There is no resistor between the 74HC595 and the LEDs. ... The LEDs are connected by a common cathode and all the current is sunk by an IRFS630B MOSFET (200V, 9A).
The LEDs are connected by a common cathode and all the current is sunk by an IRFS630B MOSFET (200V, 9A).
I was thinking "how can I save soldering 32 resistors?" and thought of doing what you did - a constant current sink. But I decided against it on the grounds that the current being sunk would be shared, and the more LEDs that lit up, the more current would need to be sunk, and that the results wouldn't be even. Your report appears to confirm that.Perhaps you can clarify how you are managing the current. In the absence of resistors are you going for a constant current sink, or relying on PWM to limit it?
The most current management I'm doing is switching it with the MOSFET which can take up to 9A
The wire (ribbon cable) in the picture is 30 Ga wire and the current is being sources/sunk through several feet of what is essentially data grade wire
QuoteThe most current management I'm doing is switching it with the MOSFET which can take up to 9AThis means you will end up frying your shift registers.I am having a bit of trouble down loading the data sheet for that FET, but almost certainly it is not a logic level FET and requires 10V to fully turn it on. You are supplying it with 5V and so it has a high, for a FET resistance. This is fine for small currents, but when a large current flows, with all your LEDs on the resistance drops a larger voltage and restrict what is flowing.EDIT - just checked the data sheet and the FET is your problem, the Gate Threshold is only when the FET starts to turn on, not when it is fully on, you need 10V on the gate to get an on resistance of 0.4R which tells me it is a very old device newer devices have a much lower Ron ( resistance when on ) than this. So either boost the gate voltage or get another FET.
Can you elaborate on frying the shift registers?
Right now I've been able to get away with no resistors between the shift register outputs and the LEDs
Also how will changing the transistor affect the current?
However, I measured the current at the output of one shift register pin (i.e. one LED) and it was like 1.5mA while lit.
any ideas what series to go with