A few quick questions on earthing

• A few quick questions on earthing
  Posted by Setanta on 17 Nov, 2015 00:47
• Howdy folks,
  
  Old fart here, but very much a “young player” when it comes to hardware (been a software guy all my life). I’m looking for some guidance when it comes to earthing. I’ve been reading up on different types of grounds and, while I’m now somewhat the wiser, there are a few things I still can’t quite figure out.
  
  Background
  
  I’m going to upgrade from my little Antex fixed temperature soldering iron to one of those cheap and nasty YiHUA 936 clones. I’m not expecting it to last very long but it will serve my purposes (and suit my tight budget) for a little while at least.
  
  When the YiHUA quits it’s my intention to replace the guts of it with a circuit I found online at http://www.zl2pd.com/SolderingStation.html - more specifically the version shown in this schematic http://www.zl2pd.com/img/SolderingStation/ATtinyBasicCct.gif. As shown the controller is driven by a 24V 3A laptop power brick. My intention is to add a rectifier and suitable smoothing capacitor to the original YiHUA transformer to provide me with 24V DC Power.
  
  Questions
  
  In the above schematic, the tip of the iron is grounded to the circuit ground rather than to earth. In both the YiHUA and the original Hakko 936 the tip is grounded to earth and not to the circuit ground. I assume that this is purely to provide some level of ESD safety.
  
  1: Does connecting the tip to the circuit ground alone provide any ESD safety at all or does it need to be connected to earth?
  
  2: If it has to be the connected to earth purely for ESD safety, do I really need it as a basic hobbyist or can I do without it? (I have seen a lot of conflicting opinions about this)
  
  Note that in everyday life I have virtually zero problem with static - possibly because I live in Northern Ireland, so humidity is always high, and the fact that the flooring, workbench etc. are all wooden.
  
  The one modification I was hoping to make to this circuit as it stands was to remove the ‘Standby’ switch and connect pin 3 to one of my metal soldering iron stands. This would mean that when the iron was placed in the stand and touched the metal, the pin would be grounded and standby mode enabled by the code in the ATTiny.
  
  3: If the tip needs to be connected to earth rather than circuit ground, would this still work (i.e. in this case would earth function as circuit ground and trip standby mode)?
  
  4: If not, is it possible/safe/desirable in this case to connect the circuit ground to earth to make it work?
  
  Thanks in advance for any help you can provide, and remember, I did warn you I was a newbie at the top of the post
  
  Setanta
• #1 Reply
  Posted by IanB on 17 Nov, 2015 01:08
• You have to be a little careful with switch mode power supplies like laptop bricks. Depending on the design there may be some very ugly voltages on the low voltage side due to RFI suppression capacitors that feed through from the mains. This is a common occurrence with USB power supplies, some of which can give you quite a tingle if you touch them. If I plug my iPhone into its charger the whole phone will make a non-contact voltage detector light up and beep.
  
  Therefore, if you want to be sure your iron will not damage delicate parts it would be safest to connect the tip to earth ground. Sometimes this is done through a moderate value resistor rather than a direct connection.
• #2 Reply
  Posted by Setanta on 17 Nov, 2015 01:26
• Quote from: IanB on 17 Nov, 2015 01:08

  You have to be a little careful with switch mode power supplies like laptop bricks. Depending on the design there may be some very ugly voltages on the low voltage side due to RFI suppression capacitors that feed through from the mains. This is a common occurrence with USB power supplies, some of which can give you quite a tingle if you touch them. If I plug my iPhone into its charger the whole phone will make a non-contact voltage detector light up and beep.
  

  
  That's part of the reason I intend to re-use the YiHUA transformer in my build along with a bridge rectifier rather than a power brick.
  
  

  Quote from: IanB on 17 Nov, 2015 01:08

  Therefore, if you want to be sure your iron will not damage delicate parts it would be safest to connect the tip to earth ground. Sometimes this is done through a moderate value resistor rather than a direct connection.
  

  
  On my travels I've seen various connection methods, ranging from direct connections to simple resistors, low-pass filter circuits, diodes etc. Most of these have been in PSUs for audio gear so I was assuming their main purpose was to avoid noise/ground loops. Ideally I'd prefer to connect the iron tip to earth one way or another, but if that means I have to forgo the automatic standby feature I had been hoping to incorporate it becomes a tricky decision for me for various reasons.
  
  Thanks for the advice