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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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12th Dec 2022, 7:10 pm | #1 |
Octode
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
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LM199/399 application
I am working on a Standard Cell replacement based on a Linear Technology data sheet circuit which I have also found to be very similar to TI's data sheet.
This all seems fine, but during some research, I found a reference in "The Art of Electronics" 2nd Edition, (Horowitz and Hill). The second last paragraph on page 339 states "One caution when using the LM399: The chip can be damaged if the heater supply hovers below 7.5 volts for any length of time". Apparently this does not appear in the third edition of H & H, although I have not been able to check myself and I haven't found anything to support this caution on the internet. Could anyone who has detailed experience of the LM399 offer any clarification please? I am not at all sure what is meant by hovering nor what constitutes "any length of time" or why a low heater voltage should damage the IC. PMM |
12th Dec 2022, 7:23 pm | #2 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
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Re: LM199/399 application
I've not used it, but I have been involved with precision, ovened quartz oscillators.
Some early generation ones used a nichrome resistive heater with a transistor circuit switching on and off, as directed by a thermostat. Small variations in the oscillator frequency could be traced to the switching. The replacement was a linear temperature controller. The heat from the power transistor was not wasted, its heatsink was the oven mass. I wondered why they retained a resistive heater section. They could have used just the power transistor alone, and had faster heat-up. The answer I got was that if the heater supply was low, there needed to be something to limit the current to a non-damaging level. I think that is what may be going on in the LM399, it the heater is all-transistor. It could be at risk of over-current. David
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