Using a Schottky Diode - Page 1
Forum section: electronics / beginners

author: atomarc
author_rank: Newbie
timestamp: July 12, 2025, 12:52:50 am
content: A newbie to this site and not versed in electronics. I have a small 90v DC gear motor I'm powering with two small offshore DC variable speed motor controllers (PWM). Two are required as the gear motor will be run at two different set speed using relays..a RUN speed and a JOG speed. I've wired the output of both controllers together and tied them to the motor leads. The controllers didn't like this and blew a fuse. This happened when only one was powering the motor. So..is it possible to wire Schottky diodes into the output of both controllers to act as 'one way valves' so they won't see each other and all can be connected to the motor leads. I can do this with relays, but if the Schottky idea is valid, it would be much simpler. Am I wasting my time. The motor is 90 volt DC at about 100 watts, and it will never see a full speed of 90 volts, more along the line of 30 volts.Thank you in advance for any insight,Stuart Miller

author: Analog Kid
author_rank: Super Contributor
timestamp: July 12, 2025, 12:58:18 am
content: Don't see why you'd need Schottkys here, unless the lower forward voltage (0.3V vs 0.7V) is an issue; just any ordinary diode, which will function as a "one-way valve".

author: BillyO
author_rank: Super Contributor
timestamp: July 12, 2025, 02:12:09 am
content: Why Schottky?What current do you need?Low forward voltage, high current Schottky diodes can have high reverse leakage current, so the questions are pertinent.

author: TimFox
author_rank: Super Contributor
timestamp: July 12, 2025, 02:23:18 am
content: High voltage Schottky diodes are expensive compared with normal PN Si rectifiers.They are used in low-voltage circuits where the lower forward voltage is important, but as mentioned above the reverse leakage (when you want them OFF) is usually higher for the Schottky diode.Schottky diodes are also used when sub-nanosecond switching is needed.

author: atomarc
author_rank: Newbie
timestamp: July 12, 2025, 03:07:24 am
content: So, no on the Schottky and yes on a simple diode. Is there a diode in series with each controller output (two)...on each controller? Is there a specific method to size the diode other than current and voltage? Thanks for your help.Stuart

author: Analog Kid
author_rank: Super Contributor
timestamp: July 12, 2025, 03:10:23 am
content: Quote from: atomarc on July 12, 2025, 03:07:24 amIn your case, no, since response time and capacitance are not relevant factors to your application.Voltage and current limits are definitely important here; you can easily figure this out.

author: Smokey
author_rank: Super Contributor
timestamp: July 12, 2025, 03:37:48 am
content: This is another one of those examples of solving the wrong problem.Get a single decent motor controller that can do both jogs and "run" speeds. (Or more likely hook the one you got up to a microcontroller that can send it two different pwm frequencies).

author: atomarc
author_rank: Newbie
timestamp: July 12, 2025, 04:24:12 am
content: Sometimes, solving the problem is directly related to the cost of solving the problem..hence the stuff from China! I appreciate the professional help you folks have provided, thank you.Stuart

author: Smokey
author_rank: Super Contributor
timestamp: July 12, 2025, 05:16:55 am
content: Quote from: atomarc on July 12, 2025, 04:24:12 amSo you're saying you can't find a single DC motor drive for a 30V 100W motor that can take a pwm input (or speed control voltage, or something?)

author: gbaddeley
author_rank: Regular Contributor
timestamp: July 12, 2025, 05:33:45 am
content: Go for at least 200 piv and a few amps, given it’s an inductive load. Eg 1N5404 jelly bean part.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 12, 2025, 11:24:41 am
content: I think, if the OP wants to stick to their original idea of using two diodes in an 'OR' configuration to prevent back-feeding, then they need a third diode (shown in red) to protect the supplies from the negative-going back EMF from the motor.Does that seem reasonable?

author: MrAl
author_rank: Super Contributor
timestamp: July 12, 2025, 11:35:08 am
content: Quote from: SteveThackery on July 12, 2025, 11:24:41 amI was going to suggest something like that also but happy you did it first. I didn't feel like drawing a schematic right now.There is a good chance that the power supplies will pick up the load in any case because the back EMF will be negative, and that will just cause one of the diodes to conduct.However, I'd use the third diode anyway because the other possible issue is the time delay between switching from one power supply to the other. That could mean that both supplies are off for a short time and who knows what problem that could cause. An extra low cost diode is well worth it.

author: atomarc
author_rank: Newbie
timestamp: July 12, 2025, 06:34:40 pm
content: Not being electronically sharp, this is how I envisioned connecting the diodes in an attempt to isolate one control from the other, they don't seem to play well together. As stated earlier, a relay could be used for this purpose, but the diode deal seemed easier. Would what I have drawn work?Thanks,Stuart

author: Analog Kid
author_rank: Super Contributor
timestamp: July 12, 2025, 06:38:32 pm
content: Yes, but assuming both controllers have positive and negative outputs, one each of the diodes connected to them (the negative rail) is backwards. Flip it around and it should work.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 12, 2025, 07:19:09 pm
content: Like this.Can someone check to see if the diagram is correct? Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: July 12, 2025, 07:24:17 pm
content: It depends on the speed controller. A H-bridge type that does active braking or that does closed loop speed control by monitoring the motor's back-EMF isn't going to 'play well' with the extra diodes.I'm with Smokey. There's probably a pot on the drive board to set the speed. Removing it from the board and adding a relay circuit to switch between two panel-mount pots of the same value for jog speed and run speed so one motor control board can do both speeds isn't the most complicated mod in the world to do. Worst case you have to switch two connections (track end and wiper) between the pots with a DPDT relay, but odds are that simply switching the wiper with a SPDT relay will be sufficient. These will be low current, probably low voltage, control circuits so a small cheap signal relay will be ideal.

author: atomarc
author_rank: Newbie
timestamp: July 12, 2025, 08:29:59 pm
content: Thanks Gentlemen,Fiddling with PC boards is out of my wheelhouse, as they say. Two DPST relays can do the job and I'm more comfortable with those items.Thank you,Stuart

author: MrAl
author_rank: Super Contributor
timestamp: July 13, 2025, 06:30:46 am
content: Quote from: SteveThackery on July 12, 2025, 07:19:09 pmHi,Why do you think you need that arrangement? Is there some reason you think you need two more diodes?

author: SteveThackery
author_rank: Super Contributor
timestamp: July 13, 2025, 10:45:53 am
content: Quote from: MrAl on July 13, 2025, 06:30:46 amYou don't need them if the OP can confirm that the drivers and the motor all share a common ground. In that case two diodes are fine, as I drew in my first diagram.But the OP then offered a second hand-drawn circuit in which he appeared to want to treat the controller outputs as isolated from each other - that is, he apparently wanted the OR function on both the positive and negative outputs from the controllers. But he made a mistake, so my second diagram corrects the mistake in his drawing.The second circuit would work, but I think the first circuit should be absolutely fine and it is what I would use. You just need to be sure it is OK to connect the negative output leads from the two controllers together.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 13, 2025, 10:50:38 am
content: Quote from: atomarc on July 12, 2025, 08:29:59 pmBe careful!If you just use relays in the way you describe, make absolutely sure they cannot both operate at the same time, because if that happens you will connect the two controller outputs together and blow that fuse again.Using the diodes connected as shown in my first diagram is actually simpler and safer.

author: atomarc
author_rank: Newbie
timestamp: July 13, 2025, 05:37:47 pm
content: I agree it seems simpler using the diodes arranged as shown, but one of the controller will have to run the little motor in JOG...forward AND reverse. I think that complicates things, right?Stuart

author: SteveThackery
author_rank: Super Contributor
timestamp: July 14, 2025, 09:52:35 am
content: Quote from: atomarc on July 13, 2025, 05:37:47 pmOh, crap, I didn’t realise the jog had to work in both directions! The OP has not mentioned that in any of his posts. If that really is required, then dump all my diagrams, forget about diodes, and use a relay.Or better still, install a proper bidirectional motor controller with variable speed. Anything else is a bodge.

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2025, 02:15:42 pm
content: Note to other novice posters:Do not worry about overspecifying the situation relevant to your question.Repliers can ignore irrelevant items, but as in this example about diodes the bi-directional polarity is a fundamental property of the situation when diodes are used.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 14, 2025, 02:49:25 pm
content: Of course you can't parallel the outputs, they won't run in sync, eventually one will push while the other will pull, creating a massive short.Why do you need two controllers?Just use one and adjust the speed, this is normally done with a potentiometer.You could replace it with resistors and relays, so each relay engages a different speed.

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2025, 04:38:00 pm
content: Also, motor controllers are generally not designed to have any switching between their outputs and the motor, and many specifically prohibit such switching as the resulting transients are likely to blow their output stage. Therefore the idea of using a relay or contactor to switch between two controllers set to different speeds is unlikely to be reliable, unless you add an interlock circuit to ensure both controllers are disabled, *AND* the motor is not turning, before switching.

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2025, 08:40:48 pm
content: Quote from: atomarc on July 13, 2025, 05:37:47 pmIt's a bad idea.The speed of most motor controllers can be set with a 0 to 10V signal or a potentiometer. Use a single motor controller and use the relays to switch the resistors to set different speeds.

author: atomarc
author_rank: Newbie
timestamp: July 15, 2025, 05:25:19 am
content: All the above are great answers, and I'm sorry I didn't include more info in the original post, but because I'm not electro-smart, I didn't know some of the info was important to the solution. I don't want to bust open these cheap, Chinese speed controllers and fiddle with the boards, so the solution for me is to use two controllers, one for JOG and the other for Run (at speed). The customer has to have the ability to tweek the pot to adjust the speed of these two controllers to match the ultimate output he wants, so they have to be infinitely adjustable..no fixed speed.I'm retrofitting a machine that looks like it was built in a barn, so it's one of those occasions where you get what you get..low tech.Stuart

author: Smokey
author_rank: Super Contributor
timestamp: July 15, 2025, 06:20:20 am
content: Quote from: atomarc on July 15, 2025, 05:25:19 am...The ... customer...be afraid... be very afraid...

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2025, 06:58:53 am
content: Quote from: atomarc on July 15, 2025, 05:25:19 amThe correct solution is to buy a single motor speed controller with an input for an external potentiometer.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 15, 2025, 09:59:49 am
content: Quote from: atomarc on July 15, 2025, 05:25:19 amImportant:does the motor need to run in both directions?