5W led star

[homersimpson2473]

The specs of my LED are White / 7-7.5v / 700ma LED star. I built this driver here on page28 5W led driver. http://www.st.com/stonline/products/literature/an/11777.pdf I built it just like the schematic. If its for my type of led or not I dont know but my guess is no. anyways my input voltage 12v and the output to the led is 11.85 volts. HOW DO I GET THE OUTPUT VOLTAGE TO 7V @ 700MA FOR MY LED? What do i need to do next please.

 

[HarryN]

Hi, welcome to cpf.

This is a good question for the battery / electronics section of the forum. Perhaps one of the moderators will move it for us.

I guess the first step is to ask a few questions:
- Is the LED is actually hooked up or not ?
- How are you measuring the voltage (exactly)?
- Is there a load on the output of the circuit - like an LED or resistor ?
- How confident are you about the components being all in working order ?
- Do you have an O scope ?
- Is this your first electronics build or are you an experienced EE ?


Please stay with us on this, it will take only a very little bit of pain to get you going here.

 

[homersimpson2473]

1)where you hook up the led, the voltage is 11.85 volts so I did not hook up the led because I did not want to burn it up.
2)I am measuring the pos led / neg led points on the circuit board using a meter and getting 11.85 volts
3)I have not put on a load to the circuit
4)these are new parts. I checked the ones that can be checked with a meter or replaced ones with new ones that cannot be checked with a meter.
5)No O scope
6)not a EE

 

[HarryN]

My guess is that the circuit is going into "direct drive" attempting to "push" the 700ma through the system.

Try putting a resistor load on the system and measure again. A few ohms will do it.

BTW, if you try to run "open circuit " with a boost regulator, it will destroy it very fast.

 

[tommylight]

That is inductive switch mode power supply, and as such it doesn't function properly without load connected at the output, especially in this case where it needs the feedback for current limiting.
Connect a incan light bulb to it and measure the voltage, an 20W hallogen would do nicely so you can know if the limiting is working properly ( it should light but be quite dim ) and measure the voltage, just to be sure it is inside the limits, it will be less than 3 V with the halogen, but at least you know the circuit is functioning properly and limiting current.

 

[homersimpson2473]

I have hooked up a 12v car dome light to it and I get 11.56volts and .020 or 20 milliamps. I have a choke in the circuit that is 68uh but might be the wrong one cuz it is as small as a 1/4watt resistor. I have been told to change it out for one that is about 1 to 1 1/2 amps or so. Does the choke have any effect on current or voltage output?

 

[tommylight]

Does your dome light come on at all ??? at 20 miliamps it should not ! I assume you do know how the current is measured, so there is definitely something wrong there. The Induction coil is very important, but very easy to make yourself just google for how to.
Do another check of everything, check if there is any alternative current on the output ( connect incan, turn on, and measure AC V, )if it is working it should give you a reading of anything between 100 and 300 mV in best cases.