BZ-140 Zener Diode equivalent or suitable substitute - Page 1
Forum section: electronics / repair

author: jastreb
author_rank: Contributor
timestamp: October 06, 2017, 04:15:39 am
content: HI everyone,I am trying to fix a Marantz SR-810 receiver. The receiver apart from other issues also has a blown diode which was so bad it was no longer possible to identify. Also not able to find a service manual for the SR-810. However I did find a service manual for an SR-1000 and it seems to be very similar build looking at pictures of the internals. The service manual suggests that the zener diode that is blown in my unit is a BZ-140 which google tells me is a 14V 1W zener diode.What would be a suitable replacement as this diode is no longer manufactured?From what I know about diodes, it will have to be 14V and at least 1W. From reading suggestion is that I should not try and replace it with a higher Wattage diode and stick to a 1W spec. Any suggestions for a modern readily available alternative? An NTE144A has been suggested in another forum but that one is also not a diode easy to find.Here is the schematic, with the diode highlighted in yellow.Cheers guys and ladies.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 06, 2017, 05:24:43 am
content: If the power supply acts as indicated in that schematic:Voltage at Zener Q807 = 14 V.Current through R806 = (34 V - 14 V) / 150 R = 133 mA.Power dissipated in R806 = (20 V)^2 / 150 R = 2.67 W. Toasty. Check the value.Current through Zener Q807 (with no current to the connector) = 133 mA.Power dissipated in Zener Q807 = 14 V * 133 mA = 1.86 W.It seems the Zener dissipates almost twice the rating of the proposed 1 W part.What is the reasoning for not going with a higher wattage rating when used as a simple resistor Zener reference regulator?My preferred Zener is the ON Semiconductor 5W Surmetic series:https://www.digikey.com/products/en?keywords=1N5351BGOS-ND

author: jastreb
author_rank: Contributor
timestamp: October 06, 2017, 09:37:35 am
content: Quote from: Tomorokoshi on October 06, 2017, 05:24:43 amThank you so much for your opinion and the detailed explanation on what the diode is expected to see. I love the maths. Great explanation. Thank you.As for why not using a larger wattage rating I was told the following: "Remember that in most cases zeners are shunt regulators, not series. You have to drag more current through a 5W zener for it to regulate well. If you replace that 1W zener with a 5w you may see poorer regulation of the supply."I did a bit more research into it and found some articles that seem to confirm what he says.

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2017, 01:19:52 am
content: Before I saw his post, I did was the same math Tomorokoshi did and came to the same conclusions; R806 as shown is under sized and Q807 should be 3 watts or more and probably 5 watts.

author: jastreb
author_rank: Contributor
timestamp: October 09, 2017, 10:28:27 am
content: So what is the final conclusion? Should I be able to user a higher wattage zener without any issues? say 5W?

author: Armadillo
author_rank: Super Contributor
timestamp: October 09, 2017, 10:38:44 am
content: It will be more that 133mA, the emitter is also supplying current.But the current is going to be supply to other circuit.You can stay with the 1W rating but go slightly higher would be a good option.Edit: 1N5351B

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2017, 06:36:21 pm
content: For the zener diode, a 5 watt part will work fine and I would use 3 watts minimum.

author: jastreb
author_rank: Contributor
timestamp: October 26, 2017, 12:57:50 am
content: ok Thank you all. I will go with a 5W and see what happens.