PID Arduino Soldering Station - MOSFET

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
Hello.
I built a soldering station based on a PID controller for a Gordak soldering iron. I used a Nokia 5110 LCD as a display. The schematic and the code are attached. The problem is that I tried two times using different IRF530 transistors and the heater of the soldering iron was staying cold. When I used a K1257 or a P55NE06, the soldering station worked normally. I also tried yesterday on a breadboard the entire soldering station and it worked using a IRF630.
Why the soldering station was not able to heat my soldering iron when I was using IRF530 ?

This is the original project: https://www.allaboutcircuits.com/projects/do-it-yourself-soldering-station-with-an-atmega8/
My code: https://pastebin.com/uuDs7ZB0
The schematic is attached.
 

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Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
Is there a solution for this problem ? Or should I just use the mosfet that works fine ?
I looked at the characteristics and I found some information, but I don't know how to put in practice...
 

ericgibbs

Joined Jan 29, 2010
9,852
hi mike,
Assuming a MCU PWM drive of say 5V, have you calculated the Vgs loss due to R2 and R3.?
What do you make the drop across the 100R.?
E
 

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
I increased the 1k (R3- the resistor from gate to source) to 10k and now it is working with IRF530. The transistor is not getting hot even without heatsink. I also tested the circuit with IRF630 and it works fine.
 

ericgibbs

Joined Jan 29, 2010
9,852
hi mike,
You were dropping from 5V to 5V *(1000/(1000+100)) = 5V * (1000/1100) =4.5V
So that 'extra' 0.5V' when using a 100R and 10K is just enough to turn on the IRF530 a little harder.
E

BTW: R3 is from Gate to 0V.
 

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
I notified right now that the IRF530 is heating up a little bit, after a few minutes of working, making a 3.5cm x 5.5cm aluminum heat sink to be at around 50-51 degrees C.
It is normal to heat up like this, even if it is used only at switching the power on/off ?
 

ericgibbs

Joined Jan 29, 2010
9,852
hi mike,
A sink temperature of around 50C is workable, if you can increase the Vgs drive voltage, so that the MOSFET turns harder On, the temperature of the MOSFET should reduce.
Try an experiment using different MOSFET's and measure the temperature.
I would recommend you use and specify a IRF630 for your design.
Lets know what the temperature is for that device.
E
Image for IRF630
 

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Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
Using the IRF630, the temperature of the mosfet was rising faster than the IRF530, and the temperature of the heatsink, after stabilization is 66-70 C.
Should I try to increase R3 (10K), to 100K ?
 

iimagine

Joined Dec 20, 2010
479
MOSFET must be turned on hard to minimize dissipation during conduction, otherwise it will have a high resistance during conduction and will dissipate considerable power as heat.
The solution in post #6 would provide 12V Vgs which is more than enough to fully turn it on.
 

ericgibbs

Joined Jan 29, 2010
9,852
hi,
Obviously the IRF630 MOSFET is passing more current so you are getting the solder iron temperature you require.
You can experiment with a100k.
E
 

ericgibbs

Joined Jan 29, 2010
9,852
hi mike,
Post #6 circuit would work, but you will have to change the sense of the PWM, also the Base resistor of 100K is much too high, use a 4k7
E

Edit:
On that circuit I assume that the 100R in the MOSFET Drain is the solder iron resistance.
 

Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
I tried with the attached schematic. It worked, but the IRF630 is getting hot again.
The IRF530 is not heating up as much as IRF630 and I can keep my finger on it.
On the schematic I used: R2=1k, QH=BC337, QL=BC327, Ro=22R.
I wanted to try this schematic because I think it works better instead of directly driving the mosfet.
Edit: Vs was 5V.
 

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Thread Starter

mike_the_begginer

Joined Dec 7, 2019
36
I used 5V supply and the 5V logic from the Arduino for that circuit.
Yes, I have an oscilloscope.
I attached the schematic.

I am attaching the output of the MCU (blue trace) and the G of the mosfet (yellow trace).
 

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ericgibbs

Joined Jan 29, 2010
9,852
hi,
OK, so please measure and post the MOSFET Drain voltage, find the low level value when the MOSFET is switch On, let us know.
E
 
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