Never Trust a Schematic Blindly

Last week I posted a long winded video on using an oscilloscope to evaluate the performance of two different transistors. The 2n2222 transistor was behaving oddly at the top end, and the MPS A42 transistor had a rather significant turn-on delay.

After discussing my findings with someone who knows much more about these things than I, I was advised to take another look at the circuit. The A42’s performance did not seem correct. (Turns out before being used as a video driver transistor, it’s 300V capability was used to switch tubes). Below is the circuit I used as a template in my testing. I used an FET I had on hand and initially replaced the 2n3903 with a 2n2222 and later with an MPS A42.

Simple PWM FET driver circuit

On a hunch I decided to replace R1 with a 10k trimmer. Bringing R1 down to about 300 ohms fixed the delay issues for the A42 and for both of the transistors and significantly squared up the corners for both.

Moral of the story… don’t blindly trust someone else’s schematic to work for you. Test. Measure. Evaluate. Fix and re-test. For my needs, replacing Q2 with a 2n2222 and R1 with a 330 Ohm resistor generates the perfect PWM driver. YMMV.

Nice Square Waves


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