Unknown Zener - Page 1
Forum section: electronics / repair

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 10, 2024, 07:46:50 pm
content: I have a very old but nice Viewsonic LCD monitor with a failed power supply (Delta ADP-60BF) that I can't find a schematic for. I believe I have found all the failed parts, but one of them is a surface mount zener that I can't determine the value of. But I do know what the output and test point values are. I'm guessing its an over-voltage protection that I may be able to take out of the circuit. Nonetheless, my question is: Would it be possible to simulate a zener with a variable power supply? There are a few others in the circuit that I have determined to be in the 10-18 Volt range. So that will be my upper limit.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 10, 2024, 08:31:31 pm
content: A simple Zener model is a regular rectifier diode in series with a power supply. The equivalent Zener voltage is the sum of the power supply and the forward drop of the diode. So a 5.6V Zener might be modeled as a silicon diode with the cathode on the positive terminal of a 4.9V power supply.That is, the cathode of the Zener is the anode of the silicon diode, and the negative terminal of the power supply is the anode of the Zener.

author: jwet
author_rank: Frequent Contributor
timestamp: February 11, 2024, 04:14:02 am
content: Be a bit careful here! Most bench supplies only "source" current with a low impedance. A standard supply won't sink current the way you might expect a "spice" type ideal voltage source might. A 2 or 4 quadrant type source could simulate a Zener but you're not likely to have that. An electronic load might be another way to get there. You can possible heavily load your supply and put the diode to that, it could sink to the extent that the load can.

author: exe
author_rank: Unknown
timestamp: February 11, 2024, 07:02:07 am
content: Quote from: Tomorokoshi on February 10, 2024, 08:31:31 pmThe model you mention would only conduct one way, while zener conducts both ways, just with a different voltage drop. Don't know if it matters here.An "adjustable zener" is TL431, a pretty ubiquitous part. You can use it to simulate zener. Can be adjusted with a pot. Just be careful with capacitive load, check this post:https://www.eevblog.com/forum/beginners/tl431-and-ceramic-caps/msg3252328/#msg3252328.I suggest put a resistor in series with TL431 to limit current it may try to sink.The downside is tl431 (and other IC regulators) have much slower clamping speed comparing to zenner. So, might not be suitable for protection, beware of that.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 11, 2024, 12:59:52 pm
content: thanks for the replies. I think I'm going to take a different approach. The zener looks like a gate voltage clamp for a switching MOSFET which needs to also be replaced. My replacement is a slightly different spec, but I'm going to install a zener a couple volts lower than Vgs max.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 11, 2024, 01:38:17 pm
content: Make an adjustable Zener to test which Vz works better:https://www.eevblog.com/forum/projects/floating-variable-zener/

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 11, 2024, 04:36:23 pm
content: Quote from: RoGeorge on February 11, 2024, 01:38:17 pmI remember seeing that post. Thanks for bringing it back up.I got the power supply back up and running, but was nearly taken down by a rookie mistake. I replaced the parts, checked everything over, powered it up and immediately cleared the fuse. I spent a couple hours running down the short and finally realized the FET was shorted to the grounded heat sink. The original MOSFET I replaced was a plastic tab 220 and my replacement was metal tab. An insulator kit solved that problem. It's all good now.