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Re: [EVDL] * Re: LED Taillights - more info

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of Danny Miller
> Sent: Wednesday, October 03, 2007 11:55 AM
> To: Electric Vehicle Discussion List
> Subject: * Re: [EVDL] LED Taillights - more info
> Well, a resistor if it doesn't have a switching current reg, but the
> resistor will burn over 4W which is a lot of heat.
> If it does have a switching power supply, and I think most do, no
> there's no simple solution of wiring this-to-that.

Any obvious reason why a simple zener regulator wouldn't work for either
type of LED lamp?

Seems like a 5W 14V zener such as the 1N5351B ($0.46 from Digikey:
BGOS-ND>) with an appropriate series resistor would yield a sufficiently
stable 13.3-14.7V supply for the LED bulb.

Something like this 200R 5W wirewound resistor ($1.30 from Digikey:
-200-ND>) would do the trick.

Something around 200 ohms should do the trick.

With a max pack voltage of 60V and min zener voltage of 13.3V, there
would be 46.7V across this resistor, for about 233mA; at a minimum pack
voltage of 42V (1.75V/cell) and max zener voltage of 14.7V, there would
be 27.3V across it for a current of about 137mA. The LED bulb would
consume about 120mA of this while the difference flows through the

Worst case dissipation in the resistor is 10.9W; worst case in the zener
(no current through LED) is 3.4W. If the LED is always lit when power
is applied to the zener/resistor, then the worst case zener dissipation
drops to 1.7W.

A 250 ohm 20W resistor (e.g. $5 at Digikey
-250-ND>) would lower the currents to 187mA and 109mA respectively (so
the LED supply voltage might droop as low as 12V at low pack
conditions), and drops the worst case dissipation to about 8.7W.

A pair of these 430 ohm 10W resistors ($0.49 each at Digikey
0-ND>) yield a much cheaper 20W 215 ohm resistor.

Throw a few uF of capacitance across the zener for good measure and
connect/disconnect the resistor to pack voltage to turn the LED bulb
on/off and at least you're only burning off this heat while the bulb is

Or, look for a 48V to 12V (ish) DC/DC. 48V is a common supply level for
telecom applications, and you can probably also find (non-isolated) 48V
to 12V DC/DCs intended for golf car applications. Here's the sort of
thing that could do the trick much more elegantly than a resistor or
resistor/zener for each bulb:


(Artesyn BXB75-48S12FLT 48V (36-72V) to 12V (adjustable) @ up to 6.3A
output DC/DC: $12 buy-it-now).



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