Zener reading a little high? - Page 1
Forum section: electronics / beginners

author: IsmAvatar
author_rank: Contributor
timestamp: May 02, 2021, 09:42:58 pm
content: I'm designing a circuit to provide voltage to the plates of some tubes, roughly 22V, 66V, and 90V DC, coming from US Mains (yes, I'm being very careful). Basing it off of someone else's design that worked for them, I came up with the image attached.After hooking everything up, (and then cleaning up from a little excitement when I found out the transistors were BCE and not EBC...) I probed some voltages, and found that the zener diode (1N4763A-TR) measured 95 volts and gradually climbed up to 99 volts, slowing along the way (very much like a big capacitor charging), after which I shut it off and let the capacitors discharge.So I guess I'm wondering why a zener diode with a 91V +/- 5% rated zener voltage is reading 95-99 volts?I do have a backup zener in case something happened with the first one or in case there was a defect, but I thought I'd check in here first before I go swapping anything out and potentially damaging both of them.In the image:HV+/- is US Mains (~120V AC, through an isolation transformer).The transistors are APT13005SU-G1DI-ND, in case it matters. And are now oriented correctly...The orange circles are my probe points.The blue labels reading v22.5, v67.5, and v90 are the *desired* output voltages. They measure close to those, albeit a bit high. They are not connected to anything yet.Capacitor C12 is actually not present... I still need to replace it after the aforementioned... excitement.

author: Jwillis
author_rank: Super Contributor
timestamp: May 02, 2021, 09:58:58 pm
content: Zener diodes above 5.1V have a positive temperature coefficient . The higher the temperature the higher the voltage goes. You need to reduce the current passing through the zener.

author: IsmAvatar
author_rank: Contributor
timestamp: May 02, 2021, 10:06:19 pm
content: I suspected it might be something along those lines. Just wasn't seeing it in my quick searches on Zeners.That makes sense. Thanks. That also gives me something to read up on to factor into my next design.

author: bob91343
author_rank: Super Contributor
timestamp: May 02, 2021, 11:15:42 pm
content: The fact that it starts out higher than expected may be a clue that the current is too high. Typically one wants a zener diode to dissipate about one-quarter of its power rating.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 03, 2021, 09:02:02 am
content: check the zener diode data sheet and check that your operating current iw in spec.Are you doing something like this?https://www.google.co.uk/url?sa=i&url=https%3A%2F%2Felectronics.stackexchange.com%2Fquestions%2F416701%2Fhigh-voltage-regulator&psig=AOvVaw2wuO0hyuYMLq9JhGuNlsXi&ust=1620118565771000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCJiStY-SrfACFQAAAAAdAAAAABAE.All the Zeners need different currents to operate correcly. Unless you use chains of equal Zeners that is.

author: SeanB
author_rank: Super Contributor
timestamp: May 03, 2021, 09:49:49 am
content: Your zener diode is being asked to dissipate 1.5W of power, well above the 1W rating. Either make R8 10k, which will still provide more than enough base current drive, or change out the zener to a series connection of 3 30V zener diodes, like 1N4751 for example.

author: IsmAvatar
author_rank: Contributor
timestamp: May 03, 2021, 09:08:06 pm
content: @Terry Bites:Unfortunately Vishay's datasheet on the diode was a bit scarce on info, giving just a list of base info about the 1N4728A thru 1N4764A.@SeanB:You're doing my homework for me, lol. Very helpful, thank you.