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Basic checks on charger, transformer. How to

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stan smythe

New Member
Trying to resolve some issues, all similar so posted all three here rather than separate.

1. Charger for mobility scooter, I am trying to source a fuse holder +fuse but none of the mobility companies seem willing to supply one.
Any ides for a source? (image 2)

2. 2nd Mobility charger, fuse in plug ok but when I use a multimeter where the mains wires attach to the circuit board I get no reading,
Cant see any break in the cable, I also tried continuity from mains plug to output lead but again no reading. Not too bothered about this one as I can just bin it but thought to check if doing this properly as it could replace the above charger.

3. Transformer for an old karaoke machine tested continuity from mains plug to output and no reading so looking to replace the transformer, I find it difficult to get an exact match re output. What parameters would be ok for a replacement eg would 1.5A be ok a match for the 1.2a Input or must it be exact. (Image 1)
fuse.jpg


trans.jpg
 

JLNY

Active Member
1. The fuse holder looks like a fairly generic screw-in cap kind. you might try just buying a generic fuse holder and seeing if you can take the cap off and thread it in correctly where the old cap was. You'll want to check the current value of the fuse from your one with an intact fuse holder to figure out what value fuse to replace the other one with. My guess is that it is a fairly regular 5x20mm size fuse, but you'll need to figure out what fuse current you need.

2/3. I'm not sure I undertstand what you are testing? How exactly are you checking the plug? If you are trying to measure continuity across the plug pins, modern Switch-Mode Power Supplies (SMPS) will probably read open-circuit on a multimeter. It might actually still work fine. A better way to check might be to try measuring if there is voltage at the output on the other side when it is plugged in. Alternatively, if you haven't already, you might simply plug it into whatever you are using and see if it charges/power up...

As for using a 1.5A power supply, the important things to read are the "output" characteristics. It doesn't matter as much what the input current is; the main things to keep in mind when replacing any power supply like this are output voltage, output current, and output polarity. The voltage must be the same voltage as the one you are replacing (in this case 12V DC). The output current rating must be the same or higher. Lastly, you will want to check whether the output is "center-positive" or "center-negative." These days, center-negative supplies are pretty uncommon, but using the wrong kind can blow up whatever you are using if you aren't careful. In your case, the PSU in the image is center-positive, so you will want to replace it with the same. If the symbol shows the dot in the middle as being connected to a minus sign and the "C" shape as connected to the plus sign, that means it's center-negative and can't be used. If the plus is connected to the dot and the minus to the "C," it is center-positive and you're fine.

trans.png
 
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Les Jones

Well-Known Member
Most Helpful Member
You should not expect to see any continuity ( I.E the resitance reading should be infinite.) between the mains input and the output of any power supplies. (Either 50 / 60 hz transformer type or switch mode.) For a power supply using a normal 60/60 hz transformer you should see some continuity between live and neutral but the resistace reading will vary with the size of the transformer. With a switch mode power supply you would probably get a very high or infinite reading between live and neutral. Your fuse will probably be a 20mm fuse or an 1 1/4" fuse. (This is the length of the fuse itself.) Here is a link to some panel fuse holders that may meet your requirements. I think the power supplies in your pictures are all switch mode types.

Les.
 

stan smythe

New Member
Many thanks for both replies as they have been very useful explanations for me.

2/3. I'm not sure I undertstand what you are testing? How exactly are you checking the plug? If you are trying to measure continuity across the plug pins, modern Switch-Mode Power Supplies (SMPS) will probably read open-circuit on a multimeter. It might actually still work fine. A better way to check might be to try measuring if there is voltage at the output on the other side when it is plugged in. Alternatively, if you haven't already, you might simply plug it into whatever you are using and see if it charges/power up...
I checked the plug fuse..ok, I checked for output voltage..nil, I tried using to power up...nothing. The other check I tried was as the main power lead is not detachable I checked for where it is attached to the circuit board when plugged in and couldnt get a voltage reading (expecting 240v)
 

Les Jones

Well-Known Member
Most Helpful Member
Switch mode power supplies can be quite difficult (And dangerous if you do not fully understand how they work.) to fault find on. It will probably be difficult to obtain a schematic so to get a rough idea about the design I suggest that you identify the switch mode control chip then try to obtain the data sheet on that device. The data sheet normally gives examples of circuits they are likely to be used in. One other problem is when a component fails in these power supplies it very often takes out other components. So if you miss replacing one of the failed components the new components that you have just fitted can be destroyed when powered on. If you do not have a good knowledge of electronics (Which your initial questions suggest.) then it would be better to start on something much less complex. If the only fault is a fauly fuse holder then that should be straightforward to replace. You may find a suitable fuse holder on ebay. Do a search for "panel fuse holder".

Les.
 
Last edited:

stan smythe

New Member
Switch mode power supplies can be quite difficult (And dangerous if you do not fully understand how they work.) to fault find on. It will probably be difficult to obtain a schematic so to get a rough idea about the design I suggest that you identify the switch mode control chip then try to obtain the data sheet on that device. The data sheet normally gives examples of circuits they are likely to be used in. One other problem is when a component fails in these power supplies it very often takes out other components. So if you miss replacing one of the failed components the new components that you have just fitted can be destroyed when powered on. If you do not have a good knowledge of electronics (Which your initial questions suggest.) then it would be better to start on something much less complex. If the only fault is a fauly fuse holder then that should be straightforward to replace. You may find a suitable fuse holder on ebay. Do a search for "panel fuse holder".

Les.
Les, you are quite correct, my `expertise` is in the mechanical side of things. I am comfortable with house electrics and in electronics at the input & output for checking, all the bits in between I leave to the experts unless very basic. I wont go much beyond getting a replacement fuse/holder for the charger.
Thanks for your input.
 

JLNY

Active Member
Yes, I am with Les on that. Unless you spot something obviously wrong like a burnt component or bad solder joint on the board, I would not bother with trying to debug the power supply at the component level, and even then, repairs like that often turn out to be more complex than they seem in the event of a cascading failure of multiple components. I suppose I should also throw in the obligatory safety disclaimer of "work on live mains circuits at your own risk," but if you say you have experience with electrical work I won't lecture you on it.

Actually, If the other charger with the working fuse holder is not working, why not just take the fuse holder cap from the broken one and try it in the one missing the fuse holder in the mean time?

Have you checked the output voltage of the 12V power supply or was this just the 24V battery charger?
 

stan smythe

New Member
Actually, If the other charger with the working fuse holder is not working, why not just take the fuse holder cap from the broken one and try it in the one missing the fuse holder in the mean time?

Have you checked the output voltage of the 12V power supply or was this just the 24V battery charger?
Unfortunately one charger is not fused at output so cannot swap, it only has a fuse in the mains plug. Something I may have missed is that the scooter has 2 x 12 V in series for a 24V drive output, I know it needs new batteries as one reads 4V and the other 10V, is it possible the charger will not activate if the batteries are dead.

To let you understand Father in law (now deceased) had this scooter and other manual mobility stuff which I have donated to the local community hospital and sheltered home. The scooter I want to get working and sell at just above what it costs me to get it going so that some person can gain from it at a cost hopefully at £200 cheaper than the going rate, As it stands the batteries will cost me around £100, a new charger around £30. the going rate for 2nd hand scooter is approx £300 so I am looking to sell at £150 if I get it going. FIL did have the batteries checked and was advised they need replacing but he wouldn't pay the cost for them.
 

JLNY

Active Member
Unless I'm misunderstanding, I'm suggesting that you swap the output fuse holder into the one without the fuse holder on the output and see if it works. It's just missing the cap and the fuse itself, right? so shouldn't it be fairly simple to take the cap off the one and put it on the other?

If not, looking at pictures online of that type of charger, I wonder if it might be possible to swap for the caps from a generic fuse holder like this one:
https://www.amazon.co.uk/Panel-Moun...8&qid=1516309015&sr=8-18&keywords=fuse+holder

Hmm, that is also a worthwhile bit of context. Doing board repairs makes even less sense knowing you will be giving this item to someone else.

4V on a lead acid battery sounds very wrong. Even 10V is pretty poor. I'm guessing that they are sealed batteries and not the kind that need refilling by the user? Sounds like they are probably shot, and you may be right that the charger is not detecting the cell properly due to the low voltage.
 

stan smythe

New Member
Unless I'm misunderstanding, I'm suggesting that you swap the output fuse holder into the one without the fuse holder on the output and see if it works. It's just missing the cap and the fuse itself, right? so shouldn't it be fairly simple to take the cap off the one and put it on the other?
In original state only one charger has a fuse at the output end so there is nothing to swap.

4V on a lead acid battery sounds very wrong. Even 10V is pretty poor. I'm guessing that they are sealed batteries and not the kind that need refilling by the user? Sounds like they are probably shot, and you may be right that the charger is not detecting the cell properly due to the low voltage.
Batteries will definitely need to be replaced unfortunately.

Image of the other charger without fuse:

2nd charger.jpg
 

JLNY

Active Member
Ah, I see. I was under the impression that the two chargers were the same model. My apologies.

As for the charger board, well... nothing immediately jumps out at me that might be amiss. As has been said, it's probably not worth messing with unless you are down to do some serious, in-depth debugging on a board like that.

Glancing over the layout on the board, the small 8-pin IC to the right of the transformer in the middle is probably your SMPS driver chip if you want to look up the part number for the datasheet, although this looks to be a double-sided board so there may be additional circuitry on the back. The other IC on the secondary side is likely a microcontroller or perhaps a dedicated chip for charging lead-acid batteries, so there is likely some degree of intelligence for detecting the battery voltage and switching the output on and off and/or managing the charging current. The power transistors/regulators clamped to the side of the case are likely for regulating the 24V output, possibly with one of them being used to regulate a separate lower voltage supply like 5V for powering the internal digital circuitry. Not sure what all those trimmers are for, but they might control certain voltage thresholds or fine-tune the charging currents, etc. I wouldn't mess with them, as they were probably factory-calibrated for this unit and likely don't need adjustment.

Unless anyone else has suggestions, I would suggest waiting until replacement batteries arrive and/or try ordering a fuse holder cap/fuses before having a look at the other charger.
 
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