IRF540 1N4148 WOES

Thread Starter

Craps1956

Joined Aug 21, 2015
31
Hi everybody, I need help with the following.

I have assembled the following circuit (Fig 1), originally obtained from:- https://www.twovolt.com/2016/11/30/10-channel-chaser-mosfets-output-driver-using-lm555-cd4017/

The only difference being R4 to R13 which are 470R because I did not have any 100R resistors at the time of assembly.
It is however working well and flashes sequentially from Q0 through to Q9.

I would like to change the flash pattern by using some 1N4148 diodes as in so many other similar circuits that may be found on the net. (ie. Fig 2)

I always make very small changes/additions/modifications to existing circuits because it is easier for me to see if I am going in the right direction so I started off by dropping in the first 1N4148, (D1, Fig 3).

And now here is the problem that I am struggling with. The LED array 1 attached to Q1, D1, R4 remains light while the rest correctly continue to flash sequentially one by one.

When I disconnect the gate of Q1 from the D1 cathode and connect it to D1 anode (effectively removing D1), everything works correctly.

I have tried swopping the direction of D1 but the LED array 1 remains off while the rest correctly continue to flash sequentially one by one.

I know that with other 4017/555 circuits the 1N4148 work with or without driver transistors, (Fig2). I don’t know too much about mosfets so I am guessing at what the problem may be. The diode voltage drop, wrong type of diode, wrong resistor values, too much or too little or no voltage at Q1 gate? Or is it something else? The 4017 is moving so fast its hard to get a steady voltage reading with a dvm.

Any help here would be much appreciated

Thanks guys
 

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crutschow

Joined Mar 14, 2008
26,726
A MOSFET gate looks like a small capacitor, so will retain the voltage last applied to it if it's not discharged in some manner.
A resistor to ground will work for that, as JBE suggested.
 

wayneh

Joined Sep 9, 2010
17,024
Choose the resistor based on how fast you need the MOSFET to turn off. If you're flashing them at a rate you can see (ie. slowly, <40Hz or so), even a 1MΩ resistor will work. You are limited on the other end by the 4017 output current limit. A <1KΩ resistor might make it difficult for the 4017 to get the gate voltage up high enough to fully turn on.

For a large load, you want the MOSFET to turn off and on quickly, to avoid being in between where it would dissipate a lot of heat.
 

ElectricSpidey

Joined Dec 2, 2017
1,525
I would switch up to OR gates to set my pattern instead of using diodes, that way the chip can discharge the gates instead of needing pull resistors.

All you need is a single 4071B to create the pattern in your first image.
 
Last edited:

sarahMCML

Joined May 11, 2019
32
Choose the resistor based on how fast you need the MOSFET to turn off. If you're flashing them at a rate you can see (ie. slowly, <40Hz or so), even a 1MΩ resistor will work. You are limited on the other end by the 4017 output current limit. A <1KΩ resistor might make it difficult for the 4017 to get the gate voltage up high enough to fully turn on.

For a large load, you want the MOSFET to turn off and on quickly, to avoid being in between where it would dissipate a lot of heat.
Another question, related to your LED's. Are they fixed modules, or can the individual LED's be wired in series? That would allow you to drive them with much less wasted power in a 5 ohm limiting resistor.
 

Thread Starter

Craps1956

Joined Aug 21, 2015
31
Another question, related to your LED's. Are they fixed modules, or can the individual LED's be wired in series? That would allow you to drive them with much less wasted power in a 5 ohm limiting resistor.
Thanks to all for your input, I now know a little bit more about mosfets, especially the "capacitor factor" and the need to discharge the it. I got a little further in the project and will experiment some more over the next few weeks in my spare time. Thank you all
 
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