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Circuit LED Problem ???

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snaoz

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HI

I made a simple circuit which works great except one thing. The problem is with the LED, when the MOSFET kicks in without a load on it except the indication LED, the LED works fine (no dimming), when i connect another load to the MOSFET like a small relay, it works fine except the LED now suddenly dims (goes almost fully dark) when the MOSFET is energized and slowly brightens to about 80% of the brightens that i would get when only load on the MOSFET is the LED by itself. What am i missing here ???

I have attached a hand drawn diagram of the circuit



Let me know in case i didn't make my self clear :oops:

Thanks in advance :)
 
Last edited:

Hi,
With a +5V supply the max Vout from a LM324 is 5v -1.5V = 3.5V
The Vgs threshold for a IRF520 is 4V, so its not being fully turned ON.
Modify the LM317 resistors to give say 9Vsup, then retry.
E
 

esp1 said:
Hi,
With a +5V supply the max Vout from a LM324 is 5v -1.5V = 3.5V
The Vgs threshold for a IRF520 is 4V, so its not being fully turned ON.
Modify the LM317 resistors to give say 9Vsup, then retry.
E
Click to expand...

the IRF520 specs says 2-4 V threshold, am i mistakenly understanding this ?.
 

hi,
The possible range of Vgs is the problem, you may have a 4Vgs threshold version, its important to note that just satisfying the Vgs threshold voltage will not give a high Ids [ Source to Drain ] current.
So when you add an extra current sink to the circuit as you described, there is insufficient Ids to meet this extra demand.
E
 

esp1 said:
hi,
The possible range of Vgs is the problem, you may have a 4Vgs threshold version, its important to note that just satisfying the Vgs threshold voltage will not give a high Ids [ Source to Drain ] current.
So when you add an extra current sink to the circuit as you described, there is insufficient Ids to meet this extra demand.
E
Click to expand...

I got the picture :grin:. But why does the brightening occurs, is it because of the mosfet capacitvily charging up and as it does that the LED gets brighter ?
 

The LM317 needs a 120 ohm resistor from its output to its ADJ terminal. Then the other resistor value needs to be re-calculated. Its datasheet shows 240 ohms for the more expensive LM117 on every schematic.

Use a "logic-level" Mosfet like a IRL510 (see the "L"?) instead of a 10V level IRF510 when the gate voltage is as low as +5V. But with your supply of only +4.8V the output of the LM324 opamp goes as high as only+3.6V so even a logic-level Mosfet might not turn on completely. Increase the supply voltage to 9V.

Heating in the Mosfet caused the LED brightening when your Mosfet was barely turned on.
 

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