumped.

In your case you have 26W or more to dump, and at an

elevated temp (inside an oven, right?).

You can read more about this project[here.

Comments Welcome!

[/img]](Nuts Volts Magazine | Nuts & Volts Magazine)

9

Wow, thanks for the awesome post.

It looks like I have quite a bit of reworking to do then.

As for why there are four points instead of the two, I also am placing a transformer on board. I could do with only 3 points, and connect the load to neutral off board, I figured it’d be easier to deal with the connections this way.

So, it seems the SMT triac (a D2PAK indeed) is out of the question, I’ll have to order some in the TO-220 package. I’ll also have to find some scrap hunk of aluminum, and prolly want to put a fan or two in there.

The optocoupler is the a through hole 6 pin dip. I noticed they also had a surface mount dip type, but the through hole version would be “better”, no?

The resistor is indeed a SMD 0805, and I posted a question about that somewhere as well, but it seems that even though 1/2W SMDs do exist, its just a wiser idea to go with tried and true through hole.

You’re power traces don’t look nearly as wide as mine do, is that true or am I missing something?

I’m a little confused as to what is meant by the following:

The clearance around the opto-coupler needs to be
6mm from low to high side. An SMT part with 150mil

body can’t do that. Take a look at the DIP06 package

and don’t run any traces under the body. (There are

modified TH packages that have 6mm clearance - the

leadframe extends out from the body).

Again, I really appreciate you’re taking time to help, I’m not sure what I would do without you guys (other than trial and error of course, which becomes expensive, and, probably begins dangerously, at least in this case).

I’ll try to come up with another design that has all of the desired components (transformer, proper resistor, and different triac package if needed).

-Nate

10

reklipz:
So, it seems the SMT triac (a D2PAK indeed) is out of the question, I’ll have to order some in the TO-220 package. I’ll also have to find some scrap hunk of aluminum, and prolly want to put a fan or two in there.

Greetings Nate,

If you want to roll your own SSR, using a triac and optocoupler,

use a TO-3 device and put it on a metal surface (heatsink) not

a PCB at 13.5A 120V.

I read that SFE made an oven controller and they gave up on

running the AC120V through the PCB (IIRC).

The easy way out is to use a commercial SSR and run your

low voltage control to it. A heatsink may still be required.

You can continue to put the AC120V transformer on the

PCB - I do that quite a lot in low power single PCB projects.

reklipz:
The resistor is indeed a SMD 0805, and I posted a question about that somewhere as well, but it seems that even though 1/2W SMDs do exist, its just a wiser idea to go with tried and true through hole.

When the Triac is turned on the voltage drop across the

resistor is nearly 170V (peak AC less diac and triac gate

drops). A 1/2W resistor with 400mil body can take this voltage.

An 0805 is only good to 150V, but under continuous high

voltage the value will shift. Derate these parts!

reklipz:
You’re power traces don’t look nearly as wide as mine do, is that true or am I missing something?

They aren't. My PCB is fused at 5Amps, so using the calculator

for 2oz cu. and 30degC rise I got 72.7mil. I used 70mil.

The duty-cycle of my project is quite low so 5A continuous is

unusual. I have tested the PCB design to 24.5A peak…

reklipz:
I’m a little confused as to what is meant by the following:

bigglez:
The clearance around the opto-coupler needs to be
6mm from low to high side.

Download this [datasheet. Notice that the devices are available in

SMT (256mil body), SOIC (256mil body) and VDE

approved 6mm creepage leadframe. The latter has the

SOIC body but extended spacing to 400mil. With it a

design can pass the 6mm creepage spec. (The open

space from copper pad to copper pad under the device).

I’m not seeking VDE approvals so I stuck with SOIC

opto-couplers and 200mil creepage under the device.

Comments Welcome!](http://rocky.digikey.com/WebLib/Lite-on/Web%20Data/MOC302x.pdf)

11

Hmm, alright, thanks for another excellent post.

I wonder, I have some relays that could handle this current / voltage, should I just give in and use one of them? They’ve even got quick connect lugs IIRC.

I was hoping to use the SSR route, as it just seems cooler, but if it’s really this much of a pain to route, I may as well just give in and use a relay.

I’ll still try my hand once more at the opto+triac design, and i’ll upload that here, but it’s gonna have to wait till the weekend probably.

-Nate

12

reklipz:
I wonder, I have some relays that could handle this current / voltage, should I just give in and use one of them? They’ve even got quick connect lugs IIRC.

Greetings Nate,

Depends upon your desire to get the project up and running.

A relay in the hand is worth two SSRs and a PCB in the future…

Relays are not without their problems. Over time the contacts

will burn out. They are mechanical and therefore noisy.

They will not allow phase-angle control which would be a

huge plus in a tightly controlled feedback system.

reklipz:
I was hoping to use the SSR route, as it just seems cooler, but if it’s really this much of a pain to route, I may as well just give in and use a relay.

Many SSRs are intended for chassis mounting with descrete

wiring. You can still put the rest of the controller on a PCB.

Even if you continue down the path of a Triac, you can use

the TO-3 chassis mount variety in place of a packaged SSR or

mechanical relay.

Comments Welcome!

13

Just because the data sheets spec it, watch out for ridiculous power ratings - I found some FETs where the manufacturer keeps claiming that they are rated at 120 amps continuous in a TO-220 package, even after we sent them a video of what really happens when you try to run 120A thru it.

Always take the lead size, contact area, and package dissipation into account.

14

bigglez:
you can use the TO-3 chassis mount variety in place of a packaged SSR or mechanical relay.

Teccor offers a 25A alternistor in a similar package:

http://parts.digikey.com/images/6/31/08/9.jpg

According to the chart, and what you said in a previous post, I’ll need to dissipate anwhere from 15 to 25 Watts. The maximum case temp for the amount of current I’ll be conducting (with some margin) is 100C.

Is anything special needed for the heatsink, or will a good hunk of aluminum with some fins work?

15

reklipz:
Teccor offers a 25A alternistor in a similar package:

According to the chart, and what you said in a previous post, I’ll need to dissipate anwhere from 15 to 25 Watts. The maximum case temp for the amount of current I’ll be conducting (with some margin) is 100C.

Is anything special needed for the heatsink, or will a good hunk of aluminum with some fins work?

Greetings Nate,

A larger contact area from the device to the heatsink

reduces the thermal resistance and lowers the junction

temperature. Running the device hotter works against you

as it lowers the device performance and will shorten

the time before failure.

The device you show is a TO218 (with isolated die), and

it has a thermal rating of 0.95 degC/w.

The average power for a 13.5A load is 15W (I said 26W

earlier and in error). So the die will be 0.95 * 15 = 14.25

degrees hotter than the case. You should not run the

case above 100degC according to the datasheet.

To get an idea of what this means, take a 15W light

bulb, turn it on, and touch the glass with your palm.

Your heatsink has to lower this energy level so that

you can safely touch it.

A heatsink that keeps the case below 100degC in a

40degC environment (your oven area in a 25deg room)

needs a thermal resistance of (100 - 40)/14.25 = 4.2 degC/W

or less. It would be wise to derate this for safety.

There are heatsinks for the TO218 that meet this spec.

4.2 * 0.8 (20% safety margin) gives us a starting point.

Look for a heatsink with 3.3degC per watt or smaller.

Here’s [one. Notice the curves indicate a 20degC rise for

14Watts with a 700 cu ft/min air flow. The fan in a PC

“silver box” PSU is rated to about 25 cu ft/min flow.

Alternatively, you can use a larger heatsink with no

forced air. This saves the cost of a fan and improves

reliability and lowers noise.

Here’s an [extrusion that will work. Notice the

equivalent surface area is 37 sq. inches! (approx

6 x 6 inch area).

If it were me I’d head in the direction of a TO3

device as it has two mounting holes and faston

crimp terminals. The device would be off the PCB,

and probably on a finned heatsing like the extrusion

above. The idea is that when it comes to heatsinks

more is better.

Another approach, not discussed yet, is to wire the

heater in smaller segments so the 13.5A is spread over

two or more circuits. Smaller PCB mounted TO-220

or even surface mounted devices could be used.

The same power is dissipated in the Triacs, but its

not all in one tiny device.

Comments Welcome!](http://www.aavidthermalloy.com/cgi-bin/exdisp_length.pl?Pnum=63730&therm=1.33185477265401&therm=&airflow=57.2&AirUnits=LFM&CType=Natural&ExLength=4.000&LengthUnits=in&SButton1=Change+Length)](http://www.aavidthermalloy.com/cgi-bin/stdisp.pl?Pnum=6400bg)

16

OK, well, I finally took time to find a TO-3 triac (mouser works wonders…).

I ordered one of them, and am working on a heatsink next. The 15W mark still stands, FYI.

Thanks so much for your help! I probably would have made my first pcb go up in flames had it not been for your help (well, all hope is not lost yet, the flames may still come, we’ll just have to wait and see! :P).

-Nate

–EDIT–

Any Idea where I can find the quick connect connectors for the triac?

–EDIT–

I think I’m going to buy this guy:

http://www.aavidthermalloy.com/cgi-bin/ … 303b00000g

~$4.50 from Mouser.

I’m going to try to mount a fan onto it, so I think this’ll be a good solution.

17

OK, well, heres my “finished” board. Oh, just realized I have to add the USB power transistor, so, nearly finished.

[

(display board has ground plane, but I didn’t hit ratsnest before export)

[

(50 mil grid)

The POWER terminal is: pin 1: neutral - pin 2: hot

The TRIAC terminal is: pin 1: load return path (to use fuse) - pin 2: triac gate trigger

The triac is the TO-3 type you suggested, mounted on the heatsink I posted.

Hows it look?](ImageShack - Best place for all of your image hosting and image sharing needs)](ImageShack - Best place for all of your image hosting and image sharing needs)

18

reklipz:
OK, well, heres my “finished” board. Oh, just realized I have to add the USB power transistor, so, nearly finished.

Greetings Nate,

Your design looks very good! I’m new to the “display board”

project, are they related?

I don’t have your schematic, not sure if you’re willing to

share the EAGLE files, so what are you using for the

temp interface? (What is IC3?).

My suggestions are:

(1) Move C14 away from the hot AC side of the transformer,

perhaps you can fit it near the 16 pin connector?

(2) Route the low voltage side of OK1 away from the hot AC

side of the transformer. Perhaps you can run the pin2

trace under the trace to R4 to compact the design?

(3) Move R4 nearer to either the 16 pin connector or

the uC, to keep its pads away from the hot AC side of

the transformer.

(4) For peace of mind you should keep a 6mm (400mil)

guard band around all hot AC traces. This is a VDE

requirement, and a tough one at that. The process of

thinking about a 6mm creepage will get you a much

better hot AC intergrity even if you can’t do 6mm for

100% of the hot AC traces.

If you are familiar with the “Mark” command in EAGLE

use it to measure line-of-sight from pads and traces to

each other on the hot AC circuits.

(5) I don’t see any test points. When it comes to SMT

layouts I find that sprinkling a few pads on ground,

power rails, and at least one unused uC port (if available)

will allow a scope probe or DMM to be used later.

My preferred testpoint is a 100mil diameter pad with

40mil drill. It can be used as-is or fitted with a[Keystone THM test point.

(6) Your screw terminal blocks look familiar, but

I’m not sure of their size. Do you really want to run 13.5A

through these? I use two sizes, the['blue’ones have single

hole mounting and can take a 14 - 22AWG wire, I use

these up to five amps (they are UL approved for 15A).

For heavier loads I use the [‘Black’ ones rated to

10A but with two electrical lugs each.

Why not run heavy wiring in the project to the heaters,

Triac, and a chassis mounted fuse (15A)? Run a second

AC circuit after the 15A fuse to the PCB and use a smaller

fuse (5A or less)? This will give the required protection

and reduce the copper needed on the PCB.

Comments Welcome!](http://www.on-shore.com/catalog2/tpg7b.pdf)](http://www.on-shore.com/catalog2/tpg7.pdf)](http://www.keyelco.com/pdfs/M55p51.pdf)

19

bigglez:
Your design looks very good!

Wow, thanks! It's my first one, so your input has done nothing but help me improve the design.

bigglez:
I’m new to the “display board” project, are they related?

Indeed they are. The 16 pin connector is just a ribbon cable that connects the two boards. The display board (in hindsight, something like user I/O board would have made more sense) consists of 4 silicon push buttons, each with a colored LED, and a Nokia LCD.

bigglez:
I don’t have your schematic, not sure if you’re willing to share the EAGLE files, so what are you using for the temp interface? (What is IC3?).

I don't have a problem sharing [[the EAGLE files](ftp://desmas-s.dyndns.org/torc.tar.bz2), but for the record, IC3 is a [[MAX6675](http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149) K-Type Thermocouple to Digital Converter from Maxim.

bigglez:
(1) Move C14 away from the hot AC side of the transformer, perhaps you can fit it near the 16 pin connector?

Perhaps I could. It would also shorten the traces a bit.

bigglez:
(2) Route the low voltage side of OK1 away from the hot AC side of the transformer. Perhaps you can run the pin2 trace under the trace to R4 to compact the design?

I'll give this a shot, although it may not be relevant after I remove the large AC polyfills (suggestion 6/7).

bigglez:
(3) Move R4 nearer to either the 16 pin connector or the uC, to keep its pads away from the hot AC side of the transformer.

Done.

bigglez:
(4) For peace of mind you should keep a 6mm (400mil) guard band around all hot AC traces. This is a VDE requirement, and a tough one at that. The process of thinking about a 6mm creepage will get you a much better hot AC intergrity even if you can’t do 6mm for 100% of the hot AC traces.

I'll keep it in mind when I make these changes, thanks for the suggestion. I think you mentioned it earlier as well, but in my haste to get something up here I neglected it.

bigglez:
If you are familiar with the “Mark” command in EAGLE use it to measure line-of-sight from pads and traces to each other on the hot AC circuits.

I've accidentally stumbled upon this one before (I think by pressing the stoplight looking thing?) It never really seemed to do anything so I've never looked into it. It seems as though I may want to, though.

bigglez:
(5) I don’t see any test points. When it comes to SMT layouts I find that sprinkling a few pads on ground, power rails, and at least one unused uC port (if available) will allow a scope probe or DMM to be used later.

Yet another excellent idea. Also reminds me that It wouldn't be a bad idea to add an ICSP port... :D

bigglez:
(6) Your screw terminal blocks look familiar, but I’m not sure of their size. Do you really want to run 13.5A through these? I use two sizes, the['blue’ones have single hole mounting and can take a 14 - 22AWG wire, I use these up to five amps (they are UL approved for 15A). For heavier loads I use the [‘Black’ ones rated to 10A but with two electrical lugs each.

Why not run heavy wiring in the project to the heaters, Triac, and a chassis mounted fuse (15A)? Run a second AC circuit after the 15A fuse to the PCB and use a smaller fuse (5A or less)? This will give the required protection and reduce the copper needed on the PCB.[/quote]
Moot point with the connectors, as the latter idea solves this issue (although I believe the connectors I was planning on ordering are rated to 13.5A). I’ve not yet looked at the chassis mounted fuse, but is there some sort of standard chassis fuse holder that I should be looking for? (And even more so, any particular type of fuse?)

On another note, I wasn’t sure which type of connector to use, so I just placed one of the Wago Screw type connectors, it looks to have the same footprint, so I wasn’t too worried. I was looking for the type that SFE uses/offers on their boards/site, so I made a guess. Do they look right?

Thanks again for all of the suggestions, I’m working on the revisions as we speak.

-Nate

*Note; I just setup the ftp server, so let me know if it works for you or not!](http://www.on-shore.com/catalog2/tpg7b.pdf)](http://www.on-shore.com/catalog2/tpg7.pdf)

](Mixed-signal and digital signal processing ICs | Analog Devices)](ftp://desmas-s.dyndns.org/torc.tar.bz2)

20

reklipz:
I don’t have a problem sharing [the EAGLE files, but for the record, IC3 is a [MAX6675 K-Type Thermocouple to Digital Converter from Maxim.

*Note; I just setup the ftp server, so let me know if it works for you or not![/quote]

Nate,

I got a file ending in BZ2. My Wintel machine has no idea what

to do with it. Some form of UNIX/Linux compression?

How about *.sch and *.brd or even *.zip?

Thanks In Advance!

Peter](Mixed-signal and digital signal processing ICs | Analog Devices)](ftp://desmas-s.dyndns.org/torc.tar.bz2)