Help please Charger problem

[Fishtankjohn]

Hi
Firstly great forum
I wondered if anyone can help me with a problem

I have a cap lamp charger
Which will not charge

It's 240v the battery is 3.7 v
I have tested the transformer which is giving less the 1 v output
What would be the best way to sort this problem

Is there anywhere I could get another transformer?
Or is there another way to sort
Could I use a 12v battery charger and a dc/dc 12v stepdown to 3.7v
Thanks in advance
John

 

[Gryd3]

You could disconnect the leads from the transformer to the circuit board and measure resistance across the leads. This should tell you if the secondary is open/short circuit.
Do this with the plug side too.
Tell us the numbers, this will confirm possible damage to the transformer.
(Just to be sure, you measured the output with the AC setting on your meter, correct?)

 

[Fishtankjohn]

Thanks for the reply
We removed the wires from the transformer to the circuit board
The meter when the leads are crossed reads .2
The resistance on the transformer was .2
The resistance on the plug was .2
The meter we are using is old so it might be a point or two out
John

 

[Gryd3]

Change your meter to Rx10 and give it another try.
Your current reading make it sound like both sides of the transformer shorted out...

 

[Fishtankjohn]

Hi
Just tried it on rx10
I got .5 on the leads to transformer
And 5 across the plug

John

 

[Gryd3]

Thank you Fishtankjohn.
The transformer you have should have close to a 50:1 turn ratio to drop the 240V down close enough to 3.7V for your charger.

The resistance on one side should be much higher than the other, neither side should show 0. If it does, adjust the range on your meter with the dial on the left hand side to see what you can get.

To confirm, when you measured the circuit board side of the circuit, you had your meter set to AC exactly as shown in your second picture above?
Note the range on your meter. (300V) If the range is set too high, values like 3V or 5V will be almost impossible to actually measure. If you are measuring the output side of the transformer, and the voltage looks really really tiny, change the AC voltage setting on the meter lower (to 50V) and re-take your measurement. If the output is still really really low, repeat the process at 0.10V and try again.

I want to make absolutely sure that the transformer is dead before I recommend buying a new one.

 

[Fishtankjohn]

I tried the voltage ac again on 50
It gave a reading of about 85v see pic
Thanks
John

 

[KrisBlueNZ]

It's normal for the secondary of a power transformer to measure less than 1Ω. I think the problem is that you measure less than 1Ω across the points that the transformer connects into. This probably means that the bridge rectifier (assuming that it uses a bridge) is short circuit.

 

[Fishtankjohn]

Oh
Which would be the bridge rectifier on the circuit board
Pic enclosed
If it is that can I get a replacement ?
Thanks
John

 

[Fishtankjohn]

Just had a look at circuit board
Near the 2 ac1 and ac2 contacts
There is a small black component with br at the side of it
Is there a way of testing it
And can you bur replacements
Thanks

 

[Fishtankjohn]

Pic of br1
John

 

[Gryd3]

Can you please snap a pic of the bottom of the board?
BR1 is probably the bridge rectifier, and yes, it can be tested.
We can use the next picture to show you how.

As far as your AC voltage reading is concerned. Because you had it set to the AC 50, you need to use the set of numbers that relate to the range you picked.
The voltage you actually read looked to me to be 17-18V unless Im having a brain fart.

oh... and BR1 is replaceable and should be cheaper than a transformer
The tricky part is making sure nothing else on the board is damaged, as sometimes the bad part we find is killed by another bad part on the board..

 

[Fishtankjohn]

Attached pic of board
Also the number on the rectifier is. Kbp02
Thanks

 

[KrisBlueNZ]

http://www.digikey.com/product-detail/en/KBP02M-E4/51/KBP02M-E4/51GI-ND/2152670

 

[Gryd3]

That part is less than a buck, but you should do another test first:

Set you meter to Rx10
I put up a picture, with the transformer disconnected, put the red probe on the metal pad I marked with a +.
Test each of the AC1 and AC2 with the black probe.
This time put the black lead on the metal pad I marked with a +, and test AC1 and AC2 again with the red probe

Give us the numbers please:
Black on +
Red on AC1 =
Red on AC2 =

Red on +
Black on AC1 =
Black on AC2 =

Black on -
Red on AC1 =
Red on AC2 =

Red on -
Black on AC1 =
Black on AC2 =

We are only expecting two numbers: Really high, and really low(or 0)
Kris may have a better method, but this is how I would test BR1.
(Or you could set your meter to DC 50 Volts, connect the transformer, plug it in and measure the two pads I marked with a + and -)

 

[Fishtankjohn]

Thanks
I will do that test tomorrow has it now 11.20 pm (uk)
I have seen something on ebay pic enclosed
Would that be the same one
Regards
John

 

[Gryd3]

Sounds good.
It's 3:35pm here, so I got time to burn.

Aside from eBay, you can also check out these suppliers.
https://www.electronicspoint.com/resources/component-suppliers.34/
Look under the UK section.

 

[KrisBlueNZ]

Kris may have a better method, but this is how I would test BR1.

If he's measuring a short between the two terminals that the transformer secondary connects onto, the bridge must be shorted. There's no need to measure the diodes individually.

I have seen something on ebay pic enclosed. Would that be the same one

Yes, that one looks good.

 

[Gryd3]

If he's measuring a short between the two terminals that the transformer secondary connects onto, the bridge must be shorted. There's no need to measure the diodes individually.

I don't recall him actually measuring the terminals on the board... perhaps I misunderstood :

Thanks for the reply
We removed the wires from the transformer to the circuit board
The meter when the leads are crossed reads .2
The resistance on the transformer was .2
The resistance on the plug was .2
The meter we are using is old so it might be a point or two out
John

I read this as removing the transformer, testing the meter by shorting the probes together, then measuring the LV side of the transformer then HV side.
Sometimes the smallest term or word can throw that off though. Perhaps the Op can clarify if he has measured the resistance across the connections on the board from where the transformer used to connect.*

I am still interested in getting a VDC measurement from that capacitor though, but we will see when the op returns.

*Note for OP: Has this been done?

 

[KrisBlueNZ]

Yes, I had assumed that "resistance on the plug" meant the resistance between the two spade lugs on the board that the wires from the transformer plug onto. You're right that it's not clear.