Zener Diode Forward Current - Page 1
Forum section: electronics / projects

author: praveen_khm
author_rank: Contributor
timestamp: March 18, 2015, 05:12:56 am
content: Hi,I am planning to use this diodehttp://www.semicon.panasonic.co.jp/ds4/DZ2J036_DED.pdffor one of the projects. This is a 3.6V zener diode and am using this with a USB board. The datasheet says that Repetitive peak forward current is 200mA. However, I am using this along with a USB to reduce 5V to 3.6V, and the output current in USB is 500mA. Is this the right diode to use, or should I use one which says maximum current rating is 500mA?Thanks,Praveen

author: tonyarkles
author_rank: Regular Contributor
timestamp: March 18, 2015, 05:30:17 am
content: Are you trying to use the Zener to reduce the 5V USB rail down to 3.6V for your circuit? If so, you should be looking at IZ instead of IF. When you're using a Zener as a regulator, it has to be reverse biased.

author: praveen_khm
author_rank: Contributor
timestamp: March 18, 2015, 05:47:22 am
content: Correct. I am using it to reduce 5V to 3.6V and it is reverse biased in the circuit. What is IZ and IF and is the part I chose not correct? Attached the schematic I am using and the Diode connections shown.

author: mij59
author_rank: Frequent Contributor
timestamp: March 18, 2015, 05:58:51 am
content: To reduce the voltage use two si-diodes in series with the 5V, a ldo voltage regulator, or a smps .How do you use the zener diode to reduce the voltage ?

author: praveen_khm
author_rank: Contributor
timestamp: March 18, 2015, 06:05:30 am
content: QuoteIt works. Zener diode breaks down once the voltage crosses 3.6V and anything in excess of 3.6V is moved to ground. I have used a normal zener diode to do this and it works. The question is related to "Repetitive peak forward current is 200mA". Does this part work, or should I use a 500mA forward current diode?

author: Psi
author_rank: Super Contributor
timestamp: March 18, 2015, 06:10:17 am
content: You're using the zener on the USB data lines, the 500mA rating refers to the power feed.I'm not sure what the max current for D+/D- is but you can be sure its a lot less than 500mAEDIT: looks like the driver output resistance is min 40ohms. So D+/D- short circuit to gnd is max 125mA and (5V-3.6)=1.4V / 40R = 35mA through the zener

author: praveen_khm
author_rank: Contributor
timestamp: March 18, 2015, 06:32:11 am
content: QuotePerfect! So, I will go ahead and order these diodes. Thank you...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2015, 10:24:00 am
content: Quote from: praveen_khm on March 18, 2015, 05:47:22 amUh... this won't work. Or it'll certainly never negotiate High Speed, if your devices are capable of it. It will be marginal at Full Speed.Typically, data lines are protected with diode clamp or low capacitance TVS type devices.The application shown is NOT being used to reduce VCC.Note that low voltage zener diodes (under 5V) have very poor characteristics, and should be avoided for voltage regulation or limiting purposes if possible. An LDO is normally used to obtain a lower voltage from the VBUS supply.Tim

author: tonyarkles
author_rank: Regular Contributor
timestamp: March 18, 2015, 04:29:20 pm
content: Yeah I really don't think this is going to work for what you're trying to do. You don't care about the forward current, you care about the Zener current, and it's 5mA (if I recall, I looked at the data sheet last night). Use an LDO instead.

author: Moondeck
author_rank: Regular Contributor
timestamp: March 18, 2015, 09:16:48 pm
content: Why the hell dont you get a level converter? Why go cheap ass crap?

author: georges80
author_rank: Frequent Contributor
timestamp: March 18, 2015, 09:37:22 pm
content: Huh??DM/DP is 3.3V or lower. What are you doing with zeners on the DM/DP lines per your drawing??cheers,george.

author: ajb
author_rank: Super Contributor
timestamp: March 18, 2015, 10:29:16 pm
content: I've seen this done in microcontroller projects that bitbang the USB interface--such as those USBASP AVR programmers. Using zeners on the data lines allows the MCU to run at 5V, which saves having to include a 3v3 regulator and allows the programmer to connect to targets running at 5V without needing level shifters. It may not be the best solution, but it's cheap and generally works well enough for such purposes.

author: georges80
author_rank: Frequent Contributor
timestamp: March 18, 2015, 10:41:37 pm
content: Quote from: ajb on March 18, 2015, 10:29:16 pmYUKPoor man's level 'shifting'. And folk wonder why they have all sorts of weird issues with their junky/clone programmers etc...Anyhow, I suppose given the OP's post he's doing something similar, using some 5V USB hack and trying to make it conform to 3.3V DM/DP differential signaling. Though the OP seems confused regarding the necessary zener specs and throwing in the nominal 500mA max USB current rating spec that has nothing to do with DM/DP.cheers,george.