What value resistor?

[Daffy D]

I know I should be able to work this out for myself, but it's been too many years and it would take too long to try remember how these things work. So a little help from the kind people on this forum, who are always ready to help, would be most welcome.

 

Here's the thing. I've got a couple of these portable light/fan units which worked quite well till the rechargeable battery died.

 

The battery was an odd shape and was unable to find a replacement. There were no markings on the battery, no indication of Voltage not even plus and minus on the terminals. So with nothing to loose I got a small 6v gell battery, hacked out the inside to make room for it, did some basic rewiring and got it to work. 

 

Though it all seems to work the LEDs burn very bright and the fan spins like it's going to take off. As I said there were no markings on the old battery and there are no markings on the fan motor to indicate voltage, but it does seem like the 6V battery I put in is too much. 

 

Now to the point, what value resistor would I need to put between the battery and the motor to reduce the voltage?

Have no idea what voltage the LEDs and motor work at but I think something around 3 or 4 volts might work.

 

Thank you gentlemen for your kind attention.  

 

[RichCor]

Looks very much like this one:

 

https://yaoota.com/en-ke/product/generic-leadsmart-3-in-1-led-portable-rechargeable-battery-f-price-from-jumia-kenya

https://www.lelong.com.my/3-1-led-portable-rechargeable-battery-fan-light-eye-protection-desk-alisonfuture-F1586187-2007-01-Sale-I.htm

 

Specifications:- The power supply mode: Direct current.- Nominal voltage: 110 - 220V.- Rated power: 3W.- Frequency: 50 / 60Hz.- SNR: Less than 36dB.- LED luminous intensity: 1200 - 1400mcd.- Battery: 4V 2500mAh lead-acid battery.- Fan blade size: 4 inches. Body Color: Blue Is Bulbs Included: Yes Is Dimmable: No Light Source: LED Bulbs Material: PP/Plastic Power Source: Rechargeable Battery Style: Modern Package weight: 0.743 kg Product Size(L x W x H): 28.00 x 18.00 x 4.00 cm / 11.02 x 7.09 x 1.57 inches Package Size(L x W x H): 25.00 x 19.00 x 9.50 cm / 9.84 x 7.48 x 3.74 inches

 

...so, origianlly a 4v 2500mAh lead-acid battery

[northsouthdevide]

Didn't it give the voltage on the charger? 

Was it USB? 

May be 5v. 

[Daffy D]

2 hours ago, RichCor said:

origianlly a 4v 2500mAh lead-acid battery

Yes exactly like that   

 

How did you find it? World wide search?????

 

 I thought it might be about 4v.

 

So what value resister would I need to bring 6v down to 4v ? 

 

 

 

 

Edited August 21, 2020 by Daffy D

[Daffy D]

2 hours ago, northsouthdevide said:

Didn't it give the voltage on the charger? 

Was it USB? 

May be 5v. 

It has built in mains charger so no help there with output voltage  

[RichCor]

6 minutes ago, Daffy D said:

So what value resister would I need to bring 6v down to 4v ? 

First, how are you planning to 'charge' this new 6v battery pack?

If you simply put a resister inline with the new battery it will reduce voltage in both directions.

[Daffy D]

2 minutes ago, RichCor said:

First, how are you planning to 'charge' this new 6v battery pack?

If you simply put a resister inline with the new battery it will reduce voltage in both directions.

Actually I bypassed the internal charger, removed it altogether and fixed a couple of contacts directly from the battery to outside on the back to charge the battery from outside.

 

The resistor would only be in line from the battery to the motor and lights.

 

 

[impulse]

Get a rheostat (potentiometer, variable resistor) and adjust it as needed.  Then measure the resistance on the pot if you want to replace it with a fixed resistor.

 

It's impossible to calculate the resistor to drop from 6V to 4V unless you know the amperage draw.

 

Or power it with a rechargeable 3.7V 18650 battery (or 2 or 3 in parallel) and call it good.

 

https://www.lazada.co.th/products/18650-37v-battery-case-4-i1432048863-s3703918233.html

 

Edited August 21, 2020 by impulse

[RichCor]

https://ohmslawcalculator.com/voltage-divider-calculator

 

Try entering 

V_source = 6  [volts]

Resistance 1 (R₁) = 10 [ohms]  <- variable (you can change this value)

Resistance2 (R₂) = 20 [ohms]  <- variable (value will be auto-calculated)

V_out = 4  [volts]

 

Variable Part Value = what size you can source the resistors. You may need to put several in series on each side of the divider to get the values you want.

 

Just remember that the 'resistor' is going to be mini-heater wasting power (resistance --> heat)

Edited August 21, 2020 by RichCor

[impulse]

11 minutes ago, RichCor said:

https://ohmslawcalculator.com/voltage-divider-calculator

 

Try entering 

V_source = 6  [volts]

Resistance 1 (R₁) = 10 [ohms]  <- variable (you can change this value)

Resistance2 (R₂) = 20 [ohms]  <- variable (value will be auto-calculated)

V_out = 4  [volts]

 

Variable Part Value = what size you can source the resistors. You may need to put several in series on each side of the divider to get the values you want.

 

Just remember that the 'resistor' is going to be mini-heater wasting power (resistance --> heat)

 

That voltage divider calculation won't work because the load you add with the fan motor will add another resistance (and an inductor)  in parallel with R2.  And the parallel impedance added by the motor will not be fixed.  The motor impedance (that's resistance plus inductance plus capacitance) will vary with the load on the fan motor.  So you can't just measure the resistance of the motor directly while it's not spinning.

[Daffy D]

Unfortunately without any markings on the motor there is no way of knowing the "amperage draw" 

 

Using a rheostat is an option, but from what I can remember these also come in a range of values so would need an approximation value anyway.

 

The battery I have fitted is 6v2.8Ah/20HR which is a bit of overkill so doubt a little power loss through a resistor would have much effect on battery performance. 

 

[DrTuner]

1 hour ago, Daffy D said:

So what value resister would I need to bring 6v down to 4v ? 

 

Depends on the power input, ie. current. But you'll be losing battery charge due to it being converted into heat in the resistor. If you want to do it right, use a switching power supply to get the voltage down, f.ex: 

https://www.lazada.co.th/products/lm2596-dc-dc-adjustable-step-down-buck-converter-4v-40v-to-125v-35v-max-3a-dc-dc-board-i379378938-s738082391.html?spm=a2o4m.searchlist.list.4.2b35701ccMDPBl&search=1

Edited August 21, 2020 by DrTuner

[impulse]

Here's the cleanest way to do it if you don't want to go with 18650 batteries...

 

https://www.lazada.co.th/products/5a-xl4015-dc-dc-buck-converter-adjustable-step-down-module-438v-96-i256915845-s396007796.html

 

Edit:  Here's a better price, though this one's only rated at 2 amps, not 5 amps...

 

https://www.lazada.co.th/products/dcdc-buck-converter-step-down-4-35v-to-12-30v-output-voltage-i1286152197-s3154358111.html

Edited August 21, 2020 by impulse

[RichCor]

Just checking over what the "Internet" suggests...

 

• A linear voltage regulator
• A switch-mode voltage regulator
• A buck-boost converter
• A series resistor
• A series diode(s)
• A resistor in parallel with the battery (arguably not exactly elegant)

 

[DrTuner]

Putting a few diodes in series could do the trick with less loss. It's about 0.6-0.7v/diode, so three of those could work. If ýou have some old board where you can find throughhole diodes that look like orange glass tubes, solder them out and connect so the black ring in each points to same direction. Then try both ways before the fan. It might be a zero-baht solution.

 

They look like these:

https://www.lazada.co.th/products/1n4148-do-35-small-signal-fast-switching-diodes-100pcs-i204431762-s308793615.html?spm=a2o4m.searchlist.list.14.58063f8cFdL7EZ&search=1

 

Plus from battery connects to the side with no black ring.

Edited August 21, 2020 by DrTuner

[maxpower]

Resistor is a bad idea for a battery device as it will dissipate and waste power. You will also needs adequate power rating.

 

If you want to keep the lead acid battery place a cheap buck converter between battery and device to lower voltage more efficiently.

 

https://www.lazada.co.th/products/dc-dc-adjustable-step-down-lm2596hvs-buck-converter-45-53v-to-3v-40v-3a-power-supply-module-i326784220-s623574799.html?spm=a2o4m.searchlist.list.47.79417dd5tvv5u6&search=1

If you feel adventurous you could convert with a tidy solution using lithium 18650 battery and holder + USB charging module with battery protection built on. Please note lithium-ion cell must be charged and protected correctly. 

 

https://www.lazada.co.th/products/tp4056-micro-usb-5v-1a-lithium-battery-charger-with-protection-i326842022-s623784502.html?spm=a2o4m.searchlist.list.17.2423b7f3uWSzhq&search=1

 

https://www.lazada.co.th/products/18650-panasonic-liitokala-3400-mah-liitokala-1-p-i1103988743-s2514868108.html?spm=a2o4m.searchlist.list.13.1f9ffc19mYOM3z&search=1

 

[impulse]

Before you buy any 18650 batteries, watch this...

 

And remember..  In Thailand, just because it says Panasonic, doesn't mean it is.  If the price is too good to be true, it probably isn't genuine.  I learned that the expensive way when I bought a few dozen 18650 batteries over the years, mostly in Chinatown.  Some of them lasted only a few charges, or were dead after storing them for a few weeks.

 

 

 

Edited August 21, 2020 by impulse

[maxpower]

14 minutes ago, impulse said:

And remember..  In Thailand, just because it says Panasonic, doesn't mean it is.

Absolutely, which is why I provided link to a store that stocks genuine Panasonic 18650 cells.

 

https://www.lazada.co.th/shop/kaixin-store/?spm=a2o4m.pdp.seller.1.6e2a9Ot39Ot33n&itemId=1103988743&channelSource=pdp

 

[Damrongsak]

Humana, humana, Humany Ohms? (Sorry, I just can resist posting this picture.)

 

 

[Crossy]

Those little switching regulators linked to in Post #12 are the correct solution, no wasted energy like a resistor or linear regulator.

 

[Daffy D]

Thank you gentlemen for your input. 

 

Seems the way to go is with a Stepdown Buck Converter, but according  the links provided the input is 4v would that work with my 6v? and how does one adjust the required input  / output voltages?

 

"DrTurner" I do have a box full of old boards that might be worth a rummage.

 

"Maxpower"   I didn't want to go with lithium batteries, they don't seem to last very long. I already have a lead acid battery in place that should last some months on standby and give several hours of use per charge.

 

Google comes up with various complex ways of reducing 6v to 4v but seems by ignoring all the electronic gizmo's a 150ohms resistor might fit the bill ?

 

When I was a kid, before electronics and things became complicated, I remember having a large rheostat that weighed a couple of pounds that we used to dim the lights when showing home movies. That worked just fine without any of that fancy electronics stuff.

 

Also as I recall my train set had a rotary rheostat to control the speed of the engine.

 

Would there be any difference between the motor in my train set and the little fan motor. "Ye canna change the laws of physics"  

 

A rheostat is just a variable resistor so there must be a single resistor value that should work in my case.

 

Ah! Well things just seem to get more complicated with the years. 

[Crossy]

36 minutes ago, Daffy D said:

Seems the way to go is with a Stepdown Buck Converter, but according  the links provided the input is 4v would that work with my 6v? and how does one adjust the required input  / output voltages?

 

Input is 4V to 40V, 6V is in that range

 

Set the output to about 3.8V and you're good to go. Blue pot adjusts the output voltage.

 

 

[Daffy D]

9 minutes ago, Crossy said:

Input is 4V to 40V, 6V is in that range

OOPS !!!! 

 

Just checking you were paying attention 

 

[Crossy]

48 minutes ago, Daffy D said:

but seems by ignoring all the electronic gizmo's a 150ohms resistor might fit the bill ?

 

If the LEDs and fan draw 13mA then yes.

 

Not a likely scenario.

 

15 ohms would be nearer (130mA) but the little switching regulator is the correct solution.

[Daffy D]

11 minutes ago, Crossy said:

15 ohms would be nearer (130mA) but the little switching regulator is the correct solution.

OK I give up a switching regulator it is 

 

Cheers 

[Crossy]

10 minutes ago, Daffy D said:

OK I give up a switching regulator it is 

 

You know it makes sense.