I am ordering some smt leds and some circuit boards for a project I am trying to create. I am not much of an electrician. What is the current draw or what type of power supply is needed for 20 smt led board.

The current draw for your LED board will depend on numerous factors. The first thing you need to consider is the information found on the data sheets associated with your LED. On the LED data sheets, the absolute maximum ratings will list the maximum safe drive current for the LED. You would never want to exceed this value. It is good practice to maintain a drive current well below this value. The second value to find on the data sheet is the typical forward current, which may be listed as the test current or test condition. This current is below the maximum rating, and LED manufacturer uses it during their test sessions. The advantage of the test condition current is that all of the other data is based on this drive current. If you choose this drive current, you can reference any item on the data sheet with confidence that the information is accurate, since the LED has already been tested at this level. If you chose a lower or higher current, you would need to calculate and then test to confirm the results, for unknown items such as forward voltage our luminous flux.

Most LED data sheets will also provide you with graphical information that allows you lookup information associated with other-than-typical drive currents. For example, suppose you wanted to drive your LED at 100mA, but the data sheet list a typical drive current of 150mA. You already know the luminous flux associated with the 150mA, but you need to find the rating associated with your current, 100mA. A basic graph shows the relationship between forward current and luminous flux, so you can easily find the data you are looking for. Be careful in this example, as you must also consider the affects of LED binning and ambient temperatures, on your flux.

After you choose a suitable drive current, you can calculate your total current based on the total number of LEDs on your board. Since current is common in series circuits, you would multiply your selected drive current by the total number of series circuits, not by the total number of LEDs. Consider a board with 20 SMT LEDs. If your LEDs are white, you might want to consider ten parallel circuits, each containing two white LEDs. Multiply your drive current (100mA) by your circuit count (ten) and your result is a 1000mA total current draw.

Now that you know your total current draw, you can continue on by choosing a suitable power supply. If your board does not include any LED drivers, you can use a single constant current source as your external power supply. If your board does feature LED driver circuity, a constant voltage source will work. The power supply voltage you choose depends on your LED circuit design. For the previous example with ten circuits of two LEDs each, you might want to consider a 9 or 12 volt power supply. If the power supply is constant current, the voltage will adjust to match the LED forward voltage, so you only have to be concerned with the maximum voltage rating. From a current standpoint, try not to exceed 75% of the power supply current rating. A power supply rated at 1.5 amps could be used to power your LED board drawing 1.0 amp, since your total current does not exceed 75% of 1.5 amps. Using this simple rule, you can ensure that your power supply will not be stressed, and will tend to operate over a wider range of ambient temperatures.