selecting correct wall wart

[justwantin]

I have finalised a little project involving 2 arrays of LEDs and need to select a 12V wall wart to power it. I have been testing with a homemade 1.5-30V powersupply that is supposed to deliver at around 1.5A but I don't really know what the actuall current is at the 12.5 I've been testing at. I am using 2 lm317's to provide 7V to one array and 5V to the other. The online array wizard I used to check my array and resistor sizes says I would be drawing 700mA and 200mA respectively.

Both lm317's have heatsinks and are just a slightly warmer than ambient to touch at, say, 20-25C after several hours running.

Would a 12V 1A wallwart be suitable for this appilcation?

 

[bigal_scorpio]

Hi Mate,

Yes 12v 1A should be fine, but beware of the type of wall wart, if its heavy it will be a transformer type which will be fine, if its light it will be switch mode and not all will work with things like LEDs as they do not have a large enough resistive load!

You can sometimes get around this by having a small resistive load in parallel with your circuit, but not always.

Hope this helps..............Al

 

[justwantin]

Thanks for that.

Would there be a problem with increasing in put to say 12V, 1.5A or 2.5A as long as my heat sinks handle the excess?.

I haven't quite wrapped my head around current yet but I believe my bench top power supply is working at >=1A and the LEDs are bright but within spec and the circuit works as intended but I have no way of checking just how much current is actually required.

Hooking up my multimeter to check amperage between PS and circuit will kill the array of 20 5mm LEDs but the 4 X 1 watt stars will remain lit. Voltage is correct 7V and 5V to both but I'm thinking that the current may be marginal.

 

[crutschow]

You can use a larger current capacity wall wart without problem as long as it is the same voltage. It won't affect the current or the heat sink dissipation. The LED circuits take only the current required.

When checking the current, your multimeter may have too high a resistance which is reducing the voltage to your LEDs and that's why they are dim or go out. As long as you have the correct voltage, the LEDs will draw the design current.

Current is determined by the voltage across the resistance in the circuit (ohm's law: I = V/R). If there are LEDs in series with a resistor, then you subtract the LED voltage from the supply voltage to get the voltage drop across the resistor.

 

[justwantin]

I have three settings for measuring Amps om my multimeter, A, mA, uA. I can only measure on this circuit with mA on this multimeter but I have been wondering for other reasons if the multimeter is going bad and will have to check it against another one. Perhaps I have to check amperage after the lm317's instead of before the input pins on the board (before both lm317s) but I would expect that this would not factor in the resistance created by the 2 lm3i7s and their 3 resistors each.

 

[crutschow]

The mA scale will typically have a 1Ω shunt resistance which drops 200mV at the full scale of 200mA.

If you A scale doesn't work you may have a blown fuse.

Measuring the current at the input of the 317s is good. That way the ammeter voltage drop does not affect the voltage to the LEDs. The difference in current between the input and output of a 317 is less than 10mA for typical circuit resistor values.

 

[justwantin]

I did a visual on the fuses a couple days ago when I changed the batteries. and they looked ok but I'll do a continuity of the fuses tonight.

I now get both arrays working but the amperage doesn't make sense I start at around 300 then fall to an average of 260mA. I say average because the 20 5mmLEDs are are in 5 arrays of 4 ea and switched randomly with a microchip. That being said the 4 1w LEDs are supposed to draw around 700.

Thanks for the input I have to go to work now.