Lm399 reverse polarity, dead heater! - Page 1
Forum section: electronics / general

author: nour
author_rank: Frequent Contributor
timestamp: May 28, 2017, 11:05:45 pm
content: Heater is dead after 15V reverse polarity, can it be used anymore at any level ?Or I will just throw it away !The Zener is still working but it is drifting!By the way, there was an Lt1001 connected at the same rail, does anyone knows if there is any chance that it survived the polarity error?

author: mimmus78
author_rank: Unknown
timestamp: May 28, 2017, 11:11:16 pm
content: It seems very often lm399 heater will blow up if polarity is reversed. It happens to many of us ...Inviato dal mio Nexus 6P utilizzando Tapatalk

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2017, 07:59:07 am
content: With the heater blown, the LM399 could still be used at room temperature. So its more like an LM129 now, an unheated burried zener reference chip in a hermetic case. Depending on how high the TC is this might be still rather useful, or hardly useful.The blown heater side might still work as a kind of temperature sensor, depending on how it is broken.With 15 V in reverse the LT1001 might still work, but one does not no for sure how well it is working now. So usually one would replace it. Even if it still works like an OP the properties might not be as good as a new one and chances for failure are higher.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: May 29, 2017, 05:50:19 pm
content: There is an easy way to convert a blowned LM399 to the LTZ-like voltage reference.

author: nour
author_rank: Frequent Contributor
timestamp: May 29, 2017, 07:25:43 pm
content: The zener itself is working fine, but it is drifting with temperature changes very easily now, may be almost 100 microvolts or more.I have tried something, I have a tiny small heater ceramic chip, I attached this chip on the top of the reference package and it is now working as a heater and the reference is stable now down to -+ 1 microvolt.I am not using any feedback or PID to control the heater, which I may us later and see how far it will work in this configuration!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2017, 07:32:40 pm
content: If one wants to add external temperature control to the LM399 without the internal heater, one might also remove the thermal shield from the LM399. In addition one could use temperature control no only for the reference chip, but could stabilize the whole circuit around it, just like it was done in the 1960's. There is no real need to run the LM399 as hot as the original heater.

author: nour
author_rank: Frequent Contributor
timestamp: May 29, 2017, 07:36:20 pm
content: Quote from: Kleinstein on May 29, 2017, 07:32:40 pmAs long as the ambient temp. is stable I think it should be ok even without a heater!

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: May 30, 2017, 12:05:24 am
content: @Nour:Hello, I would try your heater/ LM399 arrangement inverted and dead bug style with some simple insulation on top, like tissues or cotton ball, to stop convection off the leads. Since the leads haven't been soldered this should be easy to test. Andreas has measured significant variation when the orientation of a normal heated LM399 is rotated. However a static inverted and heated from below '399 should be even more stable than your pictured configuration provided you deal with the inevitable convection. Copper or tinplate metal shield covering over would further enhance the oven effect, like in a crystal oven.

author: nour
author_rank: Frequent Contributor
timestamp: May 30, 2017, 07:32:57 am
content: Quote from: chickenHeadKnob on May 30, 2017, 12:05:24 amThat was very quick setup because I wanted to test the idea. and I think it worked to some extent, like for example, I lose the stability when I am not covering the whole thing like in the third pic.Anyway, I have ordered another one and I will try to use some basic PID to control the heater to make it more stable and of course I will try different heater place to see which would be more stable!

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 30, 2017, 09:29:30 pm
content: You do realize that blowing the heater on the LM399 chip will likely degrade the zener's performance if not cause an early failure.

author: nour
author_rank: Frequent Contributor
timestamp: May 30, 2017, 10:01:55 pm
content: Quote from: Edwin G. Pettis on May 30, 2017, 09:29:30 pmWell, that's why I have ordered another one to compare it to this blown one