Hi, I'm trying to test frequency response of my transistors (2n2222). So far I have destroyed 3 boards. I am supplying signal via microcontroller and switching 12v with transistor. Base resistor is 20k, collector resistor is 1k. Capacitor is 100nf (to reduce storage time). Frequency I'm testing is between 100khz to 300khz. My circuit is Fig 02.

 

What is being destroyed on your boards?

 

What is being destroyed on your boards?

Microcontroller chips. They get very hot and eventually blue smoke. It worked fine for some time. I was able to see nice waveform on oscilloscope.

 

Microcontroller chips. They get very hot and eventually blue smoke. It worked fine for some time. I was able to see nice waveform on oscilloscope.

That tells me you are overvolting them, by a lot. I would suspect the 12vdc (or some sizable fraction of that voltage) is somehow being connected to the controller power bus while you're are probing or reconfiguring circuits.

 

That tells me you are overvolting them, by a lot. I would suspect the 12vdc (or some sizable fraction of that voltage) is somehow being connected to the controller power bus while you're are probing or reconfiguring circuits.

Hmmm. That seems likely. I might have shorted microcontroller when connecting capacitor/resistor. I can't believe I did it 3 times. Thank you very much for your help. I will get some more microcontrollers and try to be very careful and make put more space between components.

 

Hmmm. That seems likely. I might have shorted microcontroller when connecting capacitor/resistor. I can't believe I did it 3 times. Thank you very much for your help. I will get some more microcontrollers and try to be very careful and make put more space between components.

It's happened to all of us a few times. I usually design a PCB with zener voltage overload protection on the controller power bus to reduce the chance of a blown controller.


Turn off or completely disconnect the 12vdc power until you are sure things are connected correctly and use current limiting to limit power if the 12vdc power supply has it.

 

most common regulators are series regulators. they cannot handle situation when something injects current into its output, causing output to float high and smoke everything that is vulnerable. so yeah... it is a good idea to add even most basic crowbar circuit.

 

Or... Lower your transistor voltage to 5V... You don't need 12V to check frequency response/Bode plot.

 

These oversize screw heads look like they might cause shorts on the board- take care here.

 

These oversize screw heads look like they might cause shorts on the board- take care here.
View attachment 320115

They are tapered. It was 12v accidentally shorted to microcontroller pin (3 times). Im on my last Arduino nano but it's been running fine for hours.

 

Before turning on the power, please measure the resistance between 12V and 5V on the board, using I=(12V-5V)/R to check the current flow from 12V to 5V.

 

Don't replace the whole board. Either (1) buy the chips and a hot-air station (under $50) and learn to swap the chips and write a bootloader to them. You'll need this skill some day anyway and the chips should be very cheap at a place like Digikey/Mouser/etc.. Or (2) get the Arduino Uno board model that uses the DIP style chips, then you can just pop the old one out, pop the new one in, write the boot loader and continue. We all break stuff, learning to repair it will save you a ton of money going forward.

 

also get a bench supply. you can have one for as little as $30-40 with 4 digit voltage and current displays. then you can set current limit low enough that will allow you to test things without destroying them.

 

also get a bench supply. you can have one for as little as $30-40 with 4 digit voltage and current displays. then you can set current limit low enough that will allow you to test things without destroying them.

Along these lines, if you've got the pockets, I very highly recommend a power supply such as the Rigol DP832, or equivalent by whatever brand you like. These give you the option to set voltage and current limits that instantly turn off the channel if the values are surpassed. So if your circuit should never draw more than 50mA, set the limit at 50mA and if the circuit draws 51mA, it instantly turns off, potentially saving you parts. This has saved me several times over the years.