Fix inverter startup surge with NTC?? Help please... - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: December 26, 2020, 12:19:55 pm
content: Hi all,Inverter:https://export.farnell.com/ea-elektro-automatik/ea-srx-pb-300-12/inverter-dc-ac-12vdc-230vac/dp/1335838?ost=1335838I wanted to give the inverter a test run, so today I connected it to my bench supply, providing 12Vdc. The chassis was connected to mains-earth reference to ground it properly.My Keysight can provide 4A (Chan2&3 in parallel), but it was triggering OCP. Could I use a NTC on the +12vdc input line to reduce the current-surge at power on?I have this NTC, 33ohm 3A, assuming it needs less than 3A to start.https://export.farnell.com/epcos/b57213p0330m301/icl-ntc-thermistor-33ohm-3a/dp/3407656?Thanks.

author: bsodmike
author_rank: Regular Contributor
timestamp: December 27, 2020, 08:03:14 am
content: I also realised that I had not provided the OCP any delay, so I’m going to test about a 5aec delay with OCP at 2-3A.

author: dietert1
author_rank: Super Contributor
timestamp: December 27, 2020, 08:46:21 am
content: Please use stronger wiring on the 12 V input, at 300 W out the converter will take 25 A + x.That NTC will limit start current to 12 V / 33R = 0,3 A. It think for 25 A you will need something different, or many of those NTCs in parallel (i mean in case you bought a bag of them).Regards, Dieter

author: bsodmike
author_rank: Regular Contributor
timestamp: December 27, 2020, 09:19:27 am
content: Thanks Dieter,This explains why these were designed to run off automotive acid batteries. 300W/12 = ~25A?What’s the exact conversion from VA to Watts?The transformer is 400VA, so 400VA/230VAC = ~1.7A(AC)My building supply here is only 10A so this inverter is not going to do the job.Going to take a different approach.Thanks M

author: dietert1
author_rank: Super Contributor
timestamp: December 27, 2020, 12:00:03 pm
content: If that converter has reasonable efficiency like 80 or 90 %, the extra current that turns into heat will be about 2 to 5 A. So you need to calculate with 30 A continuous input current. With a supply of 10 A the converter should start up and deliver about 80 W max. You could try to use a large capacitor as a local energy buffer, like those for car audio.Somewhere in our lab there is such a converter, too. It was meant for free air audio measurements on a parking lot, but as far as i remember never used until now. I could try whether it starts up on a 13.8 V x 10 A linear supply that we have.Regards, DieterPS: The test worked in my case. I found that our converter "solartronics pure sine wave 300 VA" had a built in delay of 2 seconds after power on. Then without load it would output intermittent 230 V to check for load. After connecting a 100 W bulb, the fans turn on and a continuous output of 226 Vac appears. I think our converter includes some buffer capacitor. I connecting none and the linear power supply only has a small one, like 100 uF or so.

author: bsodmike
author_rank: Regular Contributor
timestamp: December 27, 2020, 02:14:40 pm
content: Thanks for the heads up Dieter.Yes, a large cap and I will also include a discharge resistor, say a 10Kohm in parallel.Right, I will also attach a 60W bulb as a load, that’s a good idea!’I can program my bench PSU to include a 2-5sec delay prior to OCP kicks in.Do you think a 16V 500uF cap be enough or something larger? Of course I will also rewire the 12V line with 14AWG as that’s the largest I have on hand.

author: bsodmike
author_rank: Regular Contributor
timestamp: December 27, 2020, 02:16:18 pm
content: Oh the max I can supply from my Bench PSU is 4A so the capacitor will need to supply the rest.

author: dietert1
author_rank: Super Contributor
timestamp: December 27, 2020, 04:17:45 pm
content: If the capacitor is meant to support power for a short time like 0.1 sec, then you can calculate how large it should be. Your load will have a resistance of about 12 V / 30 A = 0,4 Ohm. Then a capacitor of 0.1 sec / 0.4 Ohm = 0.25 F is required. 0.25 F is 250 000 uF.Your 500 uF capacitor would only last for less than 1 msec. And it cannot yield 25 or 30 A.Those expensive capacitors for car audio have 1 F to 4 F. Maybe better get a fresh car battery instead. A car battery could support your device for 1 or 2 hours. But watch out for safety with this type of project! I don't know whether using a car battery inside a house is risky.Regards, DieterPS: Meanwhile i opened our inverter and found 3x 2200 uF as input buffer capacitors at 13.8 V plus 2x 100 uF at 400 V (intermediarystorage).

author: bsodmike
author_rank: Regular Contributor
timestamp: December 28, 2020, 09:02:17 am
content: Hi Dieter,Thanks so much for your help and taking an interest to troubleshoot this for me. Even though I took the inverter apart previously to replace the L/N wires with longer leads (I desoldered the old cabling that lead to a receptacle, EU style 2-pin socket), right in my face is a "automotive" 40A fuseLooking again it has 2x 2200uF 25V caps on the intermediary stage and a 100uF 400V on the AC output side (assuming so given it's near the FWBR diodes).There's also an NTC right next to the L/N output, possibly to prevent the load causing a surge.Please see snaps attached.

author: bsodmike
author_rank: Regular Contributor
timestamp: December 28, 2020, 10:59:00 am
content: Hi Dieter,I'm ordering the following PSUs to help me test the inverter:https://amzn.to/34Q1DZj- this is a 12V/29A capable SMPS unit.https://amzn.to/2KJ861t- This is a 24V/21A capable SMPS.Hoping I could use the 12V/29A PSU to check that the inverter I have works. This is really that "first" test.My ultimate goal is to power the inverter through this Buck PSU (https://amzn.to/3rxIcOO). The purpose here is to enable OCP on this unit to limit the maximum current when a load is attached at the isolated AC output.I can then wire the 24V SMPS --> DROK Buck supply and generate 12VDC with OCP programmed as desired ----> Inverter -----> Isolation transformer ----> Output.Thoughts?

author: dietert1
author_rank: Super Contributor
timestamp: December 28, 2020, 11:39:11 am
content: The buck converter you linked is rated at 15 A, but the 12V Meanwell supply will be good for a full test. Another possibility are used HP server power supplies, like this one: HP DPS-1200FB. We got two of those from ebay and they are reliable and easy to use.Regards, Dieter

author: bsodmike
author_rank: Regular Contributor
timestamp: December 28, 2020, 11:41:28 am
content: Quote from: dietert1 on December 28, 2020, 11:39:11 amYes, I had already picked up the buck converter a while back and I can't see too many with higher supply capability. Thanks, I'll check those out!!

author: bsodmike
author_rank: Regular Contributor
timestamp: January 04, 2021, 06:50:37 pm
content: Hey Dieter, first off Happy new year to you!I emailed the manufacturers of the inverter and they've made the same suggestion as you didQuoteSo I found this 20000uF cap - this is the lowest voltage they have but we will only be charging it up to 12Vdc:ALS70A203KF100Electrolytic Capacitor, Long Life, 20000 µF, 100 V, ALS70 Series, ± 20%, Screw, 20000 hours @ 85°Chttps://export.farnell.com/kemet/als70a203kf100/cap-20000-f-100v-alu-elec-screw/dp/2729166I have a sneaky feeling I will need this otherwise I'm going to blow a fuse and/or trip the breaker (16A) quite easily.

author: dietert1
author_rank: Super Contributor
timestamp: January 04, 2021, 10:21:35 pm
content: They also have this one: ALS70A823DF025. It's four times more (82 000 uF) and half the money!Regards, Dieter

author: bsodmike
author_rank: Regular Contributor
timestamp: January 05, 2021, 04:11:55 am
content: Thanks, it's 25Vdc as well, awesome!I've been looking at application circuits to protect and manage the supercap - this maybe over the top but I'd sleep better at night knowing the supercap is discharged whenever the 12V supply is removed.Sure, a resistor in parallel will take care of it, but when 12v supply is restored I'm looking to add circuitry that:Ensures the supercap is charged to 12Vdc firstSupercap charging current is limited via a control-resistorOnce cap is charged, triggers a relay to enable 12Vdc supply to the inverter.When 12v supply collapses, inverter supply is removed (relay closes to default)Supercap starts to dischargeLimit current in-case supercap undergoes a short?TL;DR logic to prevent the supercap blowing up.These are just rough bullet points, I haven't yet had the time to go through it further just yet.

author: bsodmike
author_rank: Regular Contributor
timestamp: January 16, 2021, 02:50:49 pm
content: Hey Dierter, do you think having a circuit like this to ensure the Supercap stays charged before power on is a good idea?I've also placed a power diode to prevent the cap discharging back into the 12v supply. The R(i) resistor is to allow an oscilloscope to measure current through the cap with respect to mains-earth ref (I don't have a differential probe just yet).

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2021, 05:40:43 pm
content: Are you sure you don't mean PTC??https://www.vishay.com/thermistors/list/product-29165/very popular as inrush limiters......

author: bsodmike
author_rank: Regular Contributor
timestamp: January 16, 2021, 05:57:14 pm
content: An NTC can be used as well. Seehttps://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limitingMy application is at root temp so the NTC action would be high start resistance (~35ohms) and it drops once it heats up.

author: Gyro
author_rank: Super Contributor
timestamp: January 16, 2021, 06:54:42 pm
content: Quote from: Terry Bites on January 16, 2021, 05:40:43 pmThat page and datasheet seem completely screwed up.It looks like a standard PTC polyfuse where the resistance shoots up by several orders of magnitude as expected at ~150'C.I don't understand how they can be describing it as an inrush current limiter? Once it heats up from excessive current, it will stay latched 'off' only dropping its resistance enough to maintain itself in the high temperature state, passing nothing useful to the load. It's a self resetting [Edit: overcurrent] protective device.Inrush surge limiters are always NTC, with higher resistance when cold to reduce input current and then dropping as it heats up to minimise losses.

author: bsodmike
author_rank: Regular Contributor
timestamp: February 03, 2021, 02:26:14 pm
content: Made some progress. The DC PSUs by MeanWell arrived today. Thanks to COVID everything is held up, so still waiting on some parts.Video on progress made and powering it up:

author: bsodmike
author_rank: Regular Contributor
timestamp: March 11, 2021, 11:58:04 am
content: I found an old wall-wart and decided to take it apart, and found a large cavity inside, which gave me the idea of adding a simple linear voltage regulator and some small tweaks to obtain a simple 12Vdc / 1A (highly overrated) output, to switch a few relays.

author: bsodmike
author_rank: Regular Contributor
timestamp: March 11, 2021, 12:02:31 pm
content: Here are a few snaps of progress made in wrapping most of the internals with Kapton tape and adding it into the enclosure. You'll also find progress videos linked below:- Part 2:- Part 3: