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I didn't want to highjack a companion led discussion thread, so started this one 

I have this led lighted xmas tree that runs off (3)AA batteries or has a barrel connector which I could wire to a 10vdc buss with the rest of my accessories. I choose not to use another power source if a $.05-$.10 component can be installed(somewhere) which would reduce the dc voltage to close to 4.5 so as not to over power the xmas tree leds. Or, will the tree stand that higher voltage for an extended period of time?  If I haven't asked the right question or provided necessary details, let me know. Thx in advance for the help. Ted

Last edited by TedW
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Stan you're responsible for the buss(terminal strip?). I power it from the acc side of the xfrmr to an ac/dc buck converter set to 10vdc. I have a flickering 5mm led and a couple 3mm leds in a shack then two sets of three on a strip. Those lights like the 10vdc. Plenty of headroom for the tiny leds on the tree I think.  10" tall tree maybe 30 or so lights.  Should be under 400-500mA. 

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Many options and I do have a recommendation (wait for it!).  But being a discussion forum allow me to ramble..

So the task is to drop from 10V DC to 4.5V DC with a generous budget of 10 cents

The key is to know how much power (Watts) is involved - hence my question about current required by the tree.  Let's say with 30 LEDs we're talking 500 mA in round numbers.  Power = Voltage x Current, so dropping 5.5V at 500mA is a power dissipation of about 3 Watts.

1. A resistor can drop the 5.5V.  A 5 Watt resistor is common.  The resistor value is Voltage / Current so 5.5V / 0.5A = 11 Ohms.  Could be difficult to find an 11 Ohm, 5 Watt resistor.  10 Ohm and 12 Ohm are common values.  Any variation/deviation of the current would proportionally change the voltage drop.  Can probably find a 5 Watt resistor surplus for 10 cents...more like 50 cents (eBay).

2. A Zener diode can drop a fixed DC voltage.  A common value is 5.6V which is close enough.  A 5 Watt Zener is common.  The voltage drop with variation/deviation in current is relatively constant.  Not sure you can find one for 10 cents...more like 50 cents (eBay).

3. Not a single component but the venerable string-of-diodes can also drop a relatively fixed voltage.  Using 2 cent diodes that drop about 0.7V, you could string together say 8 of them to drop about 5.6V.  Note that your tree with a 4.5V 3-battery compartment probably operates fine with a fresh set of batteries of say 4.8V down to maybe 4V after some use.  The point being the lack of resolution using this method is really not an issue as you can find some number of diodes to use which will put the dropped voltage in the operating voltage range of 4.0 to 4.8V (or whatever).  Since many diodes are used, the power is spread out so low-cost generic 1 Amp diodes would be fine.  That's maybe 20 cents in parts so probably the least expensive method but some assembly hassle.

4. Bottom line.  My recommendation is to use an off-the-shelf DC-DC converter module for less than $1 (free shipping from Asia).  Put the 10V DC in, adjust the module to 4.5V DC output.  Variations/deviation in current will not affect the output voltage.  I suppose if you're a kind person you could let it slide and call this "one" component since it's an assembled module.  Additionally, because of the so-called switching technology used in this module, the 3 Watts of dissipated/wasted power of the other methods is reduced to less than 1 Watt which is a nice bonus.

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You have chosen wisely.

Odd about price - I just looked and there were dozens of eBay listings for 78 cents (free shipping from Asia).  Whatever. 

Anyway, these modules are really tiny compared to the AC-to-DC converter modules.   And the adjustment screw is only single-turn covering the entire output voltage range.  So the blue-colored multi-turn adjustment screw on the AC-to-DC module has much finer resolution (about 1 Volt per turn); you might find you just wink at this module and it changes 0.1V!  No matter - you'll be able to set it to 4.4 or 4.5 or 4.6 and that will be fine.

I suspect your xmas tree draws much less than 500 mA, so this module when set to 4.5V (or so) DC will also be able to power other 4.5V DC accessories from Lemax, Menards, etc.

stan2004 posted:

You have chosen wisely.

Odd about price - I just looked and there were dozens of eBay listings for 78 cents (free shipping from Asia).  Whatever. 

Anyway, these modules are really tiny compared to the AC-to-DC converter modules.   And the adjustment screw is only single-turn covering the entire output voltage range.  So the blue-colored multi-turn adjustment screw on the AC-to-DC module has much finer resolution (about 1 Volt per turn); you might find you just wink at this module and it changes 0.1V!  No matter - you'll be able to set it to 4.4 or 4.5 or 4.6 and that will be fine.

I suspect your xmas tree draws much less than 500 mA, so this module when set to 4.5V (or so) DC will also be able to power other 4.5V DC accessories from Lemax, Menards, etc.

I got 5 for under $6 shipped, so I don't think I got ripped off.   Thx again, and for the adjustment info. I'll b careful.   

I have ordered a couple of those computer PC power adapters discussed in another thread to get some cheap 12 VDC and 5VDC power.  I will use one of the above solutions to get 4.5 VDC for my signs.

That said, will a 5 VDC power source really damage a device designed for 4.5 VDC?  I have read articles discussing using 5 VDC wall warts to power 4.5 VDC items and they have not mentioned any issues.  Staying with 5 VDC would simplify wiring.

John H posted:

It would also be fairly simple to use a diode on the 5 volt input. That would give you the 4.5 volts you need.

GMTA?

gunrunnerjohn posted:

I used a 3A 5V wall wart and a 3A diode to drop the voltage to around 4.5 volts.  This poweres my Lemax stuff for the Christmas display.  I built an octopus cable with six plugs for various buildings coming off the 4.5 side of the supply.

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