| Electronics > Beginners |
| Trying to determine what voltages should be in a circuit |
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| gmc:
I'm busy trying to build a arduino based Hakko T12 soldering iron. I have been using the design from: https://www.hackster.io/sfrwmaker/soldering-iron-controller-for-hakko-t12-tips-on-arduino-f7a888 Wired everything up and there is some issue with the heating circuit....it's not working. I've measured the voltages as shown below: The problem I am getting is the 100R resistor in the middle of the pic is getting very hot. Voltage across it is 8.2V, so 0.6W. The circuit design uses a 1/4W resistor so I'm guessing its not supposed to get that hot and I have messed up something. Emitter voltage of 21V is incorrect, or gate voltage of 12.8 is wrong...or both. Double checked everything and still not getting anywhere. Looking at the circuit is it possible to work out that the voltages should be. I've tied the input low for testing, from the PWM (Not connected to the arduino) I'm using a IRF540, with general purpose transistors. BC547 for NPN, and BC327 PNP. Also the iron is not connected. The purpose of the pnp/npn connected to each other also has me confused. |
| bob91343:
You can't have 12.7V on the base and 21 V on the emitter. You say both transistors are KSP10 but one is NPN and the other PNP. Looks like a miswire somewhere. |
| AVGresponding:
The pnp is an S9012H, not KSP10. Are you measuring the voltages on this circuit with no PWM input? If so, that's part of the problem, though there's another issue as the gate of the fet shouldn't be drawing enough current to cause an 8v drop on the gate resistor. Gate current should be effectively 0. Check the drain and source are the right way around also |
| gmc:
Made a typo with the voltages. Should be 21.7V on the base. I've re-checked the drain and source and they are correct. Circuit is with no PWM connected. |
| fourfathom:
--- Quote from: ThickPhilM on September 26, 2019, 05:36:42 pm ---there's another issue as the gate of the fet shouldn't be drawing enough current to cause an 8v drop on the gate resistor. Gate current should be effectively 0. --- End quote --- That current through the resistor is flowing through the 18V zener, which I assume is there to protect the FET from excessive gate voltage (I haven't checked the FET specs). There's no good reason that gate resistor needs to be 100 Ohm. It could probably be 10K, and then it wouldn't be burning up. |
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