What's the circuit in LED flash light?

[peter]

In all the white-LED flashlights I bought, they all have some sort of
circuit attached to the LEDs. I thought all you needed is a resistor for
each LED to limit the current. What does the circuit do? Is it functionally
similar to a resistor but has higher efficiency (perhaps a switching current
regulator)?

 

[Dave Platt]

In all the white-LED flashlights I bought, they all have some sort of
circuit attached to the LEDs. I thought all you needed is a resistor for
each LED to limit the current. What does the circuit do? Is it functionally
similar to a resistor but has higher efficiency (perhaps a switching current
regulator)?

Yes, it's probably a boost-mode switching regulator. White-light LEDs
have a fairly high threshold voltage... they need 4-5 volts across
them to conduct, and this is more than a two-cell battery can provide.
The switching circuit boosts the voltage, and delivers a consistent
and stable current to the LEDs.

You can run white LEDs from a higher-voltage DC source (e.g. 6-9
volts) with just a series resistor. However, this is rather wasteful
of power (a good deal of the battery's power is dissipated as heat in
the resistor) and the brightness of the LEDs will vary significantly
as the battery runs down.

--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Watson A.Name - \"Watt Su]

"peter" <nospam@nospam.com> wrote in message
news:B_B5d.5707$Cn.4980@trnddc04...

In all the white-LED flashlights I bought, they all have some sort of
circuit attached to the LEDs. I thought all you needed is a resistor
for
each LED to limit the current. What does the circuit do? Is it
functionally
similar to a resistor but has higher efficiency (perhaps a switching
current
regulator)?

Depends on how many cells the light has. If it has two 1.5V cells or
less, then the light needs the circuit to boost the voltage up to 3.4VDC
or so to drive the white LED.

The Dorcy 4 white LED flashlights are only $16 or so at OSH, so you can
get a reasonably priced flashlight for under $20. If you buy one of
those 'torches' that use a lithium cell or two, then you end up paying
double for the batteries. And you keep on paying for the life of that
light.

http://www.dorcy.com/led%20new.htm just a satisfied customer.