1st Post: Dead 24V 8A Charger Questions

[Lazarus42]

Firstly, Hi everyone! I've been following the channel for donkey's years & find myself in a situation whereby this newbies questions section is absolutely spot on! Thank you!

tl;dr
 Is there a good step by step diagnostic path for fixing dead power supplies posted anywhere?
Cheers!  Laz 😀

A neighbour, showing unwarranted high expectations of my electronic prowess, has given me a mobility scooter charger (220V AC in, 24V DC at up to 8A out) to see if I can help.

Mains fuse was rated 3A so I put a 13A fuse in to cover the 8A output. A 5A would've just blown. That's the three standard choices here
The automotive blade fuse on the output is rated at 15A and is sound.
(Would a 10A be safer? I think a 10A would blow earlier otherwise we're just relying on the mains fuse, no?)

The thing is still dead but I've read it won't charge if the scooter's batteries are too low. But I would've thought it'd would at least show a power light even if it's not charging. Manual says red LED when charging going green at full charge. There's only one LED that has never lit.

The next thing I'm going to do is unhook the 12V lead acid batteries, charge them with a bog standard car battery charger to barely 12V each, put them back in circuit on the scooter and test again to see if it is because the unit can see no charge that it appears dead.
After that, it's me, a multimeter & a winning smile!
Does anyone know of a step by step diagnostic path laid out either in text or video that you'd recommend?
My meter reads capacitance but I was thinking of getting an LCR TC1 ESR tester so I can test the caps in circuit.
I opened the ends of the charger up & it passed the soot & sniff test but it's going to be fun opening the thing. Where the main board is meant to slide out on runners built into the extruded aluminium housing it's been sealed in with lashings of rubbery mastic ☹️.
Oh! It's a Strident CHE24-8 just in case anyone knows the beast & can tell me if that single LED lights upon application of power without being plugged onto the scooter.
Apologies for the ramble and it's even helpful just writing down where I'm at.
Thanks chaps!
Laz 💜☮️😎

[darkspr1te]

First rule off eev forums, thy shall test voltages :-)
Jokes aside, i would look up SMPS repair videos as most chargers are a SMPS and voltage/current controller combined , Learn electronics repair channel does some great videos on it and could do with the views (they being botted by unknowns and loosing revenue)
remember you are working with mains voltages here so if you cant be careful then dont attempt it.
As a old dog in the "you work with computers can you fix this" circles i find first i always do the sniff test across the board, dry or leaked caps will often have a slight metallic smell close up
look for obvious signs of heat on all sides of the board, look for dry joints on large and small components.


good luck


darkspr1te

[darkspr1te]

I just did a quick check on google and if you search for "24v mobility scooter charger schematic" you will get a couple of diagrams that look very close to your unit. which btw is same psu that my Till machine in the shop uses, i've only ever seen a green light but there is no doubt it's the same.


darkspr1te

[Shonky]

Mains fuse was rated 3A so I put a 13A fuse in to cover the 8A output. A 5A would've just blown. That's the three standard choices here
The automotive blade fuse on the output is rated at 15A and is sound.
(Would a 10A be safer? I think a 10A would blow earlier otherwise we're just relying on the mains fuse, no?)

Do not replace the 3A mains fuse with a 13A fuse if that's the fuse on the primary side of the power supply.

8A output at 24V does not mean 8A input current at 220-240V.

[magic]

Identify the chips, look up datasheets, study theory of operation and example schematics.


But if you are making tihs sort of mistakes

Do not replace the 3A mains fuse with a 13A fuse if that's the fuse on the primary side of the power supply.

8A output at 24V does not mean 8A input current at 220-240V.

then I don't know if you should be touching mains powered electronics.

[Eraldo]

Check the full bridge rectifier in the primary side for possible shorts. Check on the internet how to test those. If that is fine, then check the switching transistor/mosfet. Go to the datasheet and check if it's  a transistor or mosfet and test it accordingly. It shouldn't be hard to locate it since it will propably be stuck on a heatsink.

As someone else said, replacing the 3 amp fuse with 10 A one  or more is dangerous and the actual current passing through that fuse is more like 1 Amp

[Gyro]

...
The thing is still dead but I've read it won't charge if the scooter's batteries are too low. But I would've thought it'd would at least show a power light even if it's not charging. Manual says red LED when charging going green at full charge. There's only one LED that has never lit.
...

Check the physical conditon of the cable and connector wiring,  they are probably the bits most at risk of failure. As others have said, put a (good) 3A fuse back in the plug - you didn't say whether the original was blown, which would indicate a primary side fault.

If your neighbour is 'showing unwarranted high expectations of your electronic prowess', be very careful. Their house insurance company would almost certainly come after you if it,  the scooter and the house go up in flames (a possibility if you start by putting over-rated fuses in). Never be afraid to do a few simple physical and voltage checks and report that 'this is beyond my ability'. Being a helpful neighbour can easily lead you out of your comfort zone.

[snapper]

I love people who start a new post here, text a lot  and then never get in touch again 

[u666sa]

The thing is still dead

Take lots of hi-res a lot of light pics.

Fuse you replaced is one thing, there should be another fuse soldered in. Plus you gotta check bridge rectifier.

[Lazarus42]

Jokes aside, i would look up SMPS repair videos as most chargers are a SMPS and voltage/current controller combined , Learn electronics repair channel does some great videos on it and could do with the views (they being botted by unknowns and loosing revenue)
remember you are working with mains voltages here so if you cant be careful then dont attempt it.
As a old dog in the "you work with computers can you fix this" circles i find first i always do the sniff test across the board, dry or leaked caps will often have a slight metallic smell close up
look for obvious signs of heat on all sides of the board, look for dry joints on large and small components.


good luck


darkspr1te

Thank you for taking the time to put these posts together. It's hugely appreciated.
I discovered the Electronics Repair Guy already & I started at his first video taking things step by step. Such a shame some nasty, probably jealous, troll is giving him grief. He's been invaluable in enabling.e to catch up with modern equipment & materials. Things have moved on a lot since I was building HF Heathkit sound processors and ATUs with glorious air gapped variable caps that are a pleasure to use as kinetic art! That they enabled me to pull in Australian HF ATC on a long wire when I'd planned to start with a very basic setup and as I learned more I'd be able to hear further & further. Australian oceanic routes was my first "catch" on that enamel wire tensioned with bamboo canes. When a seagull hit the thing it made one hell of a twang (a Brixham hazard) but that was half way around the globe, first try. I couldn't really get much further! 😆
Anyway, that was my grounding. My trusty Antex is now replaced by a T12-952 & the new liquid fluxes have blown me away! Solder now flows beautifully. I invested in some light & magnification as tracks and surface mount components are so small & I've got 60 year old eyes now! 🤓
I told my neighbour the charger was stuffed & replaced it exactly from AliExpress. So now I've an item to learn about with no pressure from hopeful neighbours.
I know enough to stay safe. Discharging caps after it's been energised, making sure I have good quality probes on a meter Dave recommended on his channel a while ago. Auto ranging too!
I think it's the availability of boards from China that has impressed me most. Costs for charging protection and solar down in pence if you buy 10 of something you might only need one of but once you've paid postage... I'm replacing small lead acid and chunky alkaline blocks with 18650s and some stunning little USB + solar charging boards.
Things last longer, the sun making a distinct difference and things I take camping are now loads lighter and more efficient.
Oops, I'm waffling. I just wanted to say thanks for worrying, I've listened and that's why I've started at the begining of electronic repair from the proper Isle of Dogs, Gran Canaria. (Canaries being named after the dogs on the island left there by sailor's, not the islands named after the birds. And taken any urgency away from what is a steep learning curve.

Thanks again mate 😀☮️

[Lazarus42]

Thank you for taking the time to reply. I listened & not only have saved some calculators but also I'm practicing changing the triangles into formulas, plugging in some numbers and working them out properly instead of relying on the automation.
Here's a page that has helped a lot!
https://caravanchronicles.com/guides/understanding-watts-amps-volts-and-ohms/

[Lazarus42]

Thanks for the pointers about mosfets. I've been following an electronics repair course from the beginning so have plenty of information on how they do what they do and also places to track down datasheet .PDFs.
And re: your warning about my error in the rating of that fuse. I'm learning how to turn triangles into formulas and working through examples given, some I made up and mostly from irl by looking at alliances of all types around my home.
I'm getting there, taking it slowly and carefully.
I appreciate you taking time to write and tell me where I was in error. Without that step there can be no learning.
Thanks mate 😀

[Lazarus42]

Thanks Gyro, yes I agree wholeheartedly. That's why I told my  neighbour it was beyond economic repair so I ordered the  right charger with the right connector from AliExpress. He's chuffed that he has a charger that works for a substantial saving on what the mobility shop tried to sell him (disability is like golf, just the addition of the D word or the G word attracts a 100% mark-up on normal prices!

Plus, I now have a project that I can take the time to learn about safely with no pressure from eager neighbours and all the kit in the house is mine.

Thanks again Gyro, keep safe mate 😃

[Lazarus42]

I love people who start a new post here, text a lot  and then never get in touch again 

Who's that then Snapper? 🤔 What a thoughtless oik they must be eh? I missed who it was because I was in hospital for a bit. Still putting bits back together after sepsis. (Lost a couple of legs, open heart surgery, 50% kidney function and a diagnosis of Asperger's that I'm actually pleased about!)

But! From being given three days at one point I'm just pleased I'm here! It's quite the perspective shift!

I do like your screen name, Snapper by name and snappy by natu... Ahem, erm... If you spot the thoughtless git point him out mate!  😆

Keep safe Snap,
Laz 😎☮️

[Lazarus42]

Take lots of hi-res a lot of light pics.

Fuse you replaced is one thing, there should be another fuse soldered in. Plus you gotta check bridge rectifier.

Thanks mate, the bridge rectifier. Rough & ready, make sure it's putting out DC. I have no scope so checking for full wave or half wave is a bit out of reach. But I could make sure all the diodes are working as one way valves (maybe gates, valves is used elsewhere in electronics eh?) and if it's a discrete component instead of four I could desolder it and use the LCR-T4 perhaps & check the forward voltages... am I on the right path there?

Thanks for the tips, much appreciated 😃

[killingtime]

First step would be to post some hi res pictures of the psu top and bottom with the cover off. We can then tell you which components to check. There are some basic checks you an make with a DMM that will tell you if the PSU is an easy fix or a write off, like checking onboard fuses, diodes, mosfets, capacitors etc. Always start with the simple checks first.

Be careful when handling the pcb if it's a switch mode psu. The electrolytic smoothing caps on the primary side hold lethal voltages for a while after the psu has been disconnected from mains. They usually have bleeder resistors across them and are fine to touch after a few hours, but we don't know if the mfr installed them. If in doubt, short these caps out with a screwdriver or long nose pliers just to be sure.

[Lazarus42]

Thank you! I'll set it up with some good lighting tonight.
And yes, I've a healthy respect for all  capacitors. Even if something has been sat for a while I'll short across them.
My Dad used to work in the Torvac Room at S.T.C. (Standard Telephone & Cables later ITT) in Paignton, Devon metalising mylar sheets with aluminium under hard vacuum. (The  grease that machine used was great for sea fishing reels!)
Then he'd slit the sheets up and they'd get rolled up, held with a bit of yellow tape before the terminals went in & they were compressed & potted. Red & green little caps ended up on my Nan's Xmas cakes. And the rolls of metalised mylar, all light and floaty, made brilliant Xmas decorations!
Plus, bigger rolls would get sent up on kites dangling for hundreds of feet... It wasn't until later that I realised I'd been flying chaff! It must've lit up the radars of the boats in the bay!
So yeah... Caps have  een a part of my history since the early seventies! Standard's Xmas trips for the kids of the workers.
It's not there anymore. Housing estate and industrial estates  now cover what was once the biggest employer in Torbay I think.
Amazing the charge those rolls of mylar & aluminium can hold!
Heh... Sorry mate, waffling. There's not many places that I wouldn't just get blank looks if I talked about this. I was diagnosed with Asperger's recently & I keep noticing stuff that fits. It's been quite the time for "Oh! No wonder they thought I was a bit strange sometimes! 😆

I'll have a bash at those photos tonight mate...
Cheers!