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Damaged FETs?

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Overclocked

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Ive come up with the following schematic for a high-powered RGB LED controller using WS2801. For some reason the Colors flicker a lot, and are very visible. They are pretty much flashing. I tested the same circuit with a smaller LED and it doesnt flicker between changes (20mA current draw). The larger LEDs consume 350mA. The smaller LED works fine and changes through colors as it should.

Is it possible I damaged the FETs during soldering? The LEDs aren't as bright as they should be. I also noticed that when I connect a cap from each channel to ground, the flicker sort of goes away and the LEDs are a lot brighter. What is the likely hood of damaging a FET during handling?

I assembled the circuit on a piece of protoboard from radioshack.


 

Les Jones

Well-Known Member
Most Helpful Member
You do not seem to have any way of controlling the current through the LEDs. You are not using the feedback pins on the WS2801 and you do not have any current limiting resistors in series with the LEDs Check the 5 volts line with a scope to make sure it is not being dragged down by the uncontrolled current drawn by the LEDs

Les.
 

Overclocked

Member
I have a current limiting resistor on the cathode's of the RGB LED, 5 ohms for Blue and Green and 8 Ohms for Red. For external switch driving POL is driven to ground and the current feedback resistors arent used according to the datasheet. The only difference is I used NFETs instead of NPN transistors.
 

KeepItSimpleStupid

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With a low supply voltage 0.6 V matters (bipolar transistor). It's 12% of the supply voltage and even more percentage of Vf. Vf varies with color. Red, typically being the lowest Vf or brightest.
 

ronsimpson

Well-Known Member
Most Helpful Member
Data sheet attached.
I can not see the picture on post #1. Can you send it in a different format?
OR
Did you copy one of the pictures from the data sheet?
To get help you need to give us all the data. What FET part number?
 

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audioguru

Well-Known Member
Most Helpful Member
The datasheet has pretty good Chinglish. But I knew it was a Chinese document translated into English.
 

Overclocked

Member
I think I fixed it. Built it on a breadboard and tried different transistors. I tried some 2N5551 transistors I had floating around. I used 1k Pullup resistors on the bases. Its been working fine with no Hiccups (literally) for the past few hours on said bread board. I'll transfer it over to the Perf Board and also fix the current limiting resistors.
is pretty high though-around 2V.
 
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