wiring 2 loads up in in parallel to drop voltage (ed: he means in series)

[Harry Bloomfield]

BTW it is 2 times LED lamps I want to wire up to a car battery. The LED lamps take 4 AA batterys (so 1.5V times 4= 6V each)

Better to assume the car battery is more than 12v. Engine running, around 14.5v. A fully charged, off charge battery can be 12.9v too.

 

[JohnW2]

How can one connect two pulse width modulated controllers in series?

Interesting question, but I rather doubt that it's relevant to the OP's situation.

LED lamps designed to run off 4 x 1.5V batteries will very probably just have a dropping resistor (and probably a couple of LED elements in series), rather than PWM current control - in which case putting two of them in series would be no problem.

However, as said, your question is interesting, and I might do some experiments if/when I have a few moments. For no particularly convincing reason, I somewhat suspect that I might well find that two of them in series 'just works'

Kind Regards, John

 

[JohnW2]

Better to assume the car battery is more than 12v. Engine running, around 14.5v. A fully charged, off charge battery can be 12.9v too.

True, but I doubt that would, in practice, make a lot of difference, other than perhaps shortening LED life a little.

Kind Regards, John

 

[ebee]

Yes, when I use a 12V SLA burglar alarm battery I always allow for it being a 13.6 to 13.8 volts as per standard charge voltage on most control panels

 

[hllns]

What are you trying to do? There's a wide variety of cheap and efficient DC-DC converter modules to supply a correct lower voltage to both loads, the faff and uncertainty doesn't seem worth connecting things in series unless you're pulling high levels of current.

 

[JohnW2]

What are you trying to do? There's a wide variety of cheap and efficient DC-DC converter modules to supply a correct lower voltage to both loads, the faff and uncertainty doesn't seem worth connecting things in series unless you're pulling high levels of current.

What you say is strictly true but, as I've implied, if it were me I'd probably first try the simpler/cheaper approach of just putting the lamps in series - and I strongly suspect that that would work fine.

Kind Regards, John

 

[AstonHill]

If the two loads are different, then they will share the 12V unequally (e.g. one might get 8V and the other 4V).

Do you mean if one load has more amps then the other?
They are 2 identical LED lamps from Aldi

BTW, in your diagram, the two loads are in series, not parallel.

thanks!

 

[AstonHill]

A LED is a current dependent device not voltage, putting a white LED across a 3 volt battery can cause thermal run away, so we put 3 LED's and a resistor in series across a 12 volt battery the resistor is called the driver, as it limits the current to within safe limits.

But if the values where the same as in my situation 2 times 6Vs then a resistor is not needed right?
The resistor is only there to drop the voltage and not to stabalise it right?

With AC we can use a capacitor as a driver which limits current, but then frequency becomes important.

Are you just storing current with a capacitor? why only AC

We can use zener or avalanche diodes to limit the voltage where multi items are in series, but chips like the 78xx series are so cheap and now pulse width modulated chips, the old methods are not used that much any more.

I though Diodes where there to force electricity in one direction only

 

[AstonHill]

He's talking about "LED lamps", designed to run off 4 x 1.5V batteries (i.e. 6V) (hence with a resistor or other current controlling something), not LED elements.

Kind Regards, John

No they are LED lamps, or am i missing something hear?

 

[AstonHill]

What are you trying to do? There's a wide variety of cheap and efficient DC-DC converter modules to supply a correct lower voltage to both loads, the faff and uncertainty doesn't seem worth connecting things in series unless you're pulling high levels of current.

the lights where 4.99 each, not seen any decent 12V LED lamps about yet. plus they give a 3 year warranty with them

 

[AstonHill]

Better to assume the car battery is more than 12v. Engine running, around 14.5v. A fully charged, off charge battery can be 12.9v too.

good point! but the battery will not be getting any chage when the lights/ the load is on

 

[JohnW2]

No they are LED lamps, or am i missing something hear?

No, it was someone else doing the 'missing'.

Kind Regards, John

 

[davelx]

But if the values where the same as in my situation 2 times 6Vs then a resistor is not needed right?
The resistor is only there to drop the voltage and not to stabalise it right?


Are you just storing current with a capacitor? why only AC


I though Diodes where there to force electricity in one direction only

Zener diodes are a special type with a low and well defined reverse breakdown voltage so can be used to provide reference voltage in a regulator.

 

[ebee]

If the two loads are different, then they will share the 12V unequally (e.g. one might get 8V and the other 4V).

Do you mean if one load has more amps then the other?

Ref the above.
No.

If you have two resistances in series then put a voltage across them they will draw current according to the total resistance of those resistors combined.
If those resistances have quite similar values then they will quite similar voltages across each one (approx equal to half the total voltage applied across them).
If, however, their resistances are more unequal then the voltages across each one will be more unequal too but the current drawn by them will be the same as each other.

So engineering tolerances and other variables make each resistance not quite the same but when doing calculations we might ignore those differences if we consider them to be quite small depending upon how we want to use them. If the differences are relatively not quite small we might want to re-evaluate the voltage differences produced.

It just depends on how much difference we might expect it to make to our finished result and what effect it might have.

If two resistances are connected across the voltage in parallel then the opposite happens - each resistance will have the same voltage across them but each will draw slightly different currents thru them in accordance with their resistance differences. Again we evaluate if that diffence needs to concern us.

Example - if we pick resistors from a batch of say 1% tolerance resistors then for our particular use we might consider the differences acceptable and of no concern.
On the other hand, if we pick resistances from a batch of 5 or 10 or 20 percent tolerance resistors we might want to make some adjustments, but there again it might still be acceptable for the particular arrangement we have in mind.

We might also be be in that acceptance if all the resistors are from the same batch therefore more likely to actually match up than if they are from different batches/from different manufacture's/in different ambient temperatures etc etc.

Again it all depends on what you are attempting to achieve and how much difference in the finished result might concern us.

 

[FrodoOne]

?
If each string of LED elements has it's own resistor and the strings are supplied from a stable 12 Volt supply then open circuit failure of one string will not affect the voltage supplied to other strings. Short circuit failure of one or more LED elements in the same string will result in increased current through the resistor in that string. It may result in the resistor overheating and failing. The increased current load on the power supply may result in the supply being overloaded and it output voltage dropping,

We are discussing two (or, possibly, more) strings
in each 6 V "device"
with the "strings" in each 6 V "device" connected in parallel
but
with the two 6 V "devices" connected in series,
across 12 V .

One may have a "failure" of one "string", causing the problem.

Consider the situation below

If there are Four equal "Resistance" Strings
connected in "Series/Parallel"
with R1 and R2 being the "LED Strings" in one Lamp
and
R3 and R4 being the "LED Strings" in the other Lamp.

Normally, there will be 6 V across all resistors.

However. if R1 "fails" (Open Circuit)
the Voltage across R2 will increase (to 8 V)
and
the voltage across R3 and R4 (in parallel) will decrease (to 4 V.)