All good, though the ULN parts are bipolar devices, so there should not be any fear of them getting damaged by over voltage, unless it's an extreme over voltage that punctures the field oxide (way, WAY thicker than gate oxide in MOS devices).
The 2 diodes on the HV series parts (CMOS) are actually parasitic devices that are part of how the IC is manufactured. They provide ESD protection as long as there is some sort of path between VPP and GND, and they will protect you from inductive loads, though I don't expect a lot of Ldi/dt on those parts because they are rather low current compared to the ULN devices. I think a zener clamp on VPP is a good idea because it will rapidly bleed-off any excess voltage better than resistors. However, the zener wont bleed-off voltage to zero so I usually put a 1-10Meg resistor across them. On my clocks I put bleeder resistors from every power supply to chassis GND to protect from any potential ESD and to ensure every supply gets fully discharged when powered-off, and stays fully discharged. I've only had 1 Raspberry Pi fail, presumably from ESD, because it had an unknown history from the person who gave me several of them. On Saturday, March 8, 2025 at 3:26:00 PM UTC+7 Ian Sparkes wrote: > I started using ULN series devices during the supply chain problems after > Covid and won't be going back. Many "specialist" parts were not in stock > and so I decided to use "jelly bean" parts with multiple suppliers. A > driver circuit with ULN2003 or ULN2008 with appropriate clamping works > well. I clamp to 68V, and while this is above the specification for the ULN > device, I have still to see a single failure after many hundred units sold. > > Many people think that because you have 170V or whatever across the tube > that the driver has to stand this. This is not right. The tube digit has > two modes: off or on. When the digit is off, there is practically no > potential across driver because there is no current flowing. When it is on, > you're pulling the cathode to ground and it has practically no potential > across the driver. > > The reason that the above statement is not quite true and the reason > therefore for the clamp is to sink parasitic currents between a lit digit > and the unlit ones. If you don't do this, you can get way out of spec > voltages at the cathodes and this causes the output stage on the driver to > break down and let the tiny currents through. It rarely breaks the device, > but the display is "foggy". > > The principle for the HV5812 is the same. In this case you have to take > the VPP pin to some voltage that will not cause the digit to fire, but will > also drop the parasitic current via the body diode of the top MOSFET in the > output stage to VPP. The red is the path when the digit is on, the green is > the path for the parasitic current to VPP when the digit is off. > > [image: Screenshot 2025-03-08 at 09.17.21.png] > > I've seen people just using a potential divider for the derivation of VPP, > but I think I would prefer a Zener clamp. > > Having said all that, if you want a direct drive and don't want to go full > design nerd, the HV5622 and HV5530 drivers are dead easy to use and give > good results with little effort. > On Monday, 3 March 2025 at 21:46:45 UTC+1 gregebert wrote: > >> Similar here, except I use the HV5530 which is rated for 300V. I strongly >> discourage using zener diodes, or the nixie-tube itself, to drop-down the >> voltage to CMOS parts, because even the tiniest amount of leakage thru the >> diode/nixie will cause the voltage drop to be less than expected, and >> overstress the CMOS device. You could use an appropriately-sized bleeder >> resistor to ground, but it's still not rock-solid. Another approach is to >> use a zener as a clamp at each output but that is wasteful because it >> requires 1 zener per pin. >> >> On the other hand, if you are using bipolar parts, such as NPN >> transistors or 7400-series TTL parts, exceeding the voltage is OK as long >> as the current is limited to a few uA or less. Bipolar devices do not have >> oxide-insulated structures like CMOS, so they are not inherently destroyed >> by over-voltage. >> >> On Monday, March 3, 2025 at 11:39:00 AM UTC-8 newxito wrote: >> >>> I use the HV5622, which goes up to 220 V, I think there is also a PLCC >>> version. The disadvantages are the price and that it should be operated >>> with 12V according to spec. However, I never had any problems using the >>> chip with 5V. >>> JBro63 schrieb am Montag, 3. März 2025 um 19:02:56 UTC+1: >>> >>>> Hi all, >>>> >>>> Group noob here, about to start build on a few different types of >>>> display using Nixie tubes and ESP32. >>>> >>>> Planning to use K155ID1 initially (as I have a bunch) with some IN-12 >>>> and IN14 tubes but want to also try HV driver such as the 5812 or 5530 so >>>> would welcome any comment on which is the best one to go for or an >>>> alternative. I don't intend to multiplex. Any driver would need to be DIP >>>> or PLCC. >>>> >>>> Have spent many hours looking at the schematics and designs of others, >>>> I'm grasping the basics but one frustration and evident gap in my >>>> knowledge >>>> is how to pick / calculate the correct component and its size or rating >>>> for >>>> anything other than the most basic circuit. >>>> >>>> For example, with a 180v supply, calculating the anode resistor for a >>>> tube based on the datasheet is straight forward enough as the maintaining >>>> voltage and current are known. >>>> >>>> When looking at something like the HV5812, many seem to use a 60 or 70V >>>> zener diode with a resistor to keep below the max for the chip but how do >>>> you determine the current needed for the driver, diode and load to be able >>>> to calculate the current limiting resistor? The diode datasheet is simple >>>> enough but I'm lost with the sheet for the HV5812. >>>> >>>> Thanks. >>>> >>>> -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion, visit https://groups.google.com/d/msgid/neonixie-l/ac6a6b81-9f58-44d7-9040-d295acd72626n%40googlegroups.com.
