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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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Old 17th Mar 2019, 1:39 am   #21
AC/HL
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Default Re: Series connected stabilizer valves: questions

About 5.6 volts in a silicon diode according to this:
Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode.[1] The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.[2]

In a 5.6 V diode, the two effects occur together, and their temperature coefficients nearly cancel each other out, thus the 5.6 V diode is useful in temperature-critical applications.


https://en.wikipedia.org/wiki/Zener_diode
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Old 17th Mar 2019, 12:42 pm   #22
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Default Re: Series connected stabilizer valves: questions

Wow! When Skywave said that 'HV' Zeners were poor, I was thinking that 'HV' implied 20V or more. One issue which comes to mind in this context is that of where the temperature change is coming from; ambient air or the power being dissipated by the diode? I found an example on Google, "The temperature drift of the 0.5-W 18-V Zener diode (1N5248A) measured from room temperature to 120C is about 1.3 Volts". That would appear to be a (hypothetical ?) case where somebody has used a 0.5W diode where a bigger one was surely needed?

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Old 17th Mar 2019, 5:40 pm   #23
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Default Re: Series connected stabilizer valves: questions

So there's basically 3 groups of zener/voltage ref diodes if I understand correctly: 0.7-2.1V forward voltage references, 2.7-5.6V mostly zener, 5.6 and up mostly avalanche.
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Old 17th Mar 2019, 6:02 pm   #24
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Default Re: Series connected stabilizer valves: questions

Yes. And there's also the 'programmable Zener diodes' such as the TL431. But that's really an IC, not a diode. However, these are really useful for voltage stabilisation.
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Old 17th Mar 2019, 8:28 pm   #25
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Default Re: Series connected stabilizer valves: questions

I think of the TL431 as an extension of the TAA550-type principle, i.e. a very stable low-voltage internal reference with associated circuitry to make it act like an excellent Zener of a different chosen voltage- the TAA550 being fixed at a nominal 33V, the TL431 having a third pin for variability. The latter is so cheap and widely available- not to mention the very low operating current- as to almost displace conventional Zeners for many PSU reference applications, there must be umpteen millions at the heart of SMPSUs worldwide.
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Old 18th Mar 2019, 1:51 am   #26
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Arrow Re: Series connected stabilizer valves: questions

Quote:
Originally Posted by Bazz4CQJ View Post
When Skywave said that 'HV' Zeners were poor, I was thinking that 'HV' implied 20V or more.
To me, 'H.V.' for a Zener diode is much higher than 20 v.

I was thinking of my earlier attempts (ISTR last year) to produce a d.c.voltage calibrator. For that I used Zener diodes of 56 v. and even higher: they were rated at something akin to at least 1-watt. That rating was necessary because of the design I was trying to develop. So they got rather warm - and consequently the Vz drifted to such an extent that the resultant regulation was simply inadequate for the intended need.

However, I am now working on a different approach towards the same end. It's currently an active thread in Circuits & Components.

Al.
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Old 18th Mar 2019, 2:07 am   #27
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Default Re: Series connected stabilizer valves: questions

Al, not sure whether this circuit is of any interest to you? It was originally posted/discussed on the Forum back in 2015 and I built it and was quite pleased with it. As mentioned earlier, I think I used 3 x 75V Zeners, but I'd need to check that to be sure and I cannot claim to have looked in any great detail at the tempco of the circuit. However, the use of the MOSFET eases the workload of the Zeners. Apart from the MOSFET (which needs a heatsink) it all went on to a piece of Veroboard and was quite compact.

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Attached Files
File Type: pdf MOSFET Variable regulated PS0001.pdf (47.9 KB, 13 views)
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