Hello everyone:
I got some VFD tubes, and have some MAX7219 led controllers which I want to use to drive the tubes. The problem here is that the MAX7219 is designed to drive common cathode 7-segment led displays, so the pins for driving the segments are at 5V+, and the pins for multiplexing between the digits sinks current to the ground.
To interface the MAX7219 with the VFD segments I'm using a UDN2981 (8-Channel source driver) supplied with 15V. This part is working.
The problem resides with the driving of the grids, that also needs 15V, so I'm using a PNP transistor as shown in the schematic. If I measure with the oscilloscope in "A" (one of the pins MAX7219 for driving the digits) I get the expected pulsed DC signal (5V-0V). But if i directly measure it on the transistor base, or at the VFD grid, I get a DC constant current of 5V or 15V respectively, so I cannot multiplex.
What am I doing wrong?? Why am I not seeing the multiplexing pulses on the VFD grid?
Thank you in advance for your answers!
gotramaval
I got some VFD tubes, and have some MAX7219 led controllers which I want to use to drive the tubes. The problem here is that the MAX7219 is designed to drive common cathode 7-segment led displays, so the pins for driving the segments are at 5V+, and the pins for multiplexing between the digits sinks current to the ground.
To interface the MAX7219 with the VFD segments I'm using a UDN2981 (8-Channel source driver) supplied with 15V. This part is working.
The problem resides with the driving of the grids, that also needs 15V, so I'm using a PNP transistor as shown in the schematic. If I measure with the oscilloscope in "A" (one of the pins MAX7219 for driving the digits) I get the expected pulsed DC signal (5V-0V). But if i directly measure it on the transistor base, or at the VFD grid, I get a DC constant current of 5V or 15V respectively, so I cannot multiplex.
What am I doing wrong?? Why am I not seeing the multiplexing pulses on the VFD grid?
Thank you in advance for your answers!
gotramaval
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