Led strip

[333rocky333]

Lately getting lots of failed led fittings, usually it appears the drivers have failed and in the emergency fittings the electronics failed.

The led elements, varying from say 10 elements to say 40 elements are usually on there own printed circuit with + and - marked on the end of the board.
These appear fine and I'm sure still ok.

I am sure it is not as simple as applying voltage across the end, as they are expecting a constant current aprox 350ma.

My question is can a simple circuit be made to run these strips of 12 volt DC to make up some basic light boxes that can be used in a van.
Thanks in advance

 

[bernardgreen]

LED strip like this

requires a DC power supply that is constant voltage. The current through the LED elements is controlled by the resistor. The current is typically 20mA per group. A group is one resistor and 3 LED elements. The number of groups will determine how much current will be taken from the power supply

An LED driver with a constant current output will force it's rated current onto the strip irrespective of how many groups are on the strip.

If the constant current is set at 350mA and the strip has 18 groups then the strip will take the rated current from the driver with approx 20 mA through each resistor. The voltage on the driver's output will be about 12 volts

If there are less than 18 groups then the driver's output voltage will increase. For example if there are 9 groups then the driver's 350 mA will require approx 40mA to flow through each group leading to premature failure of the LED elements The voltage on the driver's output will be about 24 volts.

My question is can a simple circuit be made to run these strips of 12 volt DC to make up some basic light boxes that can be used in a van.

Connecting the strip(s) to via a switch to either a 12 volt supply or a 12 volt battery is all that is needed.

 

[ericmark]

12 volt batteries are not really 12 volt, I have many times been tempted to use lamps designed for use with a 12 volt transformer or switch mode power supply, but much would depend on how critical the current is.

While in University we were tasked with experimenting by flashing LED's to make them appear brighter, the experiment was really a failure, as could not work out a way to measure the light as seen by the human eye, it seemed brighter by flashing the LED with a 50/50 mark space ratio, at double current, but the LUX meter averaged it out, so showed same output.

However as part of the experiment with did over drive LED's which then had a reduced output, and would not recover once over driven, so ensuring the LED is not over driven is rather important.

To use a L7812 chip we can get 12 volt from a higher voltage, 15 volt if I remember right, minimum hence why we have specials for boats and caravans companies like this specialise in lamps for boats and caravans clearly with some sort of switch mode regulations so voltage can be 10 - 30 volt.

Since a diode gives around 0.6 volt drop one could add around 3 diodes so can run off 13.8 volt, and just accept when battery not on change the lights will be slightly dimmer.

Both my makes of strip lights seem to have a separate 12 volt supply transformer and controller, the Lidi version seems to have two LED's for white lights, (plus RGB for colour) and one is a low colour temperature and the other a high colour temperature and to adjust the colour temperature it makes one brighter and the other dimmer, so the strip is capable of over twice the output if all powered at the same time, which would likely burn out the power supply.

In the TCP strips I have had a power supply fail, replaced under guarantee, how once out of guarantee one could get a replacement not so sure, as I had to order a new complete unit, swap the power supplies then return the complete unit, to avoid having to remove the strip from the cupboard.

It seems to have 5 feeds and one return, but as to how much current regulations is done in the strip as shown by @bernardgreen and how much is in the controller is hard to tell.

 

[bernardgreen]

, as could not work out a way to measure the light as seen by the human eye, it seemed brighter by flashing the LED

You can use two identical LED elements, side by side, one supplied with steady state DC and the other supplied with pulsed DC from a source with variable voltage, variable mark-space ratio and variable frequency. People asked which is the brighter of the two LED elements.

This method was used to determine how to get an acceptably bright flashing LED warning beacon with minimum power from the battery to achieve a longer battery life,

 

[ericmark]

The problem is photo sensitive people, flashing lights at a high frequancy can cause problems, so really a non starter.

 

[bernardgreen]

flashing lights at a high frequancy can cause problems

A lot of vehicle lights use the pulsed method to produce a "brighter" illumination without over cooking the LED element(s). Strobe effect in a video camera can make a headlamp look like a very bright indicator.