So I considered using a Negative Temp Coefficient (NTC) inrush limiter for my clock with the three IV-11 filaments in series on the 5V rail... Found an appropriate part after a pretty broad search, but ran into a 100 piece minimum order quantity at $2.50 per each and 6-8 weeks lead time. No... I don't think so... not a good plan...
Here's plan "B" which I actually like a lot more than plan "A" above. The design below uses a mosfet with turn-on time slowed by an RC network. As shown, the slow start takes about a quarter second to rise to near 5V. I believe this will be plenty slow enough to protect the IV-11 filaments. I've previously used the same basic circuit to allow a switch mode wall wart to power a device with high initial inrush charging capacitors. The wall wart would go into hiccup mode trying to start the device in question when used without the inrush limiter. This slow start is a broadly useful circuit. (Here's a TI note describing the circuit below as well as several others: http://www.ti.com/lit/an/slva156/slva156.pdf) A few nice things about this circuit... 1) It will work to slow start most any 5 - 12 V load (though possibly with component value changes). 2) Operating voltage drop can be made very small with careful mosfet selection. My IV-11 will actually like a slightly lower operating voltage as the are currently at the upper end of spec. 3) Reset time is on the order of 1.5 seconds as shown. (Reset time is the time after power off that is needed to assure a slow start the next time power is applied. This time allows a large cap in the circuit to discharge. This is rather like the time needed for an NTC inrush protector to cool between applications of power.) 4) The circuit can be set up to run quite cool depending on operating load and selection of a mosfet with sufficiently low Ron. 5) One slow start circuit will take care of multiple loads on a common power supply... this was not going to work with the particular NTC part I had found due to excessive voltage drop if I used just one part for multiple filament strings. This one circuit can actually replace a wide variety of NTC parts for these low voltage applications. So... I'll spin up a small pcb for the slow start to be included in unused area on the next larger board that I have occasion to build. I expect there will be other uses for the little board so worth the trouble to make a small pcb for it. Will also fine tune a few values as needed but the values shown here are a not unreasonable starting place. <https://lh3.googleusercontent.com/-3aZDh7W3s8U/WdLvocqhh5I/AAAAAAAAAXI/q50HrOVh9ZY9mkqPtd5aLjq_Svvt9gMKgCLcBGAs/s1600/ExtreameSlowStart%2B%2528Medium%2529.JPG> Onward... Bob -- 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 post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/54754873-d3f7-4209-a13d-9f56c0ea9f33%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
