Soft Start Module Modification Help - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: January 15, 2020, 02:24:43 am
content: I'm building a 800W power supply and so far things are going well except the inrush current peaks at around 13.5A at the secondary. Which is a lot lower than it was before I installed a choke on the filter and a soft start on the primary.But 13.5 A is still to high . Its causing the Op amps in power supply circuit to "lock up" and stop functioning until I reset the system by switching off the power then on again before the capacitors can discharge completely.I tried resistors at the op amp inputs but it changes the dynamics of the circuit to much, Voltage and current control just won't work right.I want to modify this soft start module I have by installing more thermistors and how they are configured but I'm not sure if I'm calculating the inrush current correctly .From what I understand I can calculate the inrush current with:R = 1.414Vac/ IThe calculation seems to concur with the 13.5A at the secondary I get now with the 10 Ohms NTC configuration the soft start has right now.Around 4.3 A at the mains primary.So If I put (5) 10 Ohm NTC in series for a total of 50 Ohms I should be able to reduce the current down to around 3A at the secondary.That should reduce the primary inrush down to around 1AThe modification involves making some cuts in the PCB trace and drilling new holes eliminating NTC 2 and 3 and replacing 1 and 4 with NTC 10D-25 then adding the remaining 3 in new locations. I have lots of the 10D-25https://datasheet.lcsc.com/szlcsc/NTC-Thermistor-5R_C69391.pdfAm I on the right track? And are there other considerations I should be aware of?Like the steady state current I figure its around 6 amps .Iss= Pout/VacX 90% EfficiencyAnd how to calculate the energy through the NTCs to make sure they're rated high enough?I'm having trouble with that calculation.I get crazy numbers that don't make sense.

author: Jwillis
author_rank: Super Contributor
timestamp: January 15, 2020, 02:26:25 am
content: Sorry for some reason the picture got deleted from opening post

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 15, 2020, 01:47:33 pm
content: Is your mains voltage 120 V? It means steady state current about 7 A, and 10D-25 is rated for 7A steady state current, so it is ok (it have to work only less then 1-2 second, so current can be even higher). That means it must withstand inrush current too.But I think you not only have to modify soft start but you have to repair/modify you power supply circuit too to it wan't "lock up" because it means bad design.For example, may be you not only need resistors at OP amp inputs (up to 1 k) but protective diodes too, etc?I think you can start another topic about your power supply "lock up" problem and somebody can help.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 15, 2020, 01:57:12 pm
content: If your power supply output power is 800 W, than power at it input is larger and it is:Pin = Pout/Efficiency = Pout/0.9 or Pout/0.95 = 800/0.95 = 842 W.Efficiency of power supply depends on design, parts ets and it is usually better for high power devices (and much smaller/worst for low power devices. "Low power" I mean several watts and less).

author: Jwillis
author_rank: Super Contributor
timestamp: January 16, 2020, 12:46:51 am
content: Quote from: Vovk_Z on January 15, 2020, 01:57:12 pmThank you for the help. It's good to know I'm getting the math right for a change. I made the changes to the soft start and the inrush current is manageable now.Its difficult to get a reading right at peak voltage but measurements seem to confirm the math.I wonder if it's possible to make a circuit that switches the power on right at peak voltage ? Not for another purpose other than for testing.Total resistance at primary is 59.4 OhmsPeak voltage is 169.7 (120Vac X 1.414). Math shows 2.9A inrush .Typical measured inrush at primary is 2.5A which puts around 6A at the secondary.So the power supply circuit isn't locking up anymore.As you suggest I'll still have to look into adding more protection to the circuit.But at least things are progressing.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 16, 2020, 12:16:02 pm
content: I don't like circuits that locu up at the start - it is really annoingbut if it works ok now - then ok.Or you can use it and slowly start to collect info to get rid of that problem in other way.