Prototype boards and high current. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 13, 2021, 11:57:17 am
content: I've been getting problems with solid copper wire for high current use on FR4 stripboard gold plated tracks. Mostly warping, and harder to solder joints in the circuit. The board size I'm using 160mm X 100mm board size. Is 2mm diameter solid copper wire overkill for such a modest sized board, I should mention I'm trying to obtain 12 Volts DC up to a current rating of about 20 Amps. I've been successful up to about 12 Amps at 12 Volts DC with 1mm solid copper wire, and a negligible voltage drop under load. But 20 Amps at 12 Volts is proving quite difficult to achieve. This is for a bench linear power supply. What would be the minimum wire gauge on a 160mm X 100mm board to achieve 20 Amps at 12 Volts DC ? It's my understanding 14 AWG is the minimum, would that be right, or could I go lower in wire diameter. I'm asking as there are wide difference between chassis wiring and transmission wire runs. I'm trying to find the best solution for high current capabilities on prototype FR4 board. Any advice and help appreciated, and thanks for reading.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 14, 2021, 08:54:47 pm
content: It depends. Sometimes somewhere the smallest parasitic resistance may worsen the regulation (at regulator output). But in general 1 mm dia wire may be ok for up to 20 A together with wide PCB wires. Just try and measure PCB wires temperature, voltage drop, and a load regulation.Possibly you'll want to make PCB wires as short as possible and put two d=1 mm wires together in parallel in the common ground or 'hot' wire at the output (it depends on an exact regulator).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2021, 09:23:41 pm
content: 1mm is plenty thick for that.Current ratings on board are very simple: if it's getting noticeably hot, it may be worth bulking up.Transistor leads are far thinner than 1mm round wire, and regularly have ratings up to 100A! The key is heat sinking. The thin lead is also short, so there can be significant heat flow along its length. Which does put additional burden on your thermal design, heatsinking the transistors and connecting to them with larger traces/pours, etc.Wire ratings are tabulated for typical wiring conditions. In-wall, conduit, buried, etc. conditions are typically long lengths, so end effects can be ignored; continuous duty, so at thermal equilibrium, not pulsed/momentary; and typically residing in very stuffy environments, no circulating air (or even surrounded by thermal insulation!). Chassis wiring is shorter, typically more open, and can permit somewhat higher risk on account of the grounded or insulated enclosure (i.e., if a wire melts, it's likely to short out to another conductor and blow the fuse, resulting in a safe condition).Finally, these are largely (entirely?) motivated by two things: fire and shock hazard. If it's not causing the device to ignite (or if it does, it emits limited flames, and extinguishes promptly), and it's not creating a shock hazard for anyone handling the equipment -- it's fine. Functional demands are irrelevant to these standards. (But wires are likely to provide reliable service, under these standards.)Tim

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 15, 2021, 01:36:09 pm
content: Thank you for your replies, some interesting stuff there I didn't realise. On the 160mm X 100mm FR4 prototype board the longest wire run is no more than about 130mm long, what the circuit is, is the common 7812 voltage regulator with 6x TIP36 pass transistors to increase current. The transistors are off the board connected buy 14 AWG wire. The length of those wires is about 8 inches long to the heatsink for each transistor. So I'm fairly confident they can supply 1/6 of the power from each transistor. On the pcb are two 470uf capacitors, 6 X 0.1 ohm 5 watt ballast resistors for current sharing. And a single 10 Amp silicon rectifier diode in the - Volts rail to bump the voltage up by 0.7 Volts. And the 7812 IC and it's heatsink. I've included a couple of pictures of the pcb before population of components and wire, with the exception of the 7812 IC As this once added gives me an idea on how to lay out the rest of the components on the board. I forgot to mention the 100 ohm 0.5 watt base resistor, and I'm using 20 Amp terminal blocks for connecting the transistors base, emitter and collectors, I feel the ease of connection this way makes any maintenance quite easy with minimum need for desoldering. Where I was unsure about the wire connection gauge is connecting up the components, and input and output to the pcb. The 7812 IC uses a reclaimed graphics card heatsink, the appeture cut out is for the transistor leads to connect up to the pcb. And I've used molex connections to the transistors, the data sheet for those is 7 Amps maximum for each connector. Again I am trying for a friendly maintenance theme in the construction of this power supply. I know the highest current paths are the input rails + and - rails, and the output - Volts rail. Also the output rail directly from the collectors termination to the pcb. It's my understanding the input to the regulator on all three rails can be more modest in wire gauge. I think I've got that right.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: August 17, 2021, 06:47:03 am
content: If 1mm is enough for 12amps then 2 x 1mm should easily be enough for 20amps especially if you put them on separate tracks next to each other. As for warping the board when you're soldering them yes that can be a problem when soldering lots of copper to fr4, here's an extreme example -https://youtu.be/DbZZD6J_AK0?t=468When I've done something like that on fr4 perfboard I only solder into 4-5 hole spacings at a time then do something else while it cools down and I haven't had any trouble with warping doing it that way(I was only soldering 2 x 0.71mm single core wire though). Another thing you could do is have a wet rag/wet cotton etc bud sitting on the other end of the wire you're soldering.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 17, 2021, 12:41:36 pm
content: I have been thinking, in one power supply I think the L7812 is to far from the main board, and it might be dropping voltage. It's at least 8 inches with no capacitors near it. That's not the board in the image above. And thought about similar to what you said, I can use a solder "staple" using 1mm solid copper wire to hold the 2mm trace solid copper wire. If minimum solder on each staple and good spacing, that sounds, at least a solution that might just work. The heat from heavy soldering really does warp the FR4 boards. I am going to give it a go, it might just be the answer.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: August 17, 2021, 01:47:51 pm
content: Is it possible to use a lm317 instead to get your 12v? It has more stable output with varying current/temperature if you have >10uf on the adjust pin. You can also connect your R2(the one between adjust/ground) resistor closer to/at ground of your load for remote ground sensing so voltage drop across you traces shouldn't matter then.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 18, 2021, 12:39:16 pm
content: Yes I could use an LM317 instead of the L7812, I have the high voltage variant of the LM317 HV also. I would probably use fixed valve resistors to obtain the exact voltage. Is the L7812 not as good in its regulation then ? I would have thought there regulation tolerance would be similar in % for both these voltage regulators.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: August 18, 2021, 02:38:32 pm
content: The lm317 is a slightly different/more complex design than the 78xx's(has 3 internal capacitors vs 1-2 for the 78xx series). I have noticed the lm317m's I've used have more consistent output voltage with varying load/temp than the STM 78xx series.For using resistors to set the output voltage I've used 1% 50c ppm smd resistors and that gives an output voltage of +-20mv from what I wanted. Just be sure the place the adjust/ground resistor nearer your load for best performance.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 19, 2021, 10:37:25 pm
content: Yes I should give it a try, as I'm using current boost transistors I'd need to use a base resistor for those. It's often been mentioned that it could ruin the LM317 voltage regulation using multiple pass current transistors. I have seen a schematic that used PNP and NPN transistors to achieve the high current rating, although it does add more components to the circuit. At the moment I'm trying to incorporate a crowbar circuit on the same PCB as the PSU circuit. At the moment I'm using an add on crowbar circuit in a separate project box.