Inrush Current for off the self switching supplies - Page 1
Forum section: electronics / projects

author: kg4arn
author_rank: Unknown
timestamp: July 18, 2015, 09:22:50 am
content: Planning to use a switching supply for a project. Needs about 100W output (24V at 4 amps).Looking at these off the shelf switchers, the inrush currents are very high.http://www.jameco.com/Jameco/Products/ProdDS/2104680.pdfWith these types of units, I gather there is no inrush current limiting and one must handle the inrush current limiting external to the supply inputs?Is that correct?

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2015, 09:45:12 am
content: Its unlikely to be a problem unless you are running off an inverter. Many smaller consumer SMPSUs don't bother with inrush current limiting and reports of nuisance breaker trips when people plug in their PCs or laptops are extremely rare.If it is a serious concern in your application, look for PSUs with power factor correction as they will almost invariably have inrush current control as standard.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2015, 10:31:03 am
content: Guessing they have ~5 ohm NTCs inside for inrush. The transient is probably < 10ms. What's so bad about that? Do you have a hard limit in mind for your application (e.g. SSR) that you have to watch out for?A properly rated fuse probably takes 100ms minimum to fail at that current, so it's quite normal operation for most things.Tim

author: kg4arn
author_rank: Unknown
timestamp: July 18, 2015, 10:48:38 am
content: Many Thanks. Just did not want to trip the mains breaker. Such a short duration shouldn't be a problem then.

author: merlinb
author_rank: Contributor
timestamp: July 18, 2015, 11:10:00 am
content: I used a power supply just like this recently, and had trouble with it blowing the 500mA mains fuse I had naively chosen (the project itself consumes <10W). In the end I had to go to a 1.6A fuse which seems to handle the inrush OK.

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2015, 11:23:52 am
content: Got a brute of a robot arm 'on' the bench at work at the moment. The PSU has two mains leads fitted with 13A plugs in sockets fed from a 32A industrial socket and the inrush current when the E-Stop button is released trips 32A breakers!A 40A breaker holds it most of the time, and we cant go higher due to wiring ampacity limits.A teardown to add some appropriately rated NTC thermistors may be in its future . . . .I assure you that a 100W supply will *NOT* cause nuisance trips due to inrush current unless its criminally badly designed or the circuit is already close to being overloaded or the breaker is faulty

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2015, 01:09:03 pm
content: Yeah no, you have NO danger whatsoever of tripping breakers from a mere 100W supply, of any remotely reasonable description.Breakers are designed to withstand heavy loads for seconds at a time (such as motor starting), and even under short circuit conditions, need several cycles to open (for certain values of "short", of course -- a 120V 15A, all the way across the house, might fault under 1kA peak; a beefy 480V circuit on industrial mains can peak in the 20-100kA range however!).On those occasions when you *do* need a fast trip, this is good to keep in mind, because fuses of any kind won't do the job -- you need additional protection circuitry, or you need to make the circuit itself behave.Ian: it might help if you can get some "C" curve breakers and pop them in -- these have a longer thermal time constant (if not a higher magnetic pull-in rating), made for starting motors (may carry ~10x rated current for several seconds!). If the inrush is more due to capacitors charging, this may not help (i.e., it looks like a shorting fault and carries enough current to trip the breaker magnetically), and you'll need a precharge or soft start device instead.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2015, 02:02:32 pm
content: Unfortunately, we've got to get it to the point where it can be run off *ANY* properly installed 240V 32A industrial supply. Setting up a custom feed for it is *NOT* an option.

author: SeanB
author_rank: Super Contributor
timestamp: July 18, 2015, 02:22:32 pm
content: So the old set of power resistors and a shorting contactor driven by a 1 second power on delay timer will not work? All parts aside from the resistors are available in a DIN rail mount option, and will fit in the controller easily.

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2015, 02:46:09 pm
content: The problem is that the E-stop is handled in the PSU, but logic boards in the motor drive cabinet and the real time control processor box are powered by the same PSU, and they get power even in E-stop so the system boots and self-tests before motor power is enabled, and they may not tolerate the resulting sag in the low voltage busses without resetting so a simple soft start circuit on the power inputs probably wont work. We are talking to the manufacturer's tech support team about options to retrofit the PSU's motor supply with a soft start circuit.

author: German_EE
author_rank: Super Contributor
timestamp: July 18, 2015, 03:25:30 pm
content: I have a power supply box for my ham radio station that has three SMPS, 48V, 20V, 12V and -12V with inrush limiters are fitted to each of the supplies. They consist of a 20 ohm 10W resistor in series with the live lead and a 230V 5A relay that shorts out the resistor a fraction of a second after the power is applied. Manufacturing tolerances mean that each supply turns on at different intervals after the power switch is turned on and the fuse never blows.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 18, 2015, 03:54:34 pm
content: There are a few reports about inrush false tripping AFCIs, but even that's rare.