Connecting 200 LEDs to AC (120V)?

[Bob Scott]

That'd work, but you'd need 8 of them...


a bridge rectifier is more efficient.

Go on, please expand your statement and explain exactly how it is that using
a bridge rectifier would be more efficient than using and inductor.
Humiliate us with your profound genius.

We are on the edge of our seats awaiting your explanation, Mr. Jasen.

Impatiently waiting....
Bob

 

[nospam]

Go on, please expand your statement and explain exactly how it is that using
a bridge rectifier would be more efficient than using and inductor.
Humiliate us with your profound genius.

We are on the edge of our seats awaiting your explanation, Mr. Jasen.

Impatiently waiting....

I assume he meant it is more efficient to run the LEDs from a full wave
rectified supply rather than half wave which for a given LED maximum rated
current requires twice as many LEDs for the same light output.

--

 

[jasen]

Really? How? It adds more loss to the circuit, lowering efficiency.

it nearly doubles the power factor of the LEDs
that's got to help.

Bye.
Jasen

 

[Michael A. Terrell]

it nearly doubles the power factor of the LEDs
that's got to help.

Bye.
Jasen

You need to think this through. The LEDs can only handle so much
power, whether it is half wave, two half wave, or DC. The diodes need
to be protected from reverse voltage, so series stings of anti parallel
diodes does this, and lights the diodes of each pair on alternate half
cycles.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[nospam]

You need to think this through. The LEDs can only handle so much
power, whether it is half wave, two half wave, or DC.

You need to understand LEDs. LEDs are specified with an allowable maximum
current and that is a DC specification.

If they bother to specify anything more than a DC maximum you will likely
find it is a pulse rating for 10us or a few 10's of us width with very low
duty cycle. LEDS have a low thermal mass and can't take much more than
their rated current for much more than a fraction of a millisecond.

A typical power IR LED (which is specified for pulse operation) has a 100us
pulse current specification of twice the DC maximum but only for duty
cycles of less than 2%. At 50% duty cycle the current rating is about 15%
more than the DC rating.

Running LEDS on unsmoothed full wave rectified 50/60Hz ac already reduces
the maximum available light output, using half wave rectification halves
that output again.

--

 

[Michael A. Terrell]

You need to understand LEDs. LEDs are specified with an allowable maximum
current and that is a DC specification.

I've been designing them into equipment for 30 years. For a one off
like this, its the simplest way to do the job. Yes, there are a lot of
ways to do it, but simply adding a full wave bridge offers no real
advantage unless the output is filtered and something is done to protect
the LEDs from spikes on the AC line. Have you noticed the number of LED
traffic lights with one or more group of dead LEDs? I see them all over
the place.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Tim Auton]

I've been designing them into equipment for 30 years. For a one off
like this, its the simplest way to do the job. Yes, there are a lot of
ways to do it, but simply adding a full wave bridge offers no real
advantage unless the output is filtered and something is done to protect
the LEDs from spikes on the AC line.

Full wave lets you use half the number of LEDs. That is almost a 50%
saving on cost, assembly time and finished unit size. Those advantages
seem pretty real to me.


Tim

 

[Michael A. Terrell]

Full wave lets you use half the number of LEDs. That is almost a 50%
saving on cost, assembly time and finished unit size. Those advantages
seem pretty real to me.

Tim

The OP wanted to use 200, so where is the savings?
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Tim Auton]

The OP wanted to use 200, so where is the savings?

LOL!

I suspect 200 LEDs might have been selected over 100 because they wanted
the output of ~200 LEDs. To get that with half-wave they'd need ~400
LEDs.

But if the OP really does want to use 200 LEDs to get the output of 100
I suggest full-wave rectification and only soldering in ~100 of the LEDs
(just over 100, to cover bridge losses). The rest could be used in the
base as ballast (mass) to help stop the lamp falling over. That would be
around half the effort and half the solder (there's your savings!) and
all 200 would still have been used. They'd get over 90 spares in the
base too - you only get one in the bottom of a Maglite.


Tim

 

[Tim Auton]

But if the OP really does want to use 200 LEDs to get the output of 100
I suggest full-wave rectification and only soldering in ~100 of the LEDs
(just over 100, to cover bridge losses).

Oops. Of course you don't need more than 100 LEDs to get the output of
100 LEDs. I had written a bit about efficiency and I cut it out and...
well, you get the idea.


Tim

 

[jasen]

jasen wrote:

You need to think this through.

no, you need to think it through.

two parallel strings of 20 LED s min series each running at half the current
twice as often will have a lower voltage drop but still produce the same
output. the voltage will probably be reduced by enough that you could add
another LED and get more output.

Bye.
Jasen

 

[Michael A. Terrell]

no, you need to think it through.

two parallel strings of 20 LED s min series each running at half the current
twice as often will have a lower voltage drop but still produce the same
output. the voltage will probably be reduced by enough that you could add
another LED and get more output.

Bye.
Jasen

With the 200 LEDS in a cluster they will run quite hot. Do whatever
the hell you want, but it won't hold up if you push the LEDs to their
full ratings without some type of cooling. I've seen a lot of LEDs dim,
and die from running too hot.

Some genius got the idea to use them to show that the power was on to
the radios in the burnin cabinets at Cincinnati, Electronics. They
didn't last a year.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida