Include a NTC Current Limiter in my DIY AC PSU Chain? - Page 1
Forum section: electronics / testgear

author: t1d
author_rank: Super Contributor
timestamp: August 17, 2022, 12:57:56 pm
content: I am building my DIY AC PSU chain. It consists of:1) Emergency All Stop Button to2) Isolation Transformer to3) Variable Autotransformer to4) Dim Bulb Current Limiter (Multi-Bulb) to5) DUT.Each piece is independent, so that it can be used separately, if need be, for other purposes.Might it be helpful to include a NTC Current Limiter? Advantages/Disadvantages? If so, which component should I add it to? The input on the Dim Bulb Current Limiter? It sort of seems redundant, unless all the bulbs are bypassed...Thank you for your help.

author: Randall
author_rank: Newbie
timestamp: October 06, 2022, 03:53:32 pm
content: Bump !I'm very interested in this question?Lookin' forward to a response.....

author: thm_w
author_rank: Super Contributor
timestamp: October 06, 2022, 09:38:06 pm
content: The bulb is already going to limit current, I wouldn't bother.If you need to ramp up the voltage then you have an autotransformer that can be turned slowly.

author: srb1954
author_rank: Super Contributor
timestamp: October 07, 2022, 02:04:21 am
content: Quote from: thm_w on October 06, 2022, 09:38:06 pmThe dim bulb will eventually limit the current but it will let through quite a high surge or fault current until the filament has come up to temperature. An extra NTC resistor can compensate for this behaviour by ensuring that there is sufficient initial circuit resistance to limit the peak surge current.QuoteThat doesn't always work well with equipment incorporating switch-mode PSUs. These typically have an under-voltage lock-out and won't start until the mains supply reaches a certain level. At which point the switcher will suddenly become active and draws its maximum rated current as it attempts to deliver its rated power to the load. Or, if there is a fault in the PSU, the current draw is only limited by the relatively low impedance of the auto-transformer windings. A dim bulb tester is often better in this situation as it will allow the switch-mode PSU to start up quickly but limit the total current draw to safe levels if there is a fault condition in the PSU.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2022, 05:58:44 am
content: you might find this thread usefulhttps://www.eevblog.com/forum/repair/current-limiter-box-for-repairs/I stopped working on it because I just have not needed one

author: jonpaul
author_rank: Super Contributor
timestamp: October 07, 2022, 07:08:27 am
content: In many decades of experience in power supply consulting, design and manufacture, I never had any such complex bench power.Just...2kva Signal 120/240 V isolation transformer under benchMetered and un metered variable transformers 100w to 5 kV ( Superior PowerStat, Variac, no Chinese)Yokogawa WT110 digital power analyzer.Protection was in the variacs by fuse or CB.The insertion of lamps or PTC will affect the DUT and measurements, as they change with load current.By slowly increasing the mains at Variac output, we seldom had any blow-up.A contactor kill button is overkill. Simpler is better!Just the ramblings of an old retired EEJon

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2022, 05:00:06 pm
content: I might come back to the project if I decide to try to repair the old meanwell switcher I have, but I think I know what the problem is, its just a SOB to repair that area of the device (full disassembly), so I will have to fail at least one repair attempt before I embark on this project. Not even sure how it will react, so if I do it, I will just leave a light bulb on a bench to see if its even a valid troubleshooting aid.The more difficult/annoying something is to disassemble, the more interesting this project is. It would have come in handy for troubleshooting the amplifier I just fixed, but somehow I managed without it.