PCB Dimmer design safety review (mains powered so don't want to get shocked)

[Kamenio]

I live in the UK (mentioning for mains rating reasons) and I am looking to make a PCB to control the temperature of my soldering iron as it starts out fine then gets too hot (the iron its self is starting to melt from long term use). by the end of my project I would like to have a self contained soldering station. I have designed the PCB but would like to know if it is basically safe to use (not going to go bang, or worse, kill me) I would also like suggestions on what to use for housing for the station and what to use to mount it to the housing (as I'm working with mains input).

I believe my tracks are 3-5mm wide (can't actually remember but they are wider than in the diagram).
Not sure if its clear from the diagram but R2 is a 100k ohm potentiometer.
Can provide more information if needed.

 

[Audioguru]

Using a dimmer to adjust the temperature of your soldering iron is a Mickey Mouse way of doing it. It will cool when you begin soldering because its temperature is not detected and is not controlled.

My Weller soldering iron is 52 years old and has excellent temperature control and it still works perfectly. Its tip is always at the correct temperature and it immediately applies heat when it detects a slight temperature drop and shuts off when it detects a slight temperature rise. Its tip lasts a very long time. The soldering iron is powered from a 24V transformer and its tip can be grounded to earth if you want.

 

[chuckey]

The main objective is to keep you and the metalwork of the soldering iron away from any mains voltage, so the width of the tracks is immaterial. The creepage distance is so, any conductor or potential conductor under fault conditions must be at least 3mm away from any unearthed metal work. This really only applies to the potentiometer and its shaft, providing the PCB is put in a plastic box.
Why not just feed the iron via a mains diode to reduce its power to 1/2 while on standby with a shorting out switch to put it on full power. You could use a microswitch on the soldering iron stand.
Frank

 

[KlausST]

Hi,

Check trace width. I expect you need much wider traces.

Klaus

 

[FvM]

chuckey has already put the focus on the safe isolation point.

I presume, the potentiometer metal case (if any) doesn't have sufficient clearance and creepage to the "hot" circuit part. Consequently the potentiometer should have a plastic shaft and it's metallic case parts protected against accidental contact.

 

[t_maggot]

In general there are 2 things about personal safety when working with mains:

1.Electric shock
2.Ark blast (from a short circuit)
(or a combination of the above)

So for the first, be sure that any metal parts you may contact (as the solder tip) are grounded. Keep good distances (>3mm) between mains and other isolated parts of the circuit. If there is no circuit isolation and no good distance eg with your potentiometer, you must be careful. A metal-case potentiometer is a hazard for your application as fvm said, plastic is the only way to go. Be careful in general not to get a shock by unintentional touching of live parts of the circuit. Use of an isolation transformer for experimenting is a good practice. Also good practice is the use of a Residual-current device, but it its not fast enough to prevent the high short circuit currents.

About arc blast, fuse the line with the minimum fuses you can. The blast you can get from a mains circuit breaker of 16A is not comperable to a fused line of 0.5A. You also can use a PTC-fuse in series if you don't mind some voltage drop, it has the advantage of drastically limiting any short circuit current. Don't get your face too close in a working exposed circuit, wear plastic glasses.

And when you 're done, put the whole thing firmly in a plastic (or grounded metalic, if you keep clearances) box.

After safety you can experiment with the functionality of your circuit.

 

[Kamenio]

Thank you everyone who has replied, some comments on the above that I seemed to over look:

The potentiometer is plastic shaft and case (saw that one coming and nipped it in the bud)
not 100% sure about the spacing in the TRIAC and potentiometer pins but everything else is more than 5mm apart (due to reading that up as a safe distance when working with mains). I can earth a metal housing easily enough but not sure how easy it will be to earth the iron's rod (I'm not 100% sure of the workings of the rod, I have a rough idea but wouldn't put any faith behind it).

I was thinking of spacing it from the case using computer motherboard spacers (as i have a fair few of them and they are easy to replace)

In regards to fusing it, would I put the fuse in the circuit too (I'm obviously going to have one in the plug)

(sorry if I missed any points raised, feel free to bring up anything else I missed)

For further help, here are the parts that I have used so far:

TRIAC: [I]**broken link removed**[/I]

DIAC: [url]https://cpc.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15002&langId=69&urlRequestType=Base&partNumber=SC07604&storeId=10180[/url]

Potentiometer: [url]https://cpc.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15002&langId=69&urlRequestType=Base&partNumber=RE04393&storeId=10180[/url]

Capacitors: [url]https://cpc.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15002&langId=69&urlRequestType=Base&partNumber=CA07027&storeId=10180[/url]

I'm also happy to replace any and every part as needed.

In regards to the suggestion by chuckey (Frank), This is a nice idea that I would probably change to but at the moment, I prefer the idea of more control as well as the ability to turn it from off to full power. I find assurance in the added manual input. I do realise I could have a 3 stage switch for off>half>full but id still prefer the manual input variant (its a cheap iron anyway, its original tip only lasted about 3 jobs before it got a hole right through it, but you get what you pay for)