Successful Endeavours - Electronics Designs That Work!


Power Supply Specification

The idea for this post came from a discussion in IEEE Collabratec on how to design a Power Supply. The question of how to design a Power Supply seems innocuous enough until you really start to think back on past Power Supply designs. I was originally concerned that this was a student wanting someone else to do their coursework assignment for them but the discussion progressed into something quite useful. Here is what I posted after getting the following specification:

  • Output Voltage: -300VDC
  • Output current: 0.5-20mA
  • Tolerance: 30Volts
  • Input Voltage: 220-240 AC
Power Supply

Power Supply

Analysing Requirements

Hi …

is this project part of your course work?

The reason for this question is that the intent of coursework is to help you come to grips with what you are being taught and learn it from a practical perspective as well. Among other things, this helps a lot with retention.

I run a company that designs products for other people. I only employ graduate engineers who have demonstrated the capacity (though their academic results) and inclination (through their having done their own projects and learned how to use the teaching they have received) to do engineering and to be capable of quickly learning all the things they can’t teach in a course.

So if it is coursework, what subject is it part of?

Because if they want you to design a switching mode power supply, that is very different to an AC rectified transformer design.

You also need to be careful with a design assignment like this (coursework or a product that will be manufactured) because it is capable of killing you if you don’t use good safety practices.

I’ll assume your tolerance figure is +/-30V = +/-10% of -300VDC. So the voltage at its maximum excursion from 0V could be -330. And the maximum current is 20mA. This is 6.6W of power so it will get hot. And again, there is enough voltage to kill you.

If it is for a commercial product, then there are usually other constraints. Here are some of the questions I would be asking:

  • The input voltage range is specified as 220VAC to 240VAC but it is normal to allow for short term transients. So does the output voltage have to be clamped during mains transients?
  • Is soft start required?
  • How quickly must it respond to load transients?
  • What is the load and how much does it vary?
  • Does the input stage need to be designed so that it keeps harmonics and power factor under control (this is a legal requirement for some product types)?
  • Is there a maximum size?
  • What is the design life and/or MTBF (Mean Time Between Failure)?
  • Is fan forced convection allowed, and if so, is that even a good idea because of the MTBF or because it goes inside a sealed cabinet)?
  • What is the maximum temperature rise allowed on any of the outside surfaces?
  • What type of connections for the input and output voltages?
  • What has to happen if the output goes short circuit or open circuit (you had a minimum current of 0.5mA so is there a minimum external load and what is allowed to happen if that isn’t there)?
  • What is the environmental specification (0->70C, -20->85C, -40->85C etc)?
  • Is there a manufactured cost target?
  • Do you have to simulate it only, or are you building one and proving the performance?
  • Are there any special safety or EMC compliance requirements for this application?

And there are lots of other questions like this for a real product design.

So regardless of the reason for the design, understanding the intent of the exercise is important to delivering a satisfactory outcome.
This is one of the reasons engineering is not easy. We create the future. Others say that as well. But we also create the infrastructure and products that make a more advance future possible. And there are always lots of constraints.

I hope that has maybe encouraged you to think a bit deeper about the question. It is unlikely you will solve a problem you don’t fully understand. And an answer you don’t work through for yourself will probably not expand you understanding.

Successful Endeavours specialise in Electronics Design and Embedded Software Development, focusing on products that are intended to be Made In Australia. Ray Keefe has developed market leading electronics products in Australia for more than 30 years. This post is Copyright © 2017 Successful Endeavours Pty Ltd.

3D Printing

3D Printing has been a big topic for some time. It works in a number of different ways and you can print in metals, plastic or paper. For this post, I will focus on FDM or Fused Deposition Modelling. It is also a method within the general area of Additive Manufacturing.

We work with a wide range of Mechanical Engineering and Industrial Design companies and I had expected them to all be right into 3D Printing. But I was wrong. So we decided to do it ourselves. Here is what we were looking for:

  • cost under $5K
  • more than 1 filament material type
  • readily available filament (overnight delivery preferred)
  • 0.1mm resolution
  • fully enclosed
  • can run stand alone (you don’t need to have it permanently connected to a computer)
  • automatic material detection
  • upgradable nozzles and software

What we ended up selecting did all of the above but the price point was under $2K! The unit is an Automaker from Robox.

CEL-Robox Automaker

CEL-Robox Automaker

3D Printer Application

So after making a few of the sample models that came with the unit, it was time to start using it in a real project. One set of negatives about 3D Printing is that it is slow and noisy. So even though we selected a fully enclosed unit, it was still too noisy to sit in the middle of the office. So we relocated it to the workroom where we do the more mechanical tasks. We also had to decide how we would create 3D Models. So lets go through the whole process.

For the project, we needed a custom spacer for a 4-20mA current output voltage sensor. We were redesigning a product that had gone obsolete. Our client only needed 50 of them to replace failed units in the field and for spares over time. Having designed the Printed Circuit Board we now needed a spacer that would allow us to use an insulated case TO220 NPN transistor in place of a TO3 NPN Transistor.

3D Model Creation

The first step is to create a 3D Model of the shape we needed. This turned out to be much easier than we expected. We don’t do mechanical CAD in house so we elected to try an online tool. The tool we selected is TinkerCAD. It works in your browser and you can download an STL file which is the format most 3D Printers can use.

TinkerCAD online 3D Design

TinkerCAD online 3D Design

The software works by letting you add shapes together to make an object. You can also turn any shape into a hole. And you can set the height and location of any object including its elevation. This was more than enough to handle this project. Above is a screenshot of the completed model. We then downloaded it and fed it into the CEL-Robox 3D Printer.

3D Printing in Action

This is a small part and only took 12 minutes to print. 

3D Printed Spacer

3D Printed Spacer

So now we have our Custom Electronics Spacer. And at a small fraction of the time and cost of getting it drawn up mechanically and made by someone else.

Electronics Prototype

Time to assemble the PCB and make sure everything is OK.

3D Printed Spacer Fitted

3D Printed Spacer Fitted

This is perfect. Now we can get to testing the new design and hand it over to our client for them to evaluate and approve.

3D Printing Creates New Opportunities

Having done this one exercise, I can see enormous possibilities for this technology a Successful Endeavours. Not only can it save time and cost like it did for this project, but it allows us to do new things we might not have tried before. For lower volume products, you can afford the tooling for custom injection moulded parts, but now we don’t need to.

This is really going to be much better than I thought.

Successful Endeavours specialise in Electronics Design and Embedded Software Development. Ray Keefe has developed market leading electronics products in Australia for nearly 30 years. This post is Copyright © 2015 Successful Endeavours Pty Ltd.