Equivalent Zener diode selection - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 01:00:12 pm
content: Hi, i'm doing my first serious power supply repair that doesn't just involve filter caps. I need a Hitachi HZ11B2 zener for this old Opus 1 music system. Part's no longer made but looking at the data sheet it says min zener voltage 10.4 & max zen v 10.8. It's 500mW too. So will any old zener currently made in the range above be suitable?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2023, 01:43:18 pm
content: The exact zener type is likely not that critical. There can however be some tolerance in the actual voltage. Some zener diodes come with quite some tolerance and actual used current may be lower than the current where the voltage is specified in the datasheet.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 01:54:22 pm
content: Quote from: Kleinstein on May 12, 2023, 01:43:18 pmThanks for your help. I've had a search and can't find anything in between 10 and 11.2 volts. Could i use a 10v zener plus a normal diode to get the exactish voltage? I like to aim for the spec.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 12, 2023, 02:13:59 pm
content: Quote from: Quantumplate on May 12, 2023, 01:54:22 pmMostly yes. And it can be more temperature stable than just a single Zener. But it is hard to tell 100% without a circuit. There possibly can be a situation where it works both sides - positive and negative.

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2023, 02:17:18 pm
content: This appears to be an exact replacement for your zener:https://www.digikey.com/en/products/detail/nexperia-usa-inc/NZX11A-133/2119714Any zener that actually measures between 10.4 and 10.8V under the appropriate test conditions will be OK, but zeners typically come with nominal voltages with upper and lower limits. A 10.8V nominal zener might have upper and lower limits of 10.6 and 11.0V, so some of them will be higher than your spec. It probably would work just fine like that, but if you are particular just go with the exact replacement and then measure itin situto make sure it is in your range.

author: DBecker
author_rank: Frequent Contributor
timestamp: May 12, 2023, 02:21:31 pm
content: There are plenty of similar DO35 "10V" or "11V" zener diodes that are going to be close to that range.You'll need to check the datasheet for the tolerance at the specific current used in the circuit. It appears that diode is rated at a lower test current, 5mA.Modern designs typically use zeners as cheap-and-cheerful shunt regulation, not as a reference voltage. Thus they are rated at a higher current. If you search for readily available zeners, most are going to be rated at 20mA - 25mA current. A 11V @ 20 mA zener is going be 10.5V @ 5mA.The first one that popped up in an ebay search was a 1N5241B, nominally 11V. It's 10.450 - 11.550V @ 20mA, but in the 10.6V range at 10mA. If you only need one for a repair it's easy enough to test that specific part, and then verify in-circuit.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 02:39:32 pm
content: Quote from: Vovk_Z on May 12, 2023, 02:13:59 pmGreat to know. Here's the circuit - bit fuzzy- but it's ZD811 , the one with the long pointer going to it in the right hand half.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 02:51:50 pm
content: Quote from: DBecker on May 12, 2023, 02:21:31 pmVery good info thanks. Feeling much more clued up now.Quote from: bdunham7 on May 12, 2023, 02:17:18 pmWell done! Thanks for finding that. P&p across the pond though..

author: jwet
author_rank: Frequent Contributor
timestamp: May 12, 2023, 03:06:22 pm
content: Also look at the resistor feeding the zener and any large caps downstream- both are common failure modes. These circuits (several in the schematic) use a biased zener shunt regulator (zener + R + big C), feeding an emitter follower transistor that follows the zener (less .7v). Like others said its not terribly accurate but doesn't need to be, its just a quiet audio rail thanks to the big C.The way these circuits burn up is that a high load (caused by a transient short- maybe a bad speaker connection or a failing electrolytic downstream) draws too much current through the bias R. The bias resistor gets worked (higher current/power) and fails, usually open but occasionally by lowering its value. If it opens, the zener is saved, if it lowers, the zener gets overstressed (your case). After you replace the zener, verify that zener isn't dissipating more than say .3w (about 30 mA). You can work out the drop across that bias R with ohms law to infer the current. I see in some places in the schematic that those bias resistors are spec'd as "flameproof". If you're doing the repair you might want to over size the wattage of that R and get it off the board. Old amps are full of all this stuff.Good luck- its nice to find a real culprit but try to imagine how it failed and look at components that could contribute or it will be back in 2 months...

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 12, 2023, 03:52:57 pm
content: Quote from: Quantumplate on May 12, 2023, 02:39:32 pmOh, that's just a typical Zener-regulated power supply, so I guess any similar Zener or Zener + Diode should do.Nowadays popular Zeners are either 11 V or 12 V rated and typically have quite a tight tolerance now (+-0.2V), so it is possible but hard to find an 11.5 V rated one from a batch of 11 or 12 V ones. So if you can't find an 11.5 V rated Zener, it is much easier to use an 11 V Zener + Diode.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 04:34:01 pm
content: Quote from: jwet on May 12, 2023, 03:06:22 pmThanks, and for taking the trouble to educate me. The reason i think it failed is that three of the regulators (Q805, 806 & 808) had a thin patch of glue underneath covering several nearby components, that had gone black and corrosive, severing the wire to and half of ZD811. Other regs still have a patch of new looking clear glue. It's the -5v and -10v rails I'm missing. Seems a miracle everything was working on the unit apart from the CD player. I will have to re-do a few board wire connectors or maybe just reflow the pads that look corroded some of them. Do you mind if i ask you a couple more things - What was the glue for? Did it go black through heat? Most of the regulator transistors have a little woven cylindrical sheath of material over them...i don't know what those are for either. So much still to learn...Thanks again.

author: jwet
author_rank: Frequent Contributor
timestamp: May 12, 2023, 06:44:59 pm
content: Thank me with the thank button- trying to get my thanks up.Your new questions-Manufacturers use glue to keep parts from moving around or breaking solder connection from the mass moving around. Its more common on big parts like big electrolytics on switching power supplies etc.Shipping a box like a receiver across the world with all the planes and train and trucks is pretty rough mechanically. Its mostly harmless, a lot of modern stuff uses silicone but you have to be careful, some silicones have things that conduct until its cured. As far as the mess, a lot of times, they're not real careful applying it- they're doing with air operated gun like thing. A few years back, there was a tan colored goo used in a lot of equipment that would go bad over time and screwed up a lot of equipment. You had to get all of it off. Job security for repair techs.The black color might be heat or just poor aging. On the bad stuff, you can measure it with an ohm meter. It might be 1k or less over an inch of so of goo.The little filament cylinders over components are generally flame proofing or blast proofing for parts that run hot or can get real hot during a fault. Board mounted fuses and power resistors are often covred this way but you sometimes see them on diodes and transistors if they can explode in a fault. The idea is that if they get really over streesed, the fire won't spread if they lean up against something, they won't start a fire. Some of the R's on that board specify flame proof on the schematic.Good luck- repairing stuff is a great way to learn.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 07:04:21 pm
content: Quote from: jwet on May 12, 2023, 06:44:59 pmAh yes, i heard about the brown glue. I have some of that coming up to fix in a classic old goldstar boombox.Really appreciate your clear answers! It's tough teaching yourself this subject from scratch as i've done the past few years.Two more thanks to add to your score no problem.

author: jwet
author_rank: Frequent Contributor
timestamp: May 12, 2023, 07:13:29 pm
content: I enjoy electronics and repairing-retired now- putttering on this board is fun on a gloomy day. Thanks for the thanks and shoot me a PM if you have a specific question- happy to help especially for someone that is thankful.

author: TimFox
author_rank: Super Contributor
timestamp: May 12, 2023, 08:12:42 pm
content: You could use selected 4.7 V, 5.1 V, and 5.6 V low-power Zeners in series, if you need that tight tolerance (10.4 to 10.8 V) for the series combination of two diodes.They can be measured at your desired current with a 9 V battery, suitable resistor, and DVM.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 12, 2023, 08:51:39 pm
content: Quote from: TimFox on May 12, 2023, 08:12:42 pmThanks, i was wondering about that.Just trying to work out the desired current thru ZD811. Calling the zener voltage say 10.5v, that's 5.5v across R827 (100ohm) giving 55mA plus 2 more mA going thru R826(470ohm). So ~57mA . Dissipation thru ZD811 is then 0.057 x 10.5 = 598mW. Hmm..but these zeners are small, rated at 500mW. I appear to have gone wrong...or maybe that's why it blew...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2023, 09:03:34 pm
content: The current trough R826 would subtract from the current trough R827. Still not a large difference.The current looks quite large and thus running quite hot. I would consider getting a larger (e.g. 1.3 W) zener diode in this case. The actual heat they can disipate depends on the mounting and environment.Especially two zener diodes with a 400 mW ratings mounted close togehter may not be good for more then 500 mW.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 12, 2023, 09:14:10 pm
content: Quote from: Kleinstein on May 12, 2023, 09:03:34 pm+1. I don't see a reason why there must be 55 mA. I would understand if it was 5.5 mA or 10-20 mA. But 55 mA - why?

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2023, 09:37:07 pm
content: Quote from: Quantumplate on May 12, 2023, 08:51:39 pmAt first I thought you were wrong, but then I looked closer. I think the diagram is wrong, and if the unit is assembled according to the diagram I think that's wrong too, probably just an error. And if that is the case, this would be the likely cause of the eventual failure.Look at the positive-side regulator circuit up and to the right of this one--there the 470R and 100R resistors are the way I think they ought to be and the currents will work out like a typical zener-transistor regulator should. I'm assuing the input voltage is 13-16VDC and the output is intended to be ~10VDC.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 13, 2023, 10:48:27 am
content: Quote from: Vovk_Z on May 12, 2023, 09:14:10 pmThe voltages either end of R827 (100ohms) are -10.5 at the top (the zener voltage drop from the 0v line) and -16 at the bottom. Ohms law tells us the current thru it, which is also approx the current thru the zener.Quote from: bdunham7 on May 12, 2023, 09:37:07 pmInteresting thankyou. I can't quite work out the positive side voltages that they haven't bothered to print. Back on the neg line and the blown zener - I've checked the resistors on the board match the schematic, which they do, so it appears as you say either they underated the diode power or got the resistors the wrong way round. I've noticed over the past three years a lot of these units for sale and not working!So i think I'll just up the diode power and leave the rest as is.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 13, 2023, 08:07:20 pm
content: Quote from: Quantumplate on May 13, 2023, 10:48:27 amI know an Ohms's law, I don't get WHY there must be 55 mA but not 5.5 mA. I mean why R827 must be 100 Ohms but not 1 k for example.

author: Quantumplate
author_rank: Regular Contributor
timestamp: May 13, 2023, 08:28:44 pm
content: Quote from: Vovk_Z on May 13, 2023, 08:07:20 pmSorry, late night! Yes, i don't understand either. Also on the schematic they have -11.5v on the base of Q806 and -10.6 south of the Zener, indicating current flow into the regulator, which is not right as it's a PNP. So surely the zener voltage for ZD811 should be more like 12 or something?