On Tuesday, March 28, 2023 at 9:11:04 PM UTC-7, Anthony William Sloman wrote:
> On Wednesday, March 29, 2023 at 1:16:44 AM UTC+11, Ed Lee wrote:
> > On Tuesday, March 28, 2023 at 4:06:41 AM UTC-7, Anthony William Sloman wrote:
> > > On Tuesday, March 28, 2023 at 6:44:02 PM UTC+11, Ed Lee wrote:
> > > > On Monday, March 27, 2023 at 8:53:03 PM UTC-7, Anthony William Sloman wrote:
> > > > > On Tuesday, March 28, 2023 at 2:44:22 AM UTC+11, Ed Lee wrote:
> > > > > > On Monday, March 27, 2023 at 8:34:05 AM UTC-7, Anthony William Sloman wrote:
> > > > > > > On Tuesday, March 28, 2023 at 2:14:41 AM UTC+11, Ed Lee wrote:
> > > > > > > > On Monday, March 27, 2023 at 7:51:54 AM UTC-7, Anthony William Sloman wrote:
> > > > > > > > > On Monday, March 27, 2023 at 7:49:15 PM UTC+11, Ed Lee wrote:
> > > > > > > > > > On Sunday, March 26, 2023 at 11:19:44 PM UTC-7, Anthony William Sloman wrote:
> > > > > > > > > > > On Monday, March 27, 2023 at 6:50:54 AM UTC+11, Ed Lee wrote:
> > > > > > > > > > > > On Sunday, March 26, 2023 at 11:35:49 AM UTC-7, Anthony William Sloman wrote:
> > > > > > > > > > > > > On Monday, March 27, 2023 at 4:57:49 AM UTC+11, Ed Lee wrote:
> > > > > > > > > > > > > > On Sunday, March 26, 2023 at 10:49:41 AM UTC-7, Ed Lee wrote:
> > > > > > > > > > > > > > > I brought and use them as 12V 5W zeners in 32S4P 400V 1A. When balanced, each 1N5349 (not 1N5439) draws around 400/128 = 3.1W. Occasionally, i got smoke signals with them. No big deal when close to home, but headache trouble shooting on the road.
> > > > > > > > > > > > > How do you balance them? A 1N5439B would have a 5% tolerance, so 12V means 11.4V to 12.6V. You are trying to run then at 250mA each, but the dynamic impedance at 100mA is 2.5 ohms, so the worst case voltage difference would put most of the current through the lower voltage part.
> > > > > > > > > > > >
> > > > > > > > > > > > Yes, i am waiting for some 3.3 ohms resistors to put in series with the zeners. I can also pretest the current for each zener. I can probably order/pretest 1% tolerance.
> > > > > > > > > > > Nobody sane does.
> > > > > > > > > >
> > > > > > > > > > Actually, i don't care if they are 5% or 10%. All i care about is the trigger voltage (turn on voltage). Out of the 32 sets, they are all fairly close to 12V. They are soldered together with lots of solder as heat sink. So, it's OK if one of them get beyond 5W, as long as four of them stay under 20W. I don't care about some failure, as long as I pre-sort and pre-test them.
> > > > > > > > > It's not "trigger voltage" but "breakdown voltage", For a 12V device it is actually the avalanche voltage.
> > > > > > > > I measure the voltage to trigger the avalanche breakdown by pushing them to around 12.6V and let it settle down back to 12V. Most of them settle back to around 12.1V to 12.2V.
> > > > > > > > > You need to work how close together you need to get the breakdown voltages to get a current distribution that doesn't push any one of them above 5W.
> > > > > > > > I will measure the actual current distributions for group of four zeners.
> > > > > > > > > At least with 12V zeners breakdown voltage rises as they get hotter, but not enough that you can rely on it to get a safe current distribution with unselected diodes.
> > > > > > > > >
> > > > > > > > > There should be a mathematical model for voltage versus reverse current for the kind of avalanche diodes that you are using - LT Spice has obviously got one for modelling reverse biased Zener/avalanche diodes, but google won't find it for me.It probably would if I could find the right question to ask.
> > > > > > > >
> > > > > > > > I trust actual measurements rather than mathematical model.
> > > > > > > As you should. But electronics is all about finding mathematical models that work well enough to let you keep track of what is going on, and put together circuits that won't blow up.
> > > > > >
> > > > > > Yes, but mathematical models indicate that they should all be the same, with identical current distributions. What I need is to measure the manufacturing variations between them.
> > > > >
> > > > > Don't be silly. The mathematical model requires you to plug in the actual breakdown voltage for each device, which you have to measure or specify. If you go in for really accurate modelling you may have to allow for variations in the cross-sectional area for each device - they are made to be identical but never are - and variations in the doping profile.
> > > > >
> > > > > The main utility of mathematical model is that it allows you to interpolate accurately between a limited number of precise measurements.
> > > > >
> > > > > In your case it would let you work out how far the current distribution between two parallel diodes would vary as applied voltage changed.
> > > >
> > > > But how? The parallel zeners all have the same voltage on them; so, variations between them is zero in the math model.
> > > The mathematical models have adjustable parameters which let a generalised avalanche diode breakdown model replicate the performance of different examples of diodes with the same part numbers, but slightly different breakdown voltages. The breakdown voltage is the first adjustable parameter you need to adjust.
> > >
> > > > I have four voltmeters on segments of 8S4P. Right now, they are well balanced at 95.2V, 95.5V, 96.5V and 96.2V, or less than 0.1V from 12V on average. When there is big variation within the segment, then I check the individual module. Mathenmatically, they should be 96.0V, 96.0V, 96.0V and 96.0V.
> > >
> > > You should have four different models, each with a slightly different breakdown voltage parameter, each one modelling a different diode.
> >
> > Trigger/Threshold/Target avalanche breakdown voltages are within 11.9V and 12.1V, well within 1% to 2% of stated voltage. Even if currents/powers are off 10% to 20%, it is well within thermal rating. This is just using same batch without pre-sorting. Pre-sorting and pre-selecting should be able to improve it further.
> But you opened the thread by telling us that it didn't always work.
I also said that I am using external serial resistors to flatten the curve and pre-sorting to match them better.
> You need to work out how much more current a 11.9V diode would pass at 12.0V than a 12.0V diode at 12.0V.
I would rather measure them with meters. It's just a matter of matching the curves, no matter what the curves are.
1N2976B DO-4 5A would probably work better than 1N5349B DO-15 0.4A.
But (vertically mounted) DO-4 would make a very tall board. Ideally, i want axial mounted DO-X (just a bigger version of DO-15 in metal casing), perhaps 2A limit.