Some electronics help needed please

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G0OSE
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Some electronics help needed please

Hi guys, no matter how I try to understand electronics, I just can’t seem to.
I have a little project…..
So I am using a little camping lantern to illuminate an item, at the moment it uses 3xaa (4.5v) batteries, so I bought a little usb power board planning to wire it up inside and not have to use batteries. The power supply is a usb multi one, which apparently senses how many amps the item needs?
The reason for this (I have rechargeable batteries) is that when I use regular alkaline AA’s it keeps a pretty constant brightness, then dies after time, few days – but when I use the rechargeables it it significantly brighter (which is preferred over the alkalines which is duller), but drops a lot quicker to a pretty dull level of brightness, then when you leave it off overnight it does the same thing, bright at first then dull after a few hours. So Alkalines do a better job for the purpose keeping a steady light level, but I am not keen on putting set after set of disposables in it.
So, I thought putting the usb board in would cure my woes, albeit being 0.5v over 5v instead of 4.5v.
I opened it up today to do the job, then noticed a resistor in line which drops it to 3.3v from 4.5v (battery) so the 5v usb borad isn’t going to work as planned.
Ampage the lamp uses I have no idea, am hoping to check that if my friend has a meter, but seeing as it’s coming from aa batteries I guess it’s pretty low. The nimh ones obviously give more ampage than the alkalines as it like twice as bright, well at first anyway.
I guess the amps would need controlling too? or does it just take what it needs? (that would be from the usb power board) Any help would be appreciated.
The solution doesn’t have to be a usb one, it could be an external plug in one, but usb would be more convenient for where it is placed.
This is the lamp (well almost the same)

and this is the little board I bought

This is the lamp with fresh nimh’s
IMG20211125133017

Edited by: G0OSE on 11/25/2021 - 08:34
YuvalS
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Not sure is that what you are after but you can use a protected lithium cell (18650 for example) it is almost the same voltage 
1 lithum cell 3.7-4.1V
3 alkaline: 3*1.5 = 4.5V
3 mimh: 3*1.2 = 3.6V   
  

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thefreeman
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These 3xAA powered lights are usually direct driven, using a resistor to limit the current, alkaline cells also have very high internal resistance (like more than 1Ω) which added to the resistor limits further the current. NimH AA cells have much lower DCIR (~50mΩ) so even if the voltage at rest is lower the voltage under load is higher, more current, shooter runtime.

Anyhow what you would need to do is increase the resistor value since with the USB PSU you have a constant 5V. Measure the Vf of the LED with alkalines (if that’s a suitable brightness) and calculate the new resistor :

Rnew = (5-Vf)/I
With I = VRold/Rold

Check that the current is lower than the USB PSU rating, and power in the resistor lower than its rating too.

G0OSE
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Thanks Yuval, but I wanted to get away from batteries entirely – that said I did fancy putting a dimmable maybe anduril driver in there so I could dim it to the level I like, and also have the flashing effect lightening etc, but couldn’t source a board anywhere. That would mean a lot more hassle, new switch etc
thefreeman soloution seems better for my needs, and I guessed the reisitor value would need changing, but without know how many amps it’s drawing with the nimhs, I can’t work it out lol. When my mate pops round I’ll get him to help me measure it as he’s more electronic minded than me. I have a multimeter, so it should be doable, even if we need to unsolder a lead to do so, which I think is what will need doing (in series)

MtnDon
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Maybe a charger board like this

USB-C input and 4.2 volts output with UP to 1000 mA output. I have not tried using one to power a led light directly but have thought about it. I could connect up a trial with one to a driver and led but not until 24 to 36 hours from now.

thefreeman
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G0OSE wrote:
thefreeman soloution seems better for my needs, and I guessed the reisitor value would need changing, but without know how many amps it’s drawing with the nimhs, I can’t work it out lol. When my mate pops round I’ll get him to help me measure it as he’s more electronic minded than me. I have a multimeter, so it should be doable, even if we need to unsolder a lead to do so, which I think is what will need doing (in series)

No you can just measure the voltage across the resistor (VRold), and if you know the resistor value (code or measurement, though DMMs are bad at measuring low resistance values), you can calculate the current.
Series measurements with a DMM is not very accurate because it adds resistance to the circuit.

G0OSE
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That looks good Don, I doubt it’s drawing much more than 1A as there is no real heatsinking inside barring a very thin aluminium led board (standard cheap one)
The board I have gives no data, so i can’t tell you anything about it (amps wise) other than it’s a power board…useless I know.
Still before deciding I have to find out for sure how many amps it is drawing, or I could end up doing it all then being disappointed lol!

G0OSE
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thefreeman wrote:
G0OSE wrote:
thefreeman soloution seems better for my needs, and I guessed the reisitor value would need changing, but without know how many amps it’s drawing with the nimhs, I can’t work it out lol. When my mate pops round I’ll get him to help me measure it as he’s more electronic minded than me. I have a multimeter, so it should be doable, even if we need to unsolder a lead to do so, which I think is what will need doing (in series)

No you can just measure the voltage across the resistor (VRold), and if you know the resistor value (code or measurement, though DMMs are bad at measuring low resistance values), you can calculate the current.
Series measurements with a DMM is not very accurate because it adds resistance to the circuit.


Thevoltage across the resistor seems to be 0.72v
anyway, here is a pic of the resisitor if it helps. red yellow gold gold
IMG20211125140243
thefreeman
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Don’t desolder, turn on the light and measure the voltage across the resistor.
Then the current is V/2.4

G0OSE
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thefreeman wrote:
Don’t desolder, turn on the light and measure the voltage across the resistor.
Then the current is V/2.4

0.72v if I’m doing that correct – does that / mean ‘x’? so 1.728 amps?
If so then the nimhs give 3.6v and they are supplying 1.728 amps – is that right?
sorry for sounding so dumb, but like I said I know nothing about electronics or it’s symbols lol
kennybobby
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Or you could just add a diode between the usb board and the + terminal of the light to drop the 5V down to 4.5.

Now i used to think that i was cool,
drivin' around on fossil fuel,
until i saw what i was doin',
was drivin' down the road to ruin. --JT

G0OSE
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kennybobby wrote:
Or you could just add a diode between the usb board and the + terminal of the light to drop the 5V down to 4.5.

The LED is likely a 3.3V device and the resistor just limits the current, in this example it would be using a 33 Ohm resistor for a 40mA current. A picture of the resistor or the board inside the light would help.


The photo is above, there is no board.
I’m thinking I’m taking on more than I can understand, my head is already hurting lol, I know for those of you who understand it , it looks simple but for me, well, I’m lost lol!
If my calculations are in fact correct, I could buy a 4.5v or 3.5v 2 amp power supply (plug in type, not usb) and wire it right in?
would this – power supply be what I need?
thefreeman
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G0OSE wrote:
thefreeman wrote:
Don’t desolder, turn on the light and measure the voltage across the resistor. Then the current is V/2.4
0.72v if I’m doing that correct – does that / mean ‘x’? so 1.728 amps? If so then the nimhs give 3.6v and they are supplying 1.728 amps – is that right? sorry for sounding so dumb, but like I said I know nothing about electronics or it’s symbols lol

0.72/2.4 (the color code says 2.4Ω) = 0.3A, that’s your current.

Now measure the voltage across the LED (Vf), which should be arround 3V anyway, and you can calculate the new resistor :
Rnew = (5-Vf)/0.3
If Vf = 3V you would need 6.67Ω, 6.8Ω for an existing value.

G0OSE
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The voltage across the led is 3.23v
so that being correct, I need to order a 0.68 ohm resistor, solder that in series off the ‘+’ of the usb power board and it should be good to go?
Finally, if that is all correct, is 0.68 ohm resistor what i need?
Thankyou for all your help by the way Thumbs Up

kennybobby
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Better get a 6.2 Ohm, your maths are not correct.

Or a 6.8 Ohm, 1 Watt power rating resistor to replace the 2.4 that is in the lamp. Unsolder the old one and put in this new valued resistor.

The resistors in your link are only 1/4 Watt (0.25 Watts), you need higher power rating.

Now i used to think that i was cool,
drivin' around on fossil fuel,
until i saw what i was doin',
was drivin' down the road to ruin. --JT

thefreeman
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I don’t know how you end up with 0.68Ω,  (5-3.23)/0.3 = 5.9Ω

And with P = R x I2 , P = 5.9 x 0.32 = 0.53W, better use a 1W rated resistor.

 

G0OSE
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Sorry for my ignorance guys – I have no idea what your symbols (I don’t understand any of them) or working out mean, or how they work. I was taking the 6.8 ohm bit from the answer abover when freeman said ‘Now measure the voltage across the LED (Vf), which should be arround 3V anyway, and you can calculate the new resistor :Rnew = (5-Vf)/0.3If Vf = 3V you would need 6.67Ω, 6.8Ω for an existing value.’

So, I need a 1w resistor with 5.9 or 6.2 ohm rating which is it please? or is the difference so negligable it doesn’t matter?
Better still if you could link one off ebay uk, that would be an immense help. I for the life of me can’t find one on there. I can find 1w ones, but no 5.9 ohm ones

edit -
I have found this 6.2 ohm 1 watt – is this the one I need to order? – link

leestrong
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Why is the USB not gonna work? Just place that resistor in series with the LEDs? Alkaline batteries are 1.6V at full charge, I think 0.2V more isn’t gonna burn the LEDs out.

Perhaps you can try running it on USB for a few seconds, if it is not abnormally bright then it should be fine.

Or perhaps, you can get a constant current driver and adjust the brightness to your preference.

https://m.aliexpress.com/item/32842135081.html

Hoop
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P = power (watts), R = resistance (ohms), I = intensity (current). How it works, well, it’s a math equation. Plug in the values and solve.

kennybobby
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G0OSE wrote:
… I have found this 6.2 ohm 1 watt – is this the one I need to order? – link

Yes good find, that would work.

Now i used to think that i was cool,
drivin' around on fossil fuel,
until i saw what i was doin',
was drivin' down the road to ruin. --JT

G0OSE
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Thankyou all very much for your help everyone, it’s ordered and I’ll show you a photo when it’s all wired in!

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chops728
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Are you going to use a USB charger that’s plugged into a wall socket for your power source — My wife had several of these branches with LED lights that were driven by 3 AAA batteries — All I did was use some old phone chargers that were .75 amp / 4.2 volt — I wired them directly and now they just plug into a wall socket —- I left the original box in tact with the ON/Off timer button — works great
Any little power supply that has a output voltage close to what you need will work