Choosing power resistor for inrush limiter - Page 1
Forum section: electronics / beginners

author: Stefan
author_rank: Contributor
timestamp: July 03, 2015, 10:24:59 am
content: I'm using a 1000VA transformer I happen to have around to build an isolation transformer. Right now it's a rather heavy fuse killerThe idea is to use a series resistor as a current limiter and then bypass the resistor with a time-delay relay or something.A reply onhttps://www.eevblog.com/forum/beginners/ntc-heat-disspipaion/suggests to use a power resistor instead of an NTC, which makes a lot of sense to me.But which one to choose? If I'd like to limit the current to 2A @ 230V, the resistor dissipates a wooping 460W. A resistor that can handle this continuously would be a bit of a beast. This seems overkill as the resistor will only be in use for a short time.I think I need a power resistor that has a high termal mass, but I don't find much information in the data sheets about that.P.S.Is a 2A limit for 100ms realistic for a 1000VA toroidal on 230V?

author: oldway
author_rank: Super Contributor
timestamp: July 03, 2015, 11:05:51 am
content: You have to start from the current (more exactly, the I²dt ...) your fuse or circuit breaker can accept without blowing or tripping.You must choose the lowest possible value of resistance.For a 1000VA 230V transformer, you must use a fuse of 5A of higher.Than peak current may reach 2 x nominal current for a short time without blowing the fuse.Peak current must be limited to 10A.Peak voltage is 325V.Limiting current power resistance should then be 33R.As this resistance must only be in circuit for a short time,(1 or 2s), average power dissipation will be low and I think that a 10W power resistor will be enough.Best way is to try it...

author: mij59
author_rank: Frequent Contributor
timestamp: July 03, 2015, 11:16:18 am
content: According to trafo.nl its two 5 Ohm 5W ntc's in series link herehttp://www.ringkerntrafo.nl/shop/toebehoren/inschakelstroom-begrenzing/436/inschakelstroom-begrenzing-1000va.html

author: bktemp
author_rank: Super Contributor
timestamp: July 03, 2015, 11:27:12 am
content: Wirewound resistors are the best choice for such applications. Avoid metal oxide resistors, they can not handle high power levels. NTCs also work well, but are a bit more difficult to select.Depending on the application 10-50ohm should work fine. After a short time, short the resistor using a relais.

author: wraper
author_rank: Unknown
timestamp: July 03, 2015, 11:49:44 am
content: Limiting inrush current to 2A (even below nominal working current) is not how things are done in the industry. In commercial devices inrush current usually is a few tens of Amps.

author: Stefan
author_rank: Contributor
timestamp: July 03, 2015, 01:15:55 pm
content: Thanks for all the tips!Wire-wound resistor's data sheets actually contain information about overloading for a short period of time.Regardinghttp://www.powerguru.org/inrush-current-limiters-icl-2/, I have a hard time understanding those calculationsZ=121V/350A Reactance = Mains voltage / Inrush current. I thought that the inrush current is a result of the core saturating. It seems a bit doubious that to calculate the reactance based on that.E = 0.5*Z*I2Energy = 1/2 * Reactance * Current^2. Makes sense, but again the inrush current is used. I was under the impression that the transformer cannot store all that energy due to saturation.

author: sean0118
author_rank: Frequent Contributor
timestamp: July 03, 2015, 01:31:56 pm
content: Have you seen this video by The Post Apocalyptic Inventor? It covers everything you need, isolation transformers, current limiting and remanence.

author: kalhana
author_rank: Contributor
timestamp: July 03, 2015, 03:04:50 pm
content: Most power resistors can be overloaded for a few seconds without an issue.For example:http://onecall.farnell.com/te-connectivity-cgs/hsa50100rj/resistor-alu-housed-100r-5-50w/dp/2009359?Ntt=HSA50100RJThe above power resistor can handle up to 25x overload for 1 second (check overload vs. duration graph in datasheet)I used this in a grid connected bidirectional power converter with a relay to bypass the resistor.I would do a quick calculation (or simulation) and see how the current decays with time to make sure the ratings are never exceeded.Make sure you don't exceed the ratings or they will blow out!

author: oldway
author_rank: Super Contributor
timestamp: July 03, 2015, 03:12:53 pm
content: There are also other ways to avoid high inrush currents using triac phase control.- soft start- power on delayed 90° after 0V.Triac may be shortened by relay after start-up.