I have three LEDs each with their own battery. I want to control all three at the same time using one switch. I believe the charge in each battery is roughly the same. Would the following setup work? As far as voltage and current, is this just a parallel circuit, or some hybrid type? Thanks for any input.

 

The way you have drawn LEDs and batteries is not the accepted way of displaying them but it’s clear what you mean. You should always have a resistor in series with each LED to control the current. Each LED, depending on color, will have a voltage drop across it, from around 1.8V for red to 3.3V for blue or white. So the first step is to decide what current you plan through each LED and work out the resistor value using Ohm’s Law. What color are your LEDs and what is your battery voltage? One battery should be sufficient for all three LEDs if placed in series with the switch, unless its voltage is too low in which case you may need batteries in series.

 

When the switch is closed, each LED is connected directly across its battery. Note that there is no current sharing among the batteries, which is a good thing.

While some LEDs nave current-limiting built-in, most do not. To protect the LEDs from burning out, add a resistor between each battery + terminal and its LED. Connect each battery to its LED through a resistance to limit the current.

ak

 

The way you have drawn LEDs and batteries is not the accepted way of displaying them but it’s clear what you mean. You should always have a resistor in series with each LED to control the current. Each LED, depending on color, will have a voltage drop across it, from around 1.8V for red to 3.3V for blue or white. So the first step is to decide what current you plan through each LED and work out the resistor value using Ohm’s Law. What color are your LEDs and what is your battery voltage? One battery should be sufficient for all three LEDs if placed in series with the switch, unless its voltage is too low in which case you may need batteries in series.

Thanks, Jerry. The LEDs in my diagram represent three sets of lights (LEDs mounted inside Halloween decorations - three skeletons) - each with their own battery packs. At least on an individual skeleton basis, the resistors and current issues are already sorted out - but thank you for drawing attention to that missing detail in the diagram. I want to use a microcontroller to switch all three skeletons on using one switch. Cheers

 

When the switch is closed, each LED is connected directly across its battery. Note that there is no current sharing among the batteries, which is a good thing.

While some LEDs nave current-limiting built-in, most do not. To protect the LEDs from burning out, add a resistor between each battery + terminal and its LED. Connect each battery to its LED through a resistance to limit the current.

ak

Thanks, Ak. On the issue of current sharing, I was thinking the only current flow between the batteries would be if one was a different charge than the other...?

 

I have three LEDs each with their own battery. I want to control all three at the same time using one switch. I believe the charge in each battery is roughly the same. Would the following setup work? As far as voltage and current, is this just a parallel circuit, or some hybrid type? Thanks for any input.

View attachment 372284

 

On the issue of current sharing, I was thinking the only current flow between the batteries would be if one was a different charge than the other...?

Nope. In other circuits it is a common problem, but in your case, no.

With S1 closed, all three LED cathodes are connected to all three battery - terminals. There is no current path from any battery + terminal to any other + terminal.

ak

 

The circuits shown in the two posts might work, IFF, and ONLY IF, the wiring in each light is the same. BUT if one swith is wired in reverse then you will have a problem.
WHAT WILL WORK , if all of the items use the same number of the same kind of batteries, is to connect your external power source positive to the battery connection +terminal and your supply negative to all of the battery connections negative terminal. You would need to switch on each devices switch as well.

 

I have three LEDs each with their own battery. I want to control all three at the same time using one switch. I believe the charge in each battery is roughly the same. Would the following setup work? As far as voltage and current, is this just a parallel circuit, or some hybrid type? Thanks for any input.

View attachment 372284

Hi,

I posted a new diagram you should look at so you can understand this possible problem. Note the LEDs have been drawn like diodes because their operation more closely resembles that of a diode with a higher forward voltage characteristic.

As others have noted, you need resistors in series with each LED, but we might assume that your devices already have that built in.
Referring to the attachment, you can see the battery on the far right has a decent charge left in it as indicated by the blue line (higher up on the battery drawing means it has a decent charge). The two batteries to the left however have little or no charge left in them because they ran down before the battery on the right.
In this scenario, the current flows up out of the battery on the right and down into the two LEDs on the left as indicated with the red arrows. The current should be small, on the order of 10ua to maybe 100ua, but over time it is enough to run down the battery on the right, and if the battery voltages are high enough, it could burn out the two on the left.

The diagram is shown with the switch open, so this can happen even if the switch is 'off'. There will be leakage when the switch is 'on' as well, but it should be small relative to the forward current of the LED on the far right. It's mainly of possible concern when the switch is off and at least one battery voltage is higher than one other battery voltage.

There is a chance that it takes a long time to run the battery on the right down, but this is probably something you should check into, and also if the batteries have voltages of greater than 5v then you should not use this circuit.

BTW, they do make 3 pole switches available in many places on the web. That is the safest bet, but not sure if you can get parts easily.

 

Switching multiple packages that contain individual power sources by means of a single switch leaves power paths closed even with the switch open. HOPING that all three battery voltages are IDENTICAL is not usually adequate.

 

Welcome to AAC.

Since you intend to use an MCU it would seem a better choice to use 3 logic level N-Channel MOSFETS.

 

Here is a quick sketch of the equivalent of the post #1 circuit with the switch closed. As you can see, there is no current path among the three batteries. The single-wire connection is not enough to form a *circuit*.

Error: The battery polarity markings are correct, but the symbols are inverted. The wide bar should be on top.

ak

 

What makes the most sense is a single power source with the single switch. the multiple lights can be either in series or in parallel. Series may be more efficient, possibly, and often simpler. At least the way that I do wiring. Others results may differ.

 

Switching multiple packages wthat contain individual power sources by means of a single switch leaves power paths closed even with the switch open. HOPING that all three battery voltages are IDENTICAL is not usually adequate.

What power path is there in this circuit if the voltages are not identical? As long as the voltage difference does not exceed the reverse breakdown voltage of the LEDs, current won't flow.

 

Here is a quick sketch of the equivalent of the post #1 circuit with the switch closed. As you can see, there is no current path among the three batteries. The single-wire connection is not enough to form a *circuit*.

ak

View attachment 372332

Hi,

Not sure what you are saying here. Your diagram is a bit confusing too because you seem to have the batteries drawn with the positive terminals DOWN, but then on the far left you label one of the batteries with plus (+) on top (which would schematically be the negative terminal of the battery) and the negative lead on the bottom (which would normally be the positive lead).

You don't need the entire bunch connected you just need the three connected properly, which they seem to be in the first diagram in this thread.
But is your diagram even really equivalent? With the switch closed, all three cathodes are connected together and all the battery negative leads are connected together.
This does not mean it's the greatest idea to do it this way though.

See if the attachment makes the connections of the original circuit more clear.
Note that all of those circuits work, and the polarities of all the 'LEDs' are shown as plus and minus.

 

if the B1,B2,B3 voltage are same or similar, circuit from post #6 will work well enough.

the thing is that LEDs normally do not handle well reverse polarity. many of them can only tolerate couple of volts (5-6V). so if one of the batteries is much higher that then the others, then there will be an LED or two that will be stressed or possibly damaged (for example B1=9V, B2=6V, B3 = 24).
since B3-B2=18V and B3-B1=13V, both of which are greater than usual 5-6V max reverse voltage for LEDs, diodes LED1 and LED2 would be operating out of spec and likely get damaged. if the B3 circuit did not exist, difference B1-B2 is only 3V which is ok so LED1 and LED2 would be ok and no protection is needed.

to prevent issues due to significant voltage discrepancy one can use multi-pole switch or add regular diodes in series with batteries.

 

Your diagram is a bit confusing too because you seem to have the batteries drawn with the positive terminals DOWN, but then on the far left you label one of the batteries with plus (+) on top (which would schematically be the negative terminal of the battery) and the negative lead on the bottom (which would normally be the positive lead).

I learned the battery symbol incorrectly back in the 60's. That has plagued me ever since, lurking beneath the keyboard, waiting for me to post a quicky hand-drawn schematic.

I added an error note to the post.

ak

 

I learned the battery symbol incorrectly back in the 60's. That has plagued me ever since, lurking beneath the keyboard, waiting for me to post a quicky hand-drawn schematic.

I added an error note to the post.

ak

You've probably heard it before, but just in case, think of the short line as being the minus sign for the voltage.

 

You've probably heard it before, but just in case, think of the short line as being the minus sign for the voltage.

OR - *incorrectly* think of the short line as the button on the top of a AAA, AA, C, or D cell.

ak

 

Welcome to AAC.

Since you intend to use an MCU it would seem a better choice to use 3 logic level N-Channel MOSFETS.

Thanks for the suggestion! I assumed there was an alternative to the relay I was using for the switch, but wasn't sure what that was. Cheers