I am about to add an LED to a charger but need further opini

[alitonto]

I am trying to add an LED to a NiMh charger for 5 AA battery pack
(total voltage about 6.93) to be able to see that is actually turned
on.

I have seen some chargers that have the dropping resistor and LED in
series with each other and then this combination is in parallel with
another resistor of lower ohms which goes to the battery pack: examples

(82 ohm and LED) in parallel with (a resistor of 24 ohms) - used for
charging 4 AA batteries


(47 ohm and LED) in parallel with (a resistor of 18 ohms) - used for
charging 2 AA batteries


I can calculate the resistor for the LED but any thoughts on what the
other resistor should be? or what things to take into account in
choosing this resistor ?

Thanks
Ali

 

[alitonto]

Just to add further: the battery pack consists of 5 NiMh AA batteries
and the actual charger is just a wall plug with 9 volts dc at .500 mA
max. The batteries are inside a torch and I am wanting to add this LED
circuit inside the torch with a simple diagram.
Will the voltage across the resistor/LED combinatiion adjust itself to
be equal to the voltage across the resistor in paralell for when the
batteries are really flat and therefore higher current will be flowing?
Ali




Pete wrote:

Ol' Duffer wrote:

I have seen some chargers that have the dropping resistor and LED in
series with each other and then this combination is in parallel with
another resistor of lower ohms which goes to the battery pack

What I've often seen done is a string of three series diodes in the
output lead with a LED and series resistor in parallel with the diode
string. Just make sure that the diodes can handle the maximum charger
current.

Some of the makers who have done this have had trouble with
blowing out the LED's. Consider what happens when you plug
in a dead battery, or maybe even worse a bad one that happens
to be shorted...

Since a nicad charger is supposed to be constant current, I can't see
that being a problem unless whoever "designed" it didn't know what they
were doing...

Peter

 

[James Sweet]

alitonto wrote:

I am trying to add an LED to a NiMh charger for 5 AA battery pack
(total voltage about 6.93) to be able to see that is actually turned
on.

I have seen some chargers that have the dropping resistor and LED in
series with each other and then this combination is in parallel with
another resistor of lower ohms which goes to the battery pack: examples

(82 ohm and LED) in parallel with (a resistor of 24 ohms) - used for
charging 4 AA batteries


(47 ohm and LED) in parallel with (a resistor of 18 ohms) - used for
charging 2 AA batteries


I can calculate the resistor for the LED but any thoughts on what the
other resistor should be? or what things to take into account in
choosing this resistor ?

Thanks
Ali

Is there a resistor in the charger already which is in series with the
battery being charged? If so then measure the voltage across it,
subtract the LED's forward voltage from that and then use ohms law to
determine the resistor you need for the correct current across the LED,
5-10 mA ought to do the trick. In a pinch you can just experiment and
see what works.

 

[Jasen Betts]

["Followup-To:" header set to aus.electronics.]
On 2006-03-15, alitonto <alitonto@hotmail.com> wrote:

Just to add further: the battery pack consists of 5 NiMh AA batteries
and the actual charger is just a wall plug with 9 volts dc at .500 mA
max. The batteries are inside a torch and I am wanting to add this LED
circuit inside the torch with a simple diagram.
Will the voltage across the resistor/LED combinatiion adjust itself to
be equal to the voltage across the resistor in paralell for when the
batteries are really flat and therefore higher current will be flowing?
Ali

yes parallel combinations see equal voltage and share the current

with series combinations it's the other way around,


Bye.
Jasen