Where to buy high value zener diodes? - Page 1
Forum section: electronics / projects

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 11, 2018, 12:05:01 pm
content: HelloI am trying to source a zener diode that will take 50V and more than 10 Amps (!).The application: I have a robot arm that is being controlled by an open source servo driver. I have a smallbenchsupply capable of delivering 30V and 5A, but this isn't really man enough to drive the bot or even one motor at any kind of speed. So I have access to anex-telephone exchangepower supply, capable of delivering 48V and 10 Amps.The problem is that it has a very keen overvoltage protection, which cannot be disabled without major surgery, and the servos produce a back emf of 6V when the arm is moving under gravity. I can see this on the smallbenchsupply as it has a display - its max voltage is 30V, but the display will show 36V when the arm is coming down. Net result - theexchange supplytrips. Additional note - its a linear supply.I understand I can use a zener diode to regulate the voltage, but the internet seems awash with tiny value ones, biggest is 10W, and I am guessing I need something in the range of 500W or perhaps more. Any thoughts on where to find one, and likely cost?Cheers

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 11, 2018, 12:09:44 pm
content: Look for TVS = Transient Voltage Suppressors. In principle, they're zener diodes optimized for overvoltage protection. In the catalogs of the major distributors there's plenty suiting your requierements.

author: tszaboo
author_rank: Super Contributor
timestamp: July 11, 2018, 12:20:39 pm
content: There are no "large zeners" avaliabe today. Microsemi makes some 50W versions, otherwise they are 1W and below. If you need voltage protection, typically it is done by a crowbar circuit. I actually have the same issue, and I would love to have some zeners in the 5-10W region, but they just dont exist.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2018, 12:44:42 pm
content: Quote from: NANDBlog on July 11, 2018, 12:20:39 pmThe problem with a crowbar circuit is it's one shot: once it's tripped, everything is shut down and has to be restarted, which is no good if this happens every time the motor is back-driven.Quote from: SpannersToSparks on July 11, 2018, 12:05:01 pmHave you tested it with the telecommunications power supply?If the display is showing 36V, then it's actually generating 36V, not 6V.A large transistor, with the zener diode connected between the base and collector will work. I doubt it needs to be rated to 500W because it won't dissipate that amount of power continuously. It's fairly straightforward to design something capable of dissipating 500W for a short length of time, with long rest periods in-between,

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 11, 2018, 01:03:58 pm
content: There are many "load dump" circuits.What Hero just described is the simplest.You could increase sophistication and complexity with active circuitry like comparators and power transistors, but for what you are looking for, the zener-transistor pair is probably sufficient.

author: Nauris
author_rank: Regular Contributor
timestamp: July 11, 2018, 01:49:08 pm
content: Some avalanche-rated mosfets works as big zeners just nice. Just tie gate and source together. Something like 50W continous from TO-220 packaged one is no problem. The ones I tested were rated 30 V max voltage and actual zener voltage was about 39 V. They are certainly the cheapest big zeners available but of course that kind of use is not specified in the datasheet.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 11, 2018, 02:19:37 pm
content: Another idea for a high power zener. With appropriate precautions MOSFETs can be paralled to increase the current capability.* Edit : Schematic completed cf. NiHaoMike comment.

author: BrianHG
author_rank: Super Contributor
timestamp: July 11, 2018, 02:38:58 pm
content: A transistor boosted Zener would also work.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2018, 02:46:08 pm
content: Quote from: EmmanuelFaure on July 11, 2018, 02:19:37 pmAdd a 1k resistor and 15V zener from gate to ground to discharge and protect the gate from spikes, then optionally add a power resistor in series with the drain to reduce the power dissipation in the MOSFET.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 11, 2018, 03:19:33 pm
content: Quote from: NiHaoMike on July 11, 2018, 02:46:08 pmForgot the gate discharge resistor, my bad.I agree for the other zener preventing the gate breakdown, but when the transients are not too fast or when you have enough capacitance in the circuit, the gate breakdown voltage won't be reached without the protection zener.In my opinion the MOSFETs in through hole TO220-like packages are already the best power resistors available, no need for an additional one : They have a thermally conductive face of great surface quality, a mounting hole, can dissipate ~100W of power and cost the bargain of ~$1 each.Transient simulation attached with rise/fall times = 0.1µsec. No gate breakdown.

author: edavid
author_rank: Super Contributor
timestamp: July 11, 2018, 03:43:50 pm
content: This is confusing. Why is your servo driver trying to dump current into the power supply? Maybe it's the wrong choice for your application.Unless you need dynamic braking, I don't see the point of trying to clamp the back EMF.Have you thought about just adding some blocking diodes?

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2018, 04:09:07 pm
content: Quote from: BrianHG on July 11, 2018, 02:38:58 pmWhich is exactly what EmmanuelFaure's schematic shows.Quote from: edavid on July 11, 2018, 03:43:50 pmMy guess is the built-in servo controller is performing dynamic braking, hence the back-EMF, when the motor is free-wheeling.Using blocking diodes may cause the servo controller to shut-down, due to over-voltage.I've experienced this before. A Darlington pair with a big power transistor and zener diode did the job nicely, but in the end a more elaborate solution was chosen which sensed the current through the motor driver and shorted the supply, when it reversed, i.e. tried to back-feed into the power supply.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2018, 09:49:16 pm
content: I have needed high power zener diodes for the same application in the past and instead made a "power zener" using a bipolar transistor and zener diode.50 volts is within the secondary breakdown region for bipolar transistors but as a practical matter, this should not be an issue once power and temperature derating is taken into account. For instance something like 3 to 6 2N3773Gs in parallel would be needed depending on duty cycle.I would be interesting to use the bipolar transistors in series instead of parallel with multiple zener diodes in series to drive them. This would eliminate any secondary breakdown issues and distribute the zener current over multiple devices.Power MOSFETs which are larger than bipolar transistors are available now but they also cost more for a given power. They still might be worth using if you can get the number of devices down to just 2.