| Electronics > Projects, Designs, and Technical Stuff |
| IN-1 nixie tube - fuzzy digits when driving with TPIC6B595 |
| (1/5) > >> |
| 2shy:
Hi, inspired by Dave's series of videos I designed schematic and PCB of a simple Nixie clock and I hope that someone will be able to explain what's happening. The problem is that when I'm driving the cathodes with TPIC6B595 shift register, some of the digits are not readable/fuzzy. It's worth noticing that on different tubes different digits (in the same driving circuit) get fuzzy, eg. cathode 0 and 7, the rest is okay. I'm attaching a photo of a blurry/fuzzy digit and a well displayed digit, as well as part of the schematic. HV is 180 V DC, series resistors are 2x10k, I'm switching on the anode voltage using a pair of MPSA42 and 92. I tried to lower down the anode voltage to 165 V, but it seems that the problem remained. What I've done: I checked individual tubes with a resistor and HV power supply - they're all ok. I checked the TPIC6B595 outputs with a LED and series resistor - they're all ok, I'm driving one cathode at a time, one of them is always on, I'm filling the registers first, then turning on the anode supply, never the other way round. The displayed sequence is 0-3-4-5-6-7-8-9-0-..., because cathodes 1 and 2 are connected to the next register, which is always filled with 0's. This is for test purpose only, as I have only two shift registers soldered to the PCB for now (the nixies can be just put into PCB for now, they form a tight fit) I also recorded a video: https://youtu.be/ibOdviAl_b4 Digit 7 is not displayed correctly. If I check this tube directly with a resistor and HV power supply, digit 7 lights up correctly. Replacing tubes result in a different digit being blurry. Here is the test code (STM32): --- Code: ---void test_load_and_latch_two_registers(uint8_t output_number) { uint8_t value_to_load, j; if(output_number > 7) return; value_to_load = (1 << output_number); // second register fill with 0 for(j = 0; j < 8; j++) { set_shift_register_clock(GPIO_PIN_RESET); HAL_Delay(1); set_data(GPIO_PIN_RESET); HAL_Delay(1); set_shift_register_clock(GPIO_PIN_SET); HAL_Delay(1); } set_shift_register_clock(GPIO_PIN_RESET); // first register fill with data for(j = 0; j < 8; j++) { set_shift_register_clock(GPIO_PIN_RESET); HAL_Delay(1); if(value_to_load & (1 << j)) set_data(GPIO_PIN_SET); else set_data(GPIO_PIN_RESET); HAL_Delay(1); set_shift_register_clock(GPIO_PIN_SET); HAL_Delay(1); } set_shift_register_clock(GPIO_PIN_RESET); latch_shift_register(); HAL_Delay(1000); } --- End code --- I suppose there's a problem with floating cathodes voltage, but have no idea what is happening in this circuit. I measured voltages when a single digit is displayed in two cases. Digit 5 displayed correctly: A - 135 V C0 - 123 V C1 - 66 V C2 - 124 V C3 - 54 V C4 - 102 V C5 - 0 V C6 - 54 V C7 - 103 V C8 - 54 V C9 - 54 V Shouldn't all off cathodes be around 50-55 V? Digit 0 displayed incorrectly: A - 169 V (way too high?) C0 - 0 V C1 - 58 V C2 - 82 V C3 - 54 V C4 - 54 V C5 - 54 V C6 - 53 V C7 - 53 V C8 - 54 V C9 - 54 V As far as I understand, the off cathodes voltages in ideal case should be equal around anode voltage, but the internal clamping of TPIC6B595 should drop this floating voltage to around 50-55 V. But it seems that it doesn't work like that. I would be grateful for any hints on this, guys. |
| MasterT:
Leakage collector current of T9, probably? I'd try to lower R24 down to 2-3k |
| 2shy:
I lowered R24 to 3k3, it didn't help, actually, two more digits become fuzzy. I lowered also 220k resistors to 2x100k and 2x47k, it didn't help neither. Then I increased 220k resistors to 470k, still no change. In the evening I'll remove T3 and connect the supply directly to 180 V, to eliminate the anode driving circuit. |
| StillTrying:
"Shouldn't all off cathodes be around 50-55 V?" Shouldn't they be higher. :) Are the TPIC6B595 outputs only 50V. https://www.ti.com/lit/ds/symlink/tpic6b595.pdf |
| Prehistoricman:
Unless I'm completely misunderstanding this circuit, I don't see why the issue would have anything to do with T9, T3, or any of the plate driving circuit. You could eliminate the possibility by checking the plate voltage for both the faulty and correct states. I think your fault is much more likely to be in the shift registers. Try keeping it on the fuzzy digit and poke around at the circuit to see if you can fix it. Does shorting an 'off' digit to ground change anything? See how much current each digit is consuming (if you can be bothered :)). |
| Navigation |
| Message Index |
| Next page |