LED voltage

[davidt]

Two questions really:

1: I have some old LED's but don't know what the forward voltage or
current is for any of them. Is there anyway of finding out ?

2: If I have some LED's with a Vf of 3.4V and If of 30mA could I wire
94 of them in series and connect to mains AC supply of 230V with no
limiting resistor. I have suggested 94 because this is 320 divided by
3.4 ( where 320 is the peak V for 230V RMS).

 

[John Fields]

On Wed, 23 Jan 2008 11:20:16 -0800 (PST), davidt <dav1d@talktalk.net>
wrote:

Two questions really:

1: I have some old LED's but don't know what the forward voltage or
current is for any of them. Is there anyway of finding out ?

---
Yes. View in Courier:

+5V>--> |
| S1
O
|
+-----+
R1| |
[5000]<--+
|
+----<<---+
|A |+
[VOLTMETER] [LED]
| |
+<---<<---+
|
[AMMETER]
|
GND>----+

1. Open S1
2. Crank R1 to maximum R
3. Connect the LED
4 Close S1
5. Rotate R1 until the ammeter reads 20mA
6. Read the voltmeter

This assumes that since they're old LEDs they're rated for 20mA.
You may have some high-efficiency LEDs in there, in which case they'll
be rated for 2mA, so watch the brightness as you crank the pot since
the high-efficiency LEDs will get more or less as bright with 2mA
through them as the "standard" ones will with 20 mA through them.
---

2: If I have some LED's with a Vf of 3.4V and If of 30mA could I wire
94 of them in series and connect to mains AC supply of 230V with no
limiting resistor.

---
Yes, of course you _could_, but it's not a good idea. LEDs want to be
driven by a constant current and, being diodes, they have a very sharp
If VS Vf knee and will exhibit a very large change in forward current
for a very small change in forward voltage once the knee has been
crossed.

What I'd do would be to assume that all the LEDs were sitting at worst
case Vf low, that mains could rise to nominal +10%, and figure the
length of the string and the value of the current limiting resistor
from that.

Also, in order to eliminate flicker and to keep from reverse-voltage
stressing the LEDs, I'd full-wave rectify the mains and drive the LEDs
from the output of the bridge.

JF

 

[davidt]

Thanks JF

Great answer to both Qs, much appreciated.