20v / 2A SMPS for a Battery Charger - Explanation of Blown Resistors Please..

[reessi]

Hello,

My friend asked me to replace the wire going from this SMPS 20v adapter to the charger for a 18v Lithium Battery for a drill.

The cable had frayed and been touching, as a result the 5 Paralleled R200 (0.2 ohm) resistors had blown O/C, they are connecting the main -ve rail to the output wire.

The output was jumping around 13v and dis-appeared when a load like the charger was connected, they measured 900ohms.


I experimented by replacing at first a single 0 ohm and then a 2.2ohm in and the output was restored at 21.62v and 21.65v respectively so negligible difference.

I know I can buy the correct ones but would like to understand what they are doing and why 0.2ohms was picked and whether I can just pop in 5 x 0 ohms or anything small like 5 x 2.2ohms in.

There is another single resistor coming off the negative wire to the feedback circuit I believe.

I know the test resistor in the picture is too small and would fit the correct size for the current when I replace them.


Many thanks.

[eccerr0r]

probably current limiting, and rather psu blow than battery blow...
Use the correct resistance.

[reessi]

Fair enough. I was thinking if it wasn't for a battery charger I would just fit what I had on hand.

Interestingly it is noted that although the adapter states 20v it outputs 21.65v. But for the 18v lithium battery to be charged correctly you would need 5 x 4.2v = 21v anyway. So I am assuming that voltage output is correct.

I have also just noted that in the charger end there is another bank of 5 x R300 resistors on the same rail with similar looking feedback.


To be fair I was thinking it was for the voltage feedback but the current makes more sense.

[reessi]

So another thing that doesn't make sense is that the block states 55w 2A and 21v * 2a is 42w at least. But these 5 SMD resistors cannot be at least 10W each!!!

So how can this be... When I look them up to buy, a lot are rated at 1 or 2 w.. they are labelled as specifically current sense resistors,

[reessi]

So I looked up some more datasheets and some are saying 20a constant current but only 2w power rating, can anyone explain this.. Cheers

[eccerr0r]

It's a resistor, power rating is how much power the resistor dissipates, not the load. When you talk power supplies you talk about input/output power consumption/generation, not of individual components. Individual components need to handle the current required to take wattage from input or pass the wattage to the output however.
Since it's only a fraction of an ohm, dissipation of the resistor is not very high -- unless you force the voltage across it higher...

[stj]

you need the correct resistance, it's not current limiting.
there is a tap from both sides going back into the circuits.
it's monitoring the volt-drop to determine the current being drawn and then either using it to regulate the current or to control a safety shutdown.

[CapLeaker]

you need the correct resistance, it's not current limiting.
there is a tap from both sides going back into the circuits.
it's monitoring the volt-drop to determine the current being drawn and then either using it to regulate the current or to control a safety shutdown.

It has to be 0.2 ohm resistors and as stj already said, it’s for current sensing and at the same time they act as a fuse as you found already out.

[reessi]

Many thanks for the explanation. I have ordered some 3w ones from Aliexpress.

[eccerr0r]

it's not current limiting.
it's monitoring the volt-drop to determine the current being drawn and then either using it to regulate the current or to control a safety shutdown.

Isn't this the definition of current limiting?

[harp]

So another thing that doesn't make sense is that the block states 55w 2A and 21v * 2a is 42w at least. But these 5 SMD resistors cannot be at least 10W each!!!

Voltage you put in calculation is differential voltage from one side current sense resistor and another side. Voltage is not too high if not heavy load, so for example, if you drow 1A and got 1V voltage drop over that resistor (eg. measured 21V - 20V) he will disipating 1W.

[sam_sam_sam]

Many thanks for the explanation. I have ordered some 3w ones from Aliexpress.

Be careful about upping the current capacity of a device the choice of components for current detection is made for a reason to protect the device from major damage from misuse or major malfunction of a component

Especially if it is use for battery charging or for BMS protection boards I personally would be very careful about how much more capacity you would want and still protect from major malfunctions