1
\$\begingroup\$

On our hot tub circuit board, a trace on the PCB finally got cooked... Quite little damage, actually. The one trace is blackened, and the PCB now has a hole in it, but otherwise, everything looks okay.

I have since purchased a new relay (the old one seemed to be fine, but I decided to get a new one since it was only $5). The issue is that the copper trace that the relay was previously soldered to now isn't there anymore.

So, long story short, I'd like to know the best way to solder the thick gauge wire (I'd estimate it is 6 or 8 gauge) straight to the relay pin so that it can handle the ~24A (the heater coil is 10Ohm, supplied by 240V). This will save $400. This hot tub is now a Frankenstein, but all the issues so far have been fixable.

\$\endgroup\$
  • 3
    \$\begingroup\$ Please add a photo of the fried board. \$\endgroup\$ – Janka Jun 8 '18 at 23:42
  • \$\begingroup\$ Sorry, I am on mobile and cannot see the option for uploading pictures... I can assure you that I have experience working with PCBs (working on my masters in electrical engineering). I've just never dealt with this issue before. \$\endgroup\$ – Lerbi Jun 9 '18 at 0:02
  • \$\begingroup\$ You should repair the trace by laying a piece of copper tape where it used to be. Is this a 240V trace? \$\endgroup\$ – mkeith Jun 9 '18 at 0:59
  • 1
    \$\begingroup\$ Five bucks sounds pretty cheap for a 30A relay. Hope you are getting it from a reputable source. \$\endgroup\$ – mkeith Jun 9 '18 at 1:01
  • 1
    \$\begingroup\$ I don't think I'd consider burning a hole straight through the pcb to be "quite little damage". \$\endgroup\$ – Hearth Jun 9 '18 at 1:06
2
\$\begingroup\$

My usual technique in your situation is to clean up any burned PCB residue. If the existing trace is missing, enlarge the hole in the PCB where the trace / pad used to connect to the relay pin. Tin that pin on the relay (heck - tin them all). Remove any excess solder after tinning - you want to see the square corners of the relay contact pins.

Install the relay and solder the coil terminals. Scrape the solder mask away from both traces that connect to the relay contacts.

Take some bare solid (not stranded) 22 AWG wire and wrap at least one full turn around each of the relay's contact terminals. One and a half turns, actually. Wrap as tightly around the pin as possible. You want to wind up with two pieces of bare wire hanging from the relay pin. Obviously, the direction that the bare wire tails is pointing is towards the trace. The wire should not "wiggle" if you try to move it - you want the square corners of the relay pin digging into the copper conductor.

The reason for enlarging the hole in the PCB where the missing trace/pad was is to ensure that you can get a full wrap and a half around the pin. Push the first turn into the PCB if you have to.

Now lay those two wire tails onto the PCB trace. Cut to length, then solder the tails to the trace. Solder the wrap of wire to the relay pin. Use a good quality soldering iron that has plenty of heat capacity.

I use Metcal soldering stations, so I never have to worry about getting enough heat to the terminal to ensure a perfect solder joint. But Weller or Hako (or any of the Asian clones) might not have the ability to deliver enough heat to guarantee a perfect solder joint without risking damaging the relay. All you can do is try it.

A very short piece of #22 wire can easily handle 20 or 25 Amps so long as it is a very short length and the ends of the wire are attached to something that can take the heat away. The relay pin is one of those heatsinks, the PCB trace is the other. You have two pieces of wire, so double that current rating.

I have done exactly this kind of repair many times before, usually in older Industrial equipment where replacement boards are not available. This technique has proven to be extremely reliable.

For those who might question using such thin wire to handle those currents: think about the total cross-section of copper of 2- #22 wire conductors compared to the total cross-section of the PCB trace width. The wire has significantly more copper content than the PCB trace.

\$\endgroup\$
1
\$\begingroup\$

The best answers would be replacing the board, recreating the trace with copper tape, or using an external relay with screw/spade terminals and some 10AWG jumpers.

Assuming you're not open to those options, I would recommend treating the relay pin like a solder post and doing a J-hook termination.

enter image description here

Image source and workmanship standards here.

This might be difficult given the wire gauge and how much space you probably don't have. You would definitely need to pre-tin and bend the hook before trying to attach it to the relay. You'll cook the relay before getting a complete covering connection otherwise.

Short runs of 10AWG wire can handle 30A, so splicing in a jumper could help with space and ease of bending.

Copper ring terminals might theoretically be usable to recreate a pad and avoid the bending/jumper issues, but it's not going to be easy to find ones with a small enough hole to allow a good initial mechanical connection and solder-able surface area.

\$\endgroup\$
0
\$\begingroup\$

Try to use a relay that will have a simmilar shape as this one does:

enter image description here

It's one of the most ensured versions of casings and connecting terminals, that are out there. Also, an alternative to this you could use the one with the spade terminals as such:

enter image description here

You crimp all your wires onto female spade connectors and just hook it up. Just find a relay with characteristics that will suit you; Voltage to drive and on the "output" side the voltage and the current ratings.

\$\endgroup\$
  • \$\begingroup\$ Hi Jakey, I have thought of that, but the remaining 3 posts of the relay are still easily soldered into the board. Only the V- (assuming power goes + to -) pin needs to get soldered. With a new relay, I'd need to solder in another fat wire, which isn't the end of the world, I suppose. \$\endgroup\$ – Lerbi Jun 8 '18 at 23:52
  • 2
    \$\begingroup\$ The top one (an SSR) needs a fairly massive heat sink to dissipate ~30W, the bottom one requires no heatsink at all but may be of dubious "Hongfa" quality. I would not suggest changing relay types will-nilly, safety issues may lurk very close to the surface of your hot tub. \$\endgroup\$ – Spehro Pefhany Jun 8 '18 at 23:56
  • \$\begingroup\$ I never said to use my relays, but the relays with that kind of terminals. You can mount relay elsewhere, just solder some wires to PCB and other sides of wires hook on the relay connectors. \$\endgroup\$ – Jakey Jun 9 '18 at 11:35
0
\$\begingroup\$

You should not need 6 or 8 guage wire. 10 or 12 gauge should be sufficient, particularly if the wire is pretty well exposed to air, not in a tight space. You still may have difficulty supplying the right amount of heat to get the wire and terminal to the proper temperature without overheating anything particularly the relay pin. You might consider crimping the wire to a terminal, bending the terminal to make a good mechanical contact with the pin, and soldering that. You might also think about cutting a strip of copper and drilling a hole for the relay pin to emulate part or all of the PCB trace.

\$\endgroup\$
  • \$\begingroup\$ So you see no concern in simply soldering the stranded wire straight to the pin? That was my initial thought, but it doesn't seem very...professional. I'd imagine I could fray the end of the wire, place it around the pin, and then secure everything with solder. \$\endgroup\$ – Lerbi Jun 9 '18 at 0:48
  • \$\begingroup\$ You could put a ferule on the wire first, then solder the ferule to the pin. I agree with OP that this seems kind of unprofessional. I would try to replace or repair the PCB. Trace repair with copper tape seems doable depending on how much of the old trace is still present. \$\endgroup\$ – mkeith Jun 9 '18 at 1:04
  • \$\begingroup\$ Professional would be to buy a new board. Repairing the trace would be good, but could be pretty difficult with a 30 amp rated trace. Soldering to the relay pin could be serviceable, if you can get a good electrical and mechanical connection without melting the relay housing. If you are confident that the repair is safe and serviceable, it doesn't need to look professional. Just make sure not to sell or rent the property with that in place. \$\endgroup\$ – Charles Cowie Jun 9 '18 at 1:26
0
\$\begingroup\$

Upgrade to a panel mounted relay with screw terminals. Many options. With some searching you could find one that is a plug in type.

The PCB design is inadequate and will fail again. Temporary fix is to tin your wire and do the J joint as shown in an alternate answer direct to the terminal but this is far from optimal.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.