Converting Action Figure/Toy with Leds from Button cell batteries to 5V usb wall plug help.

[FrankTank]

I cannot get to the guts of the leds inside the toy without destroying. It has several different locations with button cell batteries (4.5v total per). I want to calculate the resistors i need for each led and just run them all or individually to a 5v usb power plug. Can i still accurately test the current, forward voltage through the battery housing? I'm sure there is already resistors within the toy but cannot access it. There also might be multiple leds in some locations and not just single leds. Can I test this without tearing it open?

 

[Alec_t]

Welcome to EP!

It has several different locations with button cell batteries (4.5v total per).

The implication is that there are three 1.5V cells in series. Can you confirm?
How much current does the toy draw (you will need to disconnect one battery terminal to check with a test meter)?

I want to calculate the resistors i need for each led

Why, if you can't access the LEDs individually?

 

[FrankTank]

Welcome to EP!

The implication is that there are three 1.5V cells in series. Can you confirm?
How much current does the toy draw (you will need to disconnect one battery terminal to check with a test meter)?


Why, if you can't access the LEDs individually?

Yes the batteries are in series. I will need to test each location for current.

My understanding was to add resistors to each location and power each location individually with 5v usb? Or if i group them all together and add 1 bigger resistor and power them via 1 usb 5v power plug. That along with the testing each loaction is what brought me to the forum for some guidance.

Thank you)

 

[Alec_t]

if i group them all together and add 1 bigger resistor

If you mean put them in parallel, that will give them all the same voltage, hence all will need individual resistors or else they must be closely matched for forward voltage (Vf). Are they all the same colour?

 

[FrankTank]

If you mean put them in parallel, that will give them all the same voltage, hence all will need individual resistors or else they must be closely matched for forward voltage (Vf). Are they all the same colour?

They are all white leds with the number unkown.

I need guidance on the multimeter placement i tried with 3 AAA batteries in series and if i did it correctly it was 4.16 actual volts supplied and 1.86v coming out of the toy with a drop of 2.3V. and when i tested the mA i got a reading of 19.48 on the 20m setting and 23.4 on the 200m setting (that doesnt seem correct tho). I also tried it directly with the usb 5v adapter and got a 3.10v coming out of the toy and a 36.5 on the 20m setting.

I really dont think i am placing the probes correctly. Every example i have found is tested straight on the circuitboard. Can you walk me through probe placement for voltage drop testing?

Also could you walk though where to places the probes or measuring the current?

 

[Alec_t]

If 3 x AAA give only 4.16V they are practically dead.
20mA or thereabouts is the typical working current of a common-or-garden LED
Tricky to advise on probe placement when we have no idea what the toy or its internals look like .

 

[FrankTank]

If 3 x AAA give only 4.16V they are practically dead.
20mA or thereabouts is the typical working current of a common-or-garden LED
Tricky to advise on probe placement when we have no idea what the toy or its internals look like .

Yes the aaa's came out of headlamp i was using so i can understand that.

I could take a picture but it is a simply a typical button cell battery compartment insert 3 batterys together a spring on the positive side and flat contact on the negative. There are no internals to see that is why i was trying for guidance.

Every example i can find is only showing me exposed circuitry. I know there are resistors inside already but the unkown is the amount of individual resistors and amount of actuall leds (they are all hidden behind diffuesers. If want to power all 6 compartments in parallel safely i thought i need to add resistors.

 

[Alec_t]

Assuming a 20mA current draw, that everything at present is powered happily by those three cells in series and gets 4.5V with fresh cells, then it is unlikely that the toy would come to harm with a 5V USB supply instead. To be on the safe side you could insert a 1N400x diode (x is in the range 1-7) in series with the 5V. That will drop the voltage at the toy down to about 4.3V.
For the record, I have a domestic doorbell which is happy on a USB 5V supply (without that diode) instead of its original 3 x AAA battery pack.

 

[Audioguru]

Button cells cannot produce much current which limits the current since the current-limiting resistor is inside the button batteries.
If an external power supply is used then it MUST use a series current-limiting resistor.

 

[Alec_t]

Good point.