# When driving a Nixie tube with the HV5622 shift register, how hot will the chip get?

I am a beginner in electronics. I am trying to drive the IN-12A Nixie Tube using the HV5622 Shift register. I am trying to calculate the temperature of the HV5622 during normal operation. For each Nixie tube, one cathode is always on. My approximate circuit schematic is shown below. How hot will the HV5622 get?

• Perfect example that you can write a good question even if you suck at English. Even before the copy-editing it still contained all the necessary information about the problem. Next person who thinks their question was closed because we discriminate against non-native speakers should get a link to this.
– pipe
Oct 22, 2017 at 9:56
• Also, it's fun - Nixie tubes! Oct 22, 2017 at 12:42
• Yes. That's why I like nixie tubes. Oct 22, 2017 at 12:48

Most of the power dissipation will be from the Vdd current in your application. If you run it at 12V Vdd, say, note that it can draw as much as 15mA, which is 180mW while clocking it at 8MHz, according to the datasheet. It's not clear (to me anyway) what the Vdd current draw is otherwise or whether there is a preferable state for the input lines. The datasheet is not very helpful, I'm afraid. For example it refers to Vin = 0, but there is no pin called Vin. It's possible the dissipation under static conditions is worse than 180mW but I would guess not (and verify!).

Edit: I looked at a similar datasheet from the original manufacturer (Supertex, who was acquired by Microchip) and Vin seems to refer to the output voltage, which is .. weird.

The output voltage is guaranteed to be less than 15V @ 100mA out and your output current will be perhaps 3mA per output. Probably the output voltage will be well under 1V, so allow less than 10mW for all 3 outputs.

There is also a bit of dissipation from leakage currents but that won't normally be significant.

• After that, what is the temperature rise of the chip at 12V Vdd? Oct 22, 2017 at 4:09
• The datasheet says 51 or 37 degrees C/W depending on PQFP or PLCC. So for the PQFP 190mW would be less than 10 degrees C rise, if your conditions are similar to those assumed in the datasheet. Not of much concern in any case. Oct 22, 2017 at 4:15
• My Nixie Tubes are using 2mA * 170VDC * 3pcs = 1.02W. Then does the HV5622 rise 58.14 degress C? Oct 22, 2017 at 9:05
• No, the heat goes almost entirely to two places- the resistor and the Nixie tube. If there is 2mA flowing then 66V is dropped across the resistor, about 103 across the Nixie and about 1V across the output. So each Nixie dissipates 0.206W, each 33K resistor 0.132W and each output 0.002W. Oct 22, 2017 at 10:58
• No heat problems will occur under normal use and you don't need a heat sink. Everything will run cool, except the resistors will get a bit warm, make sure they are rated for >150mW. 250mW would be good (and the larger size is also useful because the voltage is high) Oct 22, 2017 at 11:48