I purchased a couple 1Hz-150kHz signal generators off ebay: https://www.ebay.com/itm/Signal-Generator-PWM-Pulse-Frequency-Duty-Cycle-Adjustable-Module-LCD-3-3V-30V/372273269652?epid=4017774479&hash=item56ad374b94:g:DBUAAOSwiuxaytbq

I have been trying to use them to drive various transistors. Pretty much all of them are NPN, but they range from 90V 10A to 600V 400A IGBT. I have been trying to wrap my head around a basic circuit using LEDs and a DC motor but haven't had a ton of luck so far.

My basic understanding of a transistor is that it is a kind of solid state relay switch that is capable of cycling at incredible speeds. With that info it seems that in this circuit I should be able to replace the whole left side of the schematic with the signal generator from ebay and place any type of load where the LED is, whether it is a LED, Motor, EDM, Piezo, Induction furnace and so on. Then I could adjust the frequency using the simple generator rather than trying to adjust things with the 555 circuit.

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Assuming I wanted to use a transistor like this at 100V: https://www.digikey.com/product-detail/en/infineon-technologies/IRFB4110GPBF/IRFB4110GPBF-ND/2096587

What should I change in the diagram? Should I run 2 separate power supplies, or use a voltage regulator to drop from 100V to the 5-30V used by the generator?

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    \$\begingroup\$ if you want to use a voltage regulator to drop down that much voltage i hope it is a switching one, otherwise you will be dissipating a lot of heat. As far as driving the mosfet goes, i would suggest looking into mosfet drivers if you really plan on switching a big load relatively fast. There are gate to source and drain capacitances to worry about. Given those capacitances you also have to consider your switching frequency response. Also if you do plan to use a motor or inductive load you have to worry about the reverse inductive spike during the low side of your switching. \$\endgroup\$ Commented May 24, 2018 at 8:44
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    \$\begingroup\$ I think a little more info on your ultimate goal (load, input voltages etc...) is warranted, because a transistor is quite a bit more complicated than just a solid state relay. You may be able to hack something together and get it to "work" but then there is also the proper and safe way of going about it. Good read i stumbled across today: vishay.com/docs/68214/turnonprocess.pdf \$\endgroup\$ Commented May 24, 2018 at 8:47
  • \$\begingroup\$ Thank you Edwin, that was a great PDF to read. I need to do a little more work to define the projects as resistive vs inductive loads so I can do a better job of designing the circuit. \$\endgroup\$
    – Alan
    Commented May 25, 2018 at 22:05

2 Answers 2

  • If you want to stay safe, you should use a separate and isolated power supply for the signal generator. Use something like a wall wart.

  • The output of the signal generator can drive a typical low side switch device, albeit poorly. This includes N channel MOSFETs, power NPN BJTs, and IGBTs.

  • The signal generator probably can't source enough current to switch your switch on and off fast enough, and may not even be enough to turn it on fully (for the case of BJTs). Consider using a dedicated switch driver.


This is what worked for me; try as a first pass operate the PWM module at 12V. Connect two transistors 1 NPN and 1 PNP in the emitter follower configuration. Base to Base to PWM output of module. Emitter to Emitter to MOSFET Gate, Collector of NPN to +12 and collector of PNP to ground. You may have to add a 3.3K pull down to the gate depending on gate threshold levels. This should give you enough drive for most N channel MOSFETs. Connect the ground to the load Power supply Ground. I built one this way and it works great. I used a 75N80 MOSFET with a good avalanche rating. Works well. Be sure to decouple properly.


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