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Apologies if this question is too vague. I have incomplete information and I'm asking because I'm not sufficiently knowledgeable to answer this myself!

I have ten USB LED units and I have a 12v power supply (that is the only power supply I have available, no mains). I also have a number of LM340T 5 volt regulators. Given the components I have to hand, I want to figure out the best way to wire them up, or whether there is no best way to wire them up and I should do it differently.

There is very little info on the LED units, except that they're rated 5 Volts. I don't know the current rating. Inside are two pairs of tiny J3 transistors and resistors. I can't see the tracks, but I could work it out the circuit with a continuity tester. I imagine we're talking a few tens of mA but I don't know.

Do you think it would be a good/bad idea to use one LM340T per LED to drop the power from 12v to 5?

Do you think that the LED units (and their internal regulator circuit) would behave nicely in series to form a potential divider, three to a circuit to give 4v each?

Should I just get a proper Buck converter?

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    \$\begingroup\$ Just a note: The LM340T's will drop the voltage from 12 to 5, not the power. You could hook one up, and use your meter to measure the current usage. From there determine if the heat dissipation by the voltage regulator is sufficient. \$\endgroup\$ – JYelton Jun 19 '13 at 14:55
  • \$\begingroup\$ I don't know whether the built-in circuity in the LED unit will deal with having that much power. Would I have to introduce a current limiting resistor after the LM340T in that case? \$\endgroup\$ – Joe Jun 19 '13 at 14:58
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    \$\begingroup\$ The LED units will only use as much power as they are designed to use, so long as the supply to each remains in the USB range, ~5 Volts. Adding a current limiting resistor or for that matter the LM340T linear regulator raises the issue of power being wasted as heat at either of those components, in dropping the voltage. This heat generation is the same whether it's the regulator or the resistor, it is voltage dropped x current for each. A proper Buck converter will ensure significant heat reduction, and should be able to power all 10 lights fine if it is rated for the calculated current. \$\endgroup\$ – Anindo Ghosh Jun 19 '13 at 15:40
  • \$\begingroup\$ So are you saying that if the heat dissipation is OK (TBC), I can use the LM340T? And any idea if the individual circuits would work if I just put 3 in series? \$\endgroup\$ – Joe Jun 19 '13 at 15:48
  • \$\begingroup\$ @Joe Verifying if 3 in series will work, should be simple enough. My gut feel is it'll work fine, since the transistors and resistors inside each light are most likely a constant current source for the respective LEDs - as long as there is some headroom over the LED Vf, it should work. \$\endgroup\$ – Anindo Ghosh Jun 20 '13 at 7:02
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If the goal is simply to light them all up - why not simply get a ten port USB hub, and plug the ten LED lamps into the USB ports therein?

10 port USB hub

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  • \$\begingroup\$ Yes that is one idea. I don't necessarily have access to mains though. Are hubs automatically powered when not plugged into a computer? I'd have to take the gamble. \$\endgroup\$ – Joe Jun 19 '13 at 14:55
  • \$\begingroup\$ I don't think they draw power from the source USB, with 10x100ma = 1Amp (for normal USB devices that could be plugged in), it could blow the source USB port. You could always hot wire it. \$\endgroup\$ – Ron J. Jun 19 '13 at 15:05
  • \$\begingroup\$ No, I would be running from a power adaptor. But the hub may switch off when disconnected from the PC. Also 'hotwire' how? I am asking for a circuit in my question. \$\endgroup\$ – Joe Jun 19 '13 at 15:08
  • \$\begingroup\$ It would depend on what inside that hub. IF it needs a USB input to activate, a simple 10K resistor across the incoming data lines or taking them high/low may do the trick. My guess is that it will supply power regardless of being plugged into a PC. \$\endgroup\$ – Ron J. Jun 19 '13 at 15:12
  • \$\begingroup\$ Thanks! If I'm unable to find a better solution I might do that. A shame, as I just bought 10 USB sockets! \$\endgroup\$ – Joe Jun 19 '13 at 15:13
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You don't have a ammeter or multimeter with current sensing? Without knowing how much current it takes it's hard to suggest how many you need. You might only need one (100mA * 10 = 1 Amp, which a single LM340T can do with a proper heatsink) or you might need 5 (500mA * 10 = 5 Amps, each LM340T can only do 1A per it's datasheet).

THe problem is that we don't know you 12v Power Supply's current output capacity or the USB LED's current draw. And with a linear regulator like the LM340T, any voltage conversion is turned into wasted heat. If each USB LED takes 500mA (Unlikely, but it can), it would require 5 Amps of power from your power supply, and waste ((12V - 5V) * 5A) = 35 WATTS of POWER in heat alone (distributed between how ever many LM340Ts you use, if 5, 6 Watts each). You would need to heat sink them properly. A Switching regulator would be better, but you would still need to know how much current is being used.

Looking online, it does seem like these are high current led lamps, with 1W or higher leds in them.

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protected by W5VO Jun 19 '13 at 15:10

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