Forum Thread

Brain fart - "convert" 6VDC to ~ 3.7VDC

Dr. J 25,043 3,353 January 21, 2016 at 06:40 PM
I have a HF LED light that has a design flaw - it's a battery operated light that will not work when it's charging. too bad it only lasts a couple hours on battery - I need it to last much longer (plug in to 120VAC via a 6VDC transformer, included)

It charges with a 6VDC (500mA) charger. The battery puts out ~ 3.7VDC to the LED array. I don't know how much power/current the LED array uses itself.

My thought was to get a small DPDT toggle switch to install - it would "steer" the power from the DC transformer (120V plug in) to either the charging circuit (e.g. normal operation) OR to the LED array.

Yeah yeah I took circuits in HS and college but brain fart.... what resistors and whatnot do I need to wire in to get the 6V down to ~ 3.7V? Planning on stopping by RS tomorrow to grab the switch and could get the other parts too.

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#2
You wouldn't want to just use resistors to drop that voltage, doing it this way would require resistors with pretty high power ratings and waste a lot of electricity. You'd want to use a voltage regulator instead or build a small smps (buck most likely).

You wouldn't want to bypass all of the existing circuitry and connect directly to the array unless you know what you're doing. However, you could effectively simulate a battery, which is what I think you're actually trying to do.
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#3
Quote from Dr. J View Post :
I have a HF LED light that has a design flaw - it's a battery operated light that will not work when it's charging. too bad it only lasts a couple hours on battery - I need it to last much longer (plug in to 120VAC via a 6VDC transformer, included)

It charges with a 6VDC (500mA) charger. The battery puts out ~ 3.7VDC to the LED array. I don't know how much power/current the LED array uses itself.

My thought was to get a small DPDT toggle switch to install - it would "steer" the power from the DC transformer (120V plug in) to either the charging circuit (e.g. normal operation) OR to the LED array.

Yeah yeah I took circuits in HS and college but brain fart.... what resistors and whatnot do I need to wire in to get the 6V down to ~ 3.7V? Planning on stopping by RS tomorrow to grab the switch and could get the other parts too.
Maybe 2 LEDS in series or go to the junk cell phone store and dig around in the $1.00 parts bin [We have 2 like that here] and find a lower voltage power supply.
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#4
Hook a current meter in series between the battery and circuitry to measure how much power the leds use.

If you can post a few pics of the circuitry/light, it might be useful.
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#5
Quote from dale_101798 View Post :
Maybe 2 LEDS in series or go to the junk cell phone store and dig around in the $1.00 parts bin [We have 2 like that here] and find a lower voltage power supply.

I'd REALLY like to use the existing 6V supply. My idea is to do all the wiring internally and just have the new DPDT switch out. One position - functions just like from the factory, charges (but no light) and battery works as stated, including charging. Second position - power directly from the plug. Only issue is I need to knock it down from 6V to ~ 3.7V.
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#6
Quote from Dr. J View Post :
I'd REALLY like to use the existing 6V supply. My idea is to do all the wiring internally and just have the new DPDT switch out. One position - functions just like from the factory, charges (but no light) and battery works as stated, including charging. Second position - power directly from the plug. Only issue is I need to knock it down from 6V to ~ 3.7V.
You could either replace the entire charging circuit or attempt to add a DPDT switch that switches between the battery and a small circuit consisting of a voltage regulator, a couple resistors and capacitors and connects to the 6v supply. Radio shack sells the LM-317 adjustable voltage regulator, you need a new resistors and capacitors to get it to work. You'd need soldering tools and some perf board. See: http://www.ti.com/lit/ds/symlink/lm117.pdf

It gets a little tricky depending on the design of the circuit there could be other challenges.
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Last edited by jkee January 21, 2016 at 07:34 PM
#7
Quote from Dr. J View Post :
I'd REALLY like to use the existing 6V supply. My idea is to do all the wiring internally and just have the new DPDT switch out. One position - functions just like from the factory, charges (but no light) and battery works as stated, including charging. Second position - power directly from the plug. Only issue is I need to knock it down from 6V to ~ 3.7V.
I found a calculator and instructions [daycounter.com] you can use to determine the voltage drop requirement. With this you can figure out the correct resistance of your resistor. I believe we can safely assume that a 1/4 watt resister would be plenty heavy enough for 1 LED at your voltage.

Good luc
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#8
Quote from dale_101798 View Post :
I found a calculator and instructions [daycounter.com] you can use to determine the voltage drop requirement. With this you can figure out the correct resistance of your resistor. I believe we can safely assume that a 1/4 watt resister would be plenty heavy enough for 1 LED at your voltage.

Good luckSmilie
We aren't talking about 1 LED. We're talking about a light with with multiple leds, most likely this one with 36 leds: http://www.harborfreight.com/2-in...o-top-link
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#9
Quote from jkee View Post :
We aren't talking about 1 LED. We're talking about a light with with multiple leds, most likely this one with 36 leds: http://www.harborfreight.com/2-in...o-top-link

Nope, 120 [harborfreight.com] laugh out loud

Problem is I don't exactly know the current the thing uses (that is, the actual bulbs). Still trying to figure out how I can estimate that. It says it's meant for under-hood/auto use, but only 2 hours on batteries is pretty lame for that application.
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Last edited by Dr. J January 22, 2016 at 03:23 AM
#10
Quote from Dr. J View Post :
Nope, 120 [harborfreight.com] laugh out loud

Problem is I don't exactly know the current the thing uses (that is, the actual bulbs). Still trying to figure out how I can estimate that. It says it's meant for under-hood/auto use, but only 2 hours on batteries is pretty lame for that application.
That link would have been very helpful at the beginning of this thread.
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#11
I have that exact one - while I didn't replace the charging unit, the battery is an 18650 - you can relatively easily take out it's existing one (2000mah) and replace it with a 3400mah unit (don't buy anything rated higher, it's over-rated junk), essentially doubling it's useful lifetime.

The thing is those batteries at full charge will be at 4.2v, and at empty will be at 3v, so a simply 3.7v charger wouldn't work well, and the LEDs need a more precise current through them so they don't burn out, so that's why I didn't replace the existing circuitry (but yes, I agree that it's a design flaw that you can't charge and operate it at the same time - but also it was cheap, so I got what I paid for out of that one)
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#12
Quote from slapshot136 View Post :
I have that exact one - while I didn't replace the charging unit, the battery is an 18650 - you can relatively easily take out it's existing one (2000mah) and replace it with a 3400mah unit (don't buy anything rated higher, it's over-rated junk), essentially doubling it's useful lifetime.

The thing is those batteries at full charge will be at 4.2v, and at empty will be at 3v, so a simply 3.7v charger wouldn't work well, and the LEDs need a more precise current through them so they don't burn out, so that's why I didn't replace the existing circuitry (but yes, I agree that it's a design flaw that you can't charge and operate it at the same time - but also it was cheap, so I got what I paid for out of that one)
I had my suspicions that it might have an 18650 there, running at 3.7V.
I know that would be my preferred solution, just making the battery swappable, and having a decent 2/4 bay charger. I've had my eye on this Nitecore [amazon.com] unit, since my existing battery charger has stopped charging one of the bays.
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#13
Quote from slapshot136 View Post :
I have that exact one - while I didn't replace the charging unit, the battery is an 18650 - you can relatively easily take out it's existing one (2000mah) and replace it with a 3400mah unit (don't buy anything rated higher, it's over-rated junk), essentially doubling it's useful lifetime.

The thing is those batteries at full charge will be at 4.2v, and at empty will be at 3v, so a simply 3.7v charger wouldn't work well, and the LEDs need a more precise current through them so they don't burn out, so that's why I didn't replace the existing circuitry (but yes, I agree that it's a design flaw that you can't charge and operate it at the same time - but also it was cheap, so I got what I paid for out of that one)
Upgrading the battery is a good option. Trying to modify the circuitry in something like this is tricky, you often don't have much space to work with. if you wanted to replace the charging circuit entirely, you'd probably have to make a custom PCB.

From your numbers, we can approximate that the led light draws 1W of power. I can also say with absolute confidence that using a voltage divider or a couple diodes to drop the voltage wouldn't work. A voltage divider would require resistors rated >1W and waste a lot of power, possibly more than the power supply can output.
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#14
A white LED drops about 3.5 volts, so if it consumes 3 watts that's almost1 amp, but I don't think such an LED would be connected to a 3.7V battery through just a resistor because that wouldn't regulate the brightness very well. More likely the spotlight uses a constant current switching regulator or has several small LEDs strung in parallel (separate resistor to each one) or series (would also require switching regulator).
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