Easy question for someone.

[jhleslie@googlemail.com]

I have a 5mm red flashing LED, I've had it for probably 15 years, I
connected some others that I bought at the same time to my house
burglar alarm. They are are on the bell box and on a warning panel on
the garden shed. They make the alarm system look very scary, I think
any potential burglar would move onto another house after seeing
these.

Now I have moved to another house and I want to do the same again, the
problem is that I can't remember how to calculate the resistance that
I need to put in series.

A similar looking item that is for sale here has a specification as
follows,
Forward voltage: 2.5V
Forward current max.: 55mA

I think that this has something to do with V = I x R but I didn't pay
proper attention at school.

Can somebody tell me what a suitable resistance would be please?

Thanks in advance,
James

 

[Greg Neill]

jhleslie@googlemail.com wrote:

I have a 5mm red flashing LED, I've had it for probably 15 years, I
connected some others that I bought at the same time to my house
burglar alarm. They are are on the bell box and on a warning panel on
the garden shed. They make the alarm system look very scary, I think
any potential burglar would move onto another house after seeing
these.

Now I have moved to another house and I want to do the same again, the
problem is that I can't remember how to calculate the resistance that
I need to put in series.

A similar looking item that is for sale here has a specification as
follows,
Forward voltage: 2.5V
Forward current max.: 55mA

I think that this has something to do with V = I x R but I didn't pay
proper attention at school.

Can somebody tell me what a suitable resistance would be please?

Thanks in advance,
James

google: LED resistor size

 

On Sun, 15 Feb 2009 10:15:51 -0800 (PST), "jhleslie@googlemail.com"
<jhleslie@googlemail.com> wrote:

I have a 5mm red flashing LED, I've had it for probably 15 years, I
connected some others that I bought at the same time to my house
burglar alarm. They are are on the bell box and on a warning panel on
the garden shed. They make the alarm system look very scary, I think
any potential burglar would move onto another house after seeing
these.

Now I have moved to another house and I want to do the same again, the
problem is that I can't remember how to calculate the resistance that
I need to put in series.

A similar looking item that is for sale here has a specification as
follows,
Forward voltage: 2.5V
Forward current max.: 55mA

I think that this has something to do with V = I x R but I didn't pay
proper attention at school.

Can somebody tell me what a suitable resistance would be please?

Thanks in advance,
James

The resistor needed will depend on the voltage supply you are using
and the current you put through the LED. With current LEDs, running
it at about hlf the maximum current should provide adequate
brightness. 55/2 = 27.5 which I'll round to 27ma.

The formula would be R = E/I where I is 0.027amp, E is the difference
between the rated voltage of the LED and the supply voltage.

If your suppply is 6 volts, then E is 6 - 2.5 = 3.5 volts.

Plugging in the values: R = 3.5/0.027 = = 129.6 ohms.

The closest standard value is 120 ohms, which would limit the current
to 3.5/120 = 0.029 amp or 29ma.

Chart of standard resistance values:
http://www.logwell.com/tech/components/resistor_values.html

If the supply voltage is different, then do the math ;-)

Otherwise, use a 330 ohm resistor and live with the brightness.

John

 

[jhleslie@googlemail.com]

On 15 Feb, 18:54, n...@picaxe.us wrote:

On Sun, 15 Feb 2009 10:15:51 -0800 (PST), "jhles...@googlemail.com"



jhles...@googlemail.com> wrote:
I have a 5mm red flashing LED, I've had it for probably 15 years, I
connected some others that I bought at the same time to my house
burglar alarm. They are are on the bell box and on a warning panel on
the garden shed. They make the alarm system look very scary, I think
any potential burglar would move onto another house after seeing
these.

Now I have moved to another house and I want to do the same again, the
problem is that I can't remember how to calculate the resistance that
I need to put in series.

A similar looking item that is for sale here has a specification as
follows,
Forward voltage:    2.5V
Forward current max.:    55mA

I think that this has something to do with V = I x R but I didn't pay
proper attention at school.

Can somebody tell me what a suitable resistance would be please?

Thanks in advance,
James

The resistor needed will depend on the voltage supply you are using
and the current you put through the LED.  With current LEDs, running
it at about hlf the maximum current should provide adequate
brightness.  55/2 = 27.5 which I'll round to 27ma.

The formula would be R = E/I where I is 0.027amp, E is the difference
between the rated voltage of the LED and the supply voltage.

If your suppply is 6 volts, then E is 6 - 2.5 = 3.5 volts.

Plugging in the values: R = 3.5/0.027 = = 129.6 ohms.

The closest standard value is 120 ohms, which would limit the current
to 3.5/120 = 0.029 amp or 29ma.

Chart of standard resistance values:http://www.logwell.com/tech/components/resistor_values.html

If the supply voltage is different, then do the math ;-)

Otherwise, use a 330 ohm resistor and live with the brightness.

John

That's brilliant John... Thanks very much. Actually the voltage is 12v
so I presume that the sum is
R= (12-2.5) / 0.027 = 350 ohms
It was the bit about the difference in the two voltages that I had
forgotten in the 35 years since I learnt it at school.
Thanks again,
James

 

[IanM]

jhleslie@googlemail.com wrote:

On 15 Feb, 18:54, n...@picaxe.us wrote:
On Sun, 15 Feb 2009 10:15:51 -0800 (PST), "jhles...@googlemail.com"



jhles...@googlemail.com> wrote:
I have a 5mm red flashing LED, I've had it for probably 15 years, I
connected some others that I bought at the same time to my house
burglar alarm. They are are on the bell box and on a warning panel on
the garden shed. They make the alarm system look very scary, I think
any potential burglar would move onto another house after seeing
these.
Now I have moved to another house and I want to do the same again, the
problem is that I can't remember how to calculate the resistance that
I need to put in series.
A similar looking item that is for sale here has a specification as
follows,
Forward voltage: 2.5V
Forward current max.: 55mA
I think that this has something to do with V = I x R but I didn't pay
proper attention at school.
Can somebody tell me what a suitable resistance would be please?
Thanks in advance,
James
The resistor needed will depend on the voltage supply you are using
and the current you put through the LED. With current LEDs, running
it at about hlf the maximum current should provide adequate
brightness. 55/2 = 27.5 which I'll round to 27ma.

The formula would be R = E/I where I is 0.027amp, E is the difference
between the rated voltage of the LED and the supply voltage.

If your suppply is 6 volts, then E is 6 - 2.5 = 3.5 volts.

Plugging in the values: R = 3.5/0.027 = = 129.6 ohms.

The closest standard value is 120 ohms, which would limit the current
to 3.5/120 = 0.029 amp or 29ma.

Chart of standard resistance values:http://www.logwell.com/tech/components/resistor_values.html

If the supply voltage is different, then do the math ;-)

Otherwise, use a 330 ohm resistor and live with the brightness.

John

That's brilliant John... Thanks very much. Actually the voltage is 12v
so I presume that the sum is
R= (12-2.5) / 0.027 = 350 ohms
It was the bit about the difference in the two voltages that I had
forgotten in the 35 years since I learnt it at school.
Thanks again,
James

*FLASHING* LEDS have an integral controller chip and are usually
designed to run from a fixed supply *without* a series resistor. They
usually run from a 9 to 12 V supply.

Distinguishing them from ordinary LEDs without blowing the ordinary ones
isn't so easy. If you slowly increase the current towards If(max)
through a LED it will either get brighter and brighter while the Vf
remains near its rated value, or you will find the current doesn't
continue to increase, but the Vf increases towards the supply voltage
and the LED starts flashing. Doing this without a very controllable
current limit on your bench supply is a PITA and you'd usually be better
off buying new ones or guessing if its a flashing one, trying 12V with a
220R series resistor *briefly* and throwing the ones you cant make sense of.

Finally, a normal LED in series with a flashing LED will flash with it
but may glow very faintly when the flashing LED is off. Don't exceed
the maximum voltage rating of the flashing LED across the whole pair.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL: