Is this softstart circuit safe? (clearance, creepage and slots) - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: February 15, 2019, 10:58:03 am
content: Dear forum,I want to make a softstart circuit. Since it is connected my mains, I wonder what are safety requirements. Like, what should be distance between primary and secondary, are slots needed and other questions. Please confirm that the design doesn't have obvious safety flaws.Softstart circuit is used to switch on big toroid of my lab power supply.I ensured minimal creepage of 6mm between primary and secondary.There is also a slot that separates the shortest path between primary and secondary.The minimal distance between mains wiring is 2.5mm.Transformer is claimed to comply EN 61558-2-1 and EN 61558-2-6.Relay is following parameters in datasheet: Dielectric strength: 5,000 VAC, Surge strength: 10,000 VQuestion: Are these design and layout safe?. Do I need MOVs or something to protect the transformer?PS Fuses are installed in mains emi filter on a separate board.PPS It also serves as an AUX supply, hence additional terminals.

author: floobydust
author_rank: Super Contributor
timestamp: February 15, 2019, 08:29:43 pm
content: I'll stick my neck out and offer some ideas.For 240VAC mains, overvoltage category II (2.5kV) in a lab or workbench, 1.6mm is the minimum clearance and creepage distance. This is assuming dust, dirt and humidity are outside the enclosure and altitude <2,000m.I'm not sure if the AUX 1 or 2 side is protective earth-grounded or not.If not, then the power transformer and relay must both meet double-insulated safety standards (check datasheets) and you multiply their PCB spacings requirements by 2X. So for reinforced insulation, this becomes 3.2mm between primary-secondary components. But still 1.6mm between L and N on mains.Your slots as drawn are not needed because you have plenty of clearance and creepage there.Slots are 1 or 1.5mm wide and weaken the PCB to bending and flex. Typically slots are under parts that can get trapped flux (no wash) or have a creepage problem due to uninsulated regions.Your PCB mounting standoffs/spacers must keep clearances OK if they are metal screws. The hole near pad OUT2_2 could be a bit close if a metal spacer, nut, washer etc. could slide close.The on-off switch collar should be earth-grounded usually this is done on the metal chassis it is mounted to.The power transformer may need a fuse on its primary. Check its datasheet if "inherently short-circuit protected" then you do not require a fuse for it. The off board "big fuse" does not count because it is high enough amperage to still allow the little transformer to burn up.On your circuit, there is no capacitor across the switch, so it will arc and get beat up by the toroid when switching power off. I would add a X class cap at least 22-47nF across the switch.If C4 100nF is MLCC then your timing value will be much less than expected, and R3 1MEG is already large. 100msec is not much time for soft-start. I would change C4 to 1uF.

author: exe
author_rank: Unknown
timestamp: February 15, 2019, 09:55:25 pm
content: Thank you very much for valuable feedback.Question about switch: it is metal, but it's meant to be activated by a plastic push rod. Do I still need to earth it?

author: floobydust
author_rank: Super Contributor
timestamp: February 15, 2019, 10:34:07 pm
content: Quote from: exe on February 15, 2019, 09:55:25 pmTo protect against electric shock, you rely on PE grounding or insulation.If no fingers can touch the metal switch, then there is no shock hazard. The plastic push-rod is the insulator and no need to ground the switch.If the switch was only metal, you would check its safety agency approvals. Many small switches have a "250VAC" rating but that is for the contacts, not the switch housing and sometimes there are no VDE/UL/CSA approvals so I don't trust that make of switch.

author: exe
author_rank: Unknown
timestamp: February 16, 2019, 06:45:41 pm
content: I'm a bit confused how to make a snubber for the switch. Do you think just a 22nF cap would do the job? No resistor, nothing else? People say resistor needed to reduce peak current when switch is closing. Alternative suggestions are to put cap in parallel with trans, but I have doubts, as it becomes a resonant circuit, imho. So, some calculation is required. I heard that cap can only be 1-2nF as it becomes a capacitor divider with the switch. Since switch has only few pF, it supposed to be enough. But I didn't really see any proof that this gonna work.As for other concerns. Why I have slots is because I may need active cooling, fan may suck quite a bit of dust (but I'm trying to put heatsink with the fan outside of enclosure). I also want to make a "robust" design that doesn't make assumptions, just an "unconditionally save". It is 2.5mm wide because even if the board gets fried by a lightning and there is a deposit of conductive carbon, there is still enough clearance. I'm not sure if this reasoning is correct, let me know if I'm wrong. I just want to have one softstart circuit that I could use in all my devices without much thinking. Trapping flux is not a problem as I do hand-soldering and washing. So, for a production unnecessary slots can be bad, but so far I'm only gonna make two of these boards. I already spend two weeks for it, so a few more hours for assembly is not a problem.Slots indeed weaken the board. I hoped that relay and transformer would reduce board stress a little bit. Now I'm not so sure that stressing them is good idea.I moved spade connectors away from mounting holes. Even though I use nylon spacers and screws to mount boards, someone else may use this board. So, thanks for advice!The small transformer that provides isolated floating supply (no connection to ground) is inherently short-circuit proof. It doesn't mean it cannot fail. I used to have a 0.1A PTC, but removed it for the sake of simplicity. I think even if there is short inside, mains will either melt wiring inside, or fuses will blow.Why delay is 0.1s. It's five mains cycle, simulation showed it should be enough to avoid the biggest portion of inrush. I still changed values of time-setting resistor and capacitor (it's 1uF cap and 100k resistor instead of 100nF cap and 1MEG res) as 1MEG resistor seems to me a bit too much and may be susceptible to noise? Or may be not as 100nF was still quite a lot to be quickly charged by emi?

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2019, 04:24:13 am
content: How many VA is your toroid transformer?An X-capacitor right across the on/off switch works, done for decades on electronic equipment. It's the smallest size option and while not perfect, it does suppress most or all of the arc upon switch off. In audio gear, this arc causes a loud pop sound due to the burst of EMI.The capacitor value depends on the toroid's primary inductance and you would have to make measurements for the perfect value. You will see 10-100nF and 33-100R 1/2W used. Yes, try 22-33nF and no resistor or add 47R if you have room. There are always side-effects when adding parts, but no need to worry about pF's here.Slots are OK if you want them, under the transformer and relay I don't see air flow.For the soft-start timing, there is the power transformer's inrush current, plus the main filter capacitor's charging inrush. I'm not sure if you have huge filter capacitors. They may need more than 5 cycles to charge up.Amtherm inrush limitershas more info on their website. The bigger 555 timing cap allows simply changing the resistor if more time is needed. I use 500msec up.Temp and voltage variation of ceramic caps, or why your 4.7uF part becomes 0.33uFVishay Sprague Capacitance Change with Applied DC Voltage

author: exe
author_rank: Unknown
timestamp: February 18, 2019, 09:23:56 pm
content: Capacitor you suggesting (X2 for 250V AC) is quite big. I'm also worried that at 47nF it starts conducting noticeable current (although, I didn't take into account inductance of transformer, just assumed it is a short between live and neutral). Looks like I need a soft stop circuit. Probably, I need to make my pcb larger. Question, can output caps smoothen the inductive kick? Secondaries are magnetically coupled with primary...My current transformer is 120VA, but I provision for potential future upgrade to ~240VA. For 120VA people say softstart is not needed. So, this project is more for education purpose, rather than an actual need. This is going to be my first ordered pcb.There is no air flow under transformer and relay. Does this render slots useless?Ceramic caps are carefully chosen not to have DC bias problems (at 5V). I only buy from kemet and murata X7R and C0G as they have online simulators so I can estimated capacity and impedance under for the expected operating point. No Y5V rubbish. Nonetheless, I'm going to verify performance once the final ciruit is built (parts the current design were verified on breadboard and simulator).The total capacitance is gonna be 10000-20000uF. Quite a lot. I heard relay contacts need a bit of current for self-cleaning. That's why delay is so small.PS I realized that switch disconnects only one wire, not two as some safety standards suggest. The good thing is that I need just one snubber.