# Not so easy led driving.

Discussion in 'Electronic Design' started by Fred Bartoli, Mar 29, 2007.

1. ### Fred BartoliGuest

I've a situation where a string of 5 Leds (2mA) has to be powered from
48V up to 1kV (HV safety indicators). That's about 20mW for the leds,
and 2W heat under 1kV.

The system ATM is a simple CC source (1BJT + 2 mosfets) but we now need
as less heat generation as possible and a more compact design.

The first idea was a resistive drop down to 50V or so, and from there
have a small step down switcher current regulator providing the 2mA current.
At 50% efficiency that would have been 0.8mA for 1.2W saving, and
possibly more.

....but all the little switchers I could find either have huge current
draw or low max input voltage, or something.

Before I start rolling my own, which seems not so simple (first attempts
show a serious lack of 'efficiency')
I have to ask if I missed the good IC? (or idea)

2. ### martin griffithGuest

anything here
http://www.powerint.com/appcircuits.htm

martin

3. ### colinGuest

"Fred Bartoli"
how about more leds in series using less current for the same light output ?

Colin =^.^=

4. ### Fred BartoliGuest

colin a écrit :
One Led is also an opto (visual feed back for the operator and to the
calculator too).
And all that is doubled for redundancy :-(

6. ### James ArthurGuest

Must you power the LEDs from the 48-1kV? If not, you could make an
isolated supply to power the LEDs, and a separate circuit to sense the
high voltage: (warning: crude sketch)

+15v (isolated)
|
+HV >------. +--+--+
| | |
| | LED1
R1 | | :
| | LEDn
+---------|\ |
| | >----'
R2 Vref >-|/
| |
-HV>-------+----------+

The isolated supply could be very simple, and unregulated.

That would solve the heat problem, and could also be smaller than
the space otherwise needed to dissipate 1-or-2 watts.

Cheers,
James Arthur

7. ### Terry GivenGuest

or just a self-oscillating flyback converter driving the LED, using a
suitably high voltage device. just let it clag out naturally when Vin
gets low enough.

I built a 1W 100V - 1kVdc flyback with an emitter switched BU508 once.
and it was harder to do than the 100W version.

Cheers
Terry