Newbie question: Can I Light a 12VDC LED from 24VAC source?

T

taurus

Guest
Newbie question:

I'd like to have an led light from 24VAC (when an HVAC thermostat closes).

I have a few 12VDC leds (automotive) - is it relatively easy to get these to
light from 24VAC? What might the circuit look like - diodes, resistors??

Or are there such things as 24VAC leds?

Thanks
 
"taurus" <none@nowhere.gy> schreef in bericht
news:bZX7j.2965$ox1.1525@pd7urf3no...
Newbie question:

I'd like to have an led light from 24VAC (when an HVAC thermostat closes).

I have a few 12VDC leds (automotive) - is it relatively easy to get these
to light from 24VAC? What might the circuit look like - diodes,
resistors??

Or are there such things as 24VAC leds?

Thanks
Click to expand...
Most simple solution: A rectifier bridge and two 12V LEDs in series.

petrus bitbyter
 
On Wed, 12 Dec 2007 22:04:21 +0100, "petrus bitbyter"
<pieterkraltlaatditweg@enditookhccnet.nl> wrote:

"taurus" <none@nowhere.gy> schreef in bericht
news:bZX7j.2965$ox1.1525@pd7urf3no...
Newbie question:

I'd like to have an led light from 24VAC (when an HVAC thermostat closes).

I have a few 12VDC leds (automotive) - is it relatively easy to get these
to light from 24VAC? What might the circuit look like - diodes,
resistors??

Or are there such things as 24VAC leds?

Thanks


Most simple solution: A rectifier bridge and two 12V LEDs in series.

petrus bitbyter
Click to expand...
---
Disagree:

For If = 20mA
/
24VAC>---[620R]--+--[LED>]--+
1/2W | |
+--[<LED]--+
|
24VAC>----------------------+




--
JF
 
On Wed, 12 Dec 2007 17:11:09 -0600, John Fields
<jfields@austininstruments.com> wrote:

Most simple solution: A rectifier bridge and two 12V LEDs in series.

petrus bitbyter

---
Disagree:

For If = 20mA
/
24VAC>---[620R]--+--[LED>]--+
1/2W | |
+--[<LED]--+
|
24VAC>----------------------+
Click to expand...
 
On Wed, 12 Dec 2007 17:11:09 -0600, John Fields
<jfields@austininstruments.com> wrote:

Most simple solution: A rectifier bridge and two 12V LEDs in series.

petrus bitbyter

---
Disagree:

For If = 20mA
/
24VAC>---[620R]--+--[LED>]--+
1/2W | |
+--[<LED]--+
|
24VAC>----------------------+
Click to expand...
Blah last post got sent too quick.

24v AC can't be used to drive a single 12v LED for a good reason. The
AC voltage is usually measured in RMS (Root Mean Square) and is not
the same as peak voltage. With half a wave to light the LED, you
would have an "average" voltage of 24v and peak voltage of about 34v
and this will fry 12v LED quickly. A 620 ohms resistor above will
help but l'd think 1k resistor is safer.

Putting 2 LEDs close together, 1 in reverse from other, will give the
illusion they are always lit. If you use only 1 LED, you'll get a
noticeable flicker like cheap LED christmas lights.
 
On Thu, 13 Dec 2007 03:04:42 GMT, Impmon <impmon@digi.mon> wrote:

On Wed, 12 Dec 2007 17:11:09 -0600, John Fields
jfields@austininstruments.com> wrote:

Most simple solution: A rectifier bridge and two 12V LEDs in series.

petrus bitbyter

---
Disagree:

For If = 20mA
/
24VAC>---[620R]--+--[LED>]--+
1/2W | |
+--[<LED]--+
|
24VAC>----------------------+

Blah last post got sent too quick.

24v AC can't be used to drive a single 12v LED for a good reason.
The AC voltage is usually measured in RMS (Root Mean Square) and is not
the same as peak voltage. With half a wave to light the LED, you
would have an "average" voltage of 24v and peak voltage of about 34v
and this will fry 12v LED quickly. A 620 ohms resistor above will
help but l'd think 1k resistor is safer.
Click to expand...
---
What kills LEDs (when charge is flowing through them in the forward
direction) is overheating because of excessive power dissipation,
and as far as heating the LED and the resistor goes, 24VRMS is
exactly the same as 24VDC. Moreover, since each LED is only on 50%
of the time they'll each be dissipating 240mW instead of 480.
---

Putting 2 LEDs close together, 1 in reverse from other, will give the
illusion they are always lit. If you use only 1 LED, you'll get a
noticeable flicker like cheap LED christmas lights.
Click to expand...
---
I don't understand. Are you saying the mine _aren't_ in parallel
opposition?



--
JF
 
Thanks for taking the time to reply - I'm off to the components store now!
"taurus" <none@nowhere.gy> wrote in message
news:bZX7j.2965$ox1.1525@pd7urf3no...
Newbie question:

I'd like to have an led light from 24VAC (when an HVAC thermostat closes).

I have a few 12VDC leds (automotive) - is it relatively easy to get these
to light from 24VAC? What might the circuit look like - diodes,
resistors??

Or are there such things as 24VAC leds?

Thanks
Click to expand...
 
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