DIY double insulated PSU to replace batteries - Page 1
Forum section: electronics / projects

author: ecclectrics
author_rank: Contributor
timestamp: September 13, 2022, 05:16:46 pm
content: Hi All,I have several small measurement units which are powered from 9V blocks. Since the batteries tend to be dead when I need them, I want to build a small power supply to replace them. This supply needs to be floating for the units to work, hence I want to build it according to double insulated standards. My understanding thereof is as follows:A basic insulation is provided by using insulated wires inside the PSU and obeying creepages/clearances for PCB traces.A second protective insulation is basically provided by putting the whole circuit in an insulating plastic enclosure. Trouble begins as soon as one drills the first hole in this enclosure to connect the PSU output or anything else.To ensure that also the output etc has a second protective insulation, a suitably rated transformer must be used. These carry the symbols "isolating transformer" or "safety isolating transformer". The datasheet often states "prepared for protection class II" and they typically have their windings on split bobbins.A last consideration is that if the mains cable should come loose inside the equipment it must not come into touch with anything connected to the PSU output. To this end I want to separate the interior of the enclosure into two parts, one on the primary side of the transformer and the other on its secondary side as sketched in the attached drawing. The only electrical connection between both will be on the PCB.My questions are:Is my understanding of the requirements for double insulated construction correct?Do the proposed measures qualify for that purpose?Is there anything else I would need to consider?Any help is greatly appreciated!Regards,RolandP.S. Maybe I should add that this is strictly a hobby project and no agency approvals will be needed. I just have the ambition to understand and build it according to the state of the art.

author: mariush
author_rank: Super Contributor
timestamp: September 13, 2022, 05:57:41 pm
content: transformer -> bridge rectifier (or 4 diodes) -> capacitor to smooth out the rectified voltage -> 9v regulator (or whatever voltage you want)You can do it without big heavy transformers using digital isolators, but they're very inefficient and low power and a bit noisy (you need to add some capacitors and inductors on output to clean the output) ... and last but not least EXPENSIVE.For example, see Adum5000 :https://www.analog.com/media/en/technical-documentation/data-sheets/adum5000.pdfIt's 5v or 3.3v input, 5v or 3.3v output , up to 500mW (100mA on 5v but that would be enough for most things powered by a 9v battery) ... and would do that with around 33% efficiencyOnce you get your isolated 5v 100mA, you could use a simple basic voltage doubler chip (charge pump) and a couple ceramic capacitors to double the 5v to around 8-9vThe 5v input for the chip could be a 5v charger you don't trust, or could be a capacitive dropper + zenner or 5v regulator deal

author: ledtester
author_rank: Super Contributor
timestamp: September 13, 2022, 06:51:00 pm
content: You could also build your 9V supply out of rechargable batteries.Rechargable batteries in the 9V form-factor are pretty expensive but you could make a 9V supply out of some Li-ion cells. 2 Li-ion cells gives a nominal voltage of 7.2 to 8V. Or use 3 cells and a regulator.

author: thm_w
author_rank: Super Contributor
timestamp: September 13, 2022, 09:14:52 pm
content: Any reason you can't just use a 9V wallwart and have a 9V battery snap terminal on the end of it?

author: TimNJ
author_rank: Super Contributor
timestamp: September 13, 2022, 10:47:13 pm
content: I do not know the nature of your "measurement units", but just be mindful of leakage effects in the AC/DC converter and how that might affect whatever you are measuring. If it is very sensitive stuff, there may be good reason to use a battery based supply.

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 12:40:47 am
content: Dont build safety critical systems until you are very comfortable with the terminology and details. As above:Quote from: thm_w on September 13, 2022, 09:14:52 pm99% of the time building a power supply is a waste of time and money, just buy something for the task. There are very very small class II mains adaptors available (but not quite as small as a 9V battery).

author: wilhe_jo
author_rank: Regular Contributor
timestamp: September 14, 2022, 08:12:23 am
content: Quote from: ledtester on September 13, 2022, 06:51:00 pmYou can get them ready made... including some types with micro-usb port for chargingI have both (those with external charger and those with a built-in one)... both work flawlessly... in my comb-generator, they do better than poor "real" ones as they tend to be able to supply more current.73

author: macboy
author_rank: Super Contributor
timestamp: September 15, 2022, 01:38:24 pm
content: There is more to it than galvanic isolation.The circuitry of the "measurement units" is referenced to one of the inputs, typically the negative or shield, but not always. It is floating from actual earth ground. When battery powered, the capacitance between this reference and ground is almost non-existent. This will no longer be the case after powering it from any type of power supply connected to mains. It can be floating of course, using galvanic isolation (transformer), but there will be some added capacitance to ground. This causes a frequency-dependent AC leakage current to ground. There may also be some non-negligible DC leakage current, especially at higher voltages. Unless you have a Megger to test this, it is impossible to say. Manufacturers of sensitive floating measurement devices (bench multimeter is a good example) use a specially shielded transformer to provide both better isolation (higher withstand voltage) and lower capacitance/leakage to ground. These special transformers can be expensive; I recall a few people around here giving up on repairing 6.5 or 7.5 digit bench meters due to the cost of a replacement transformer.Specs like Class-II are designed for human safety, to ensure that a person will not be electrocuted. The amount of AC and DC leakage currents allowed by those specs may be insignificant to a person's body but may be significant to the measurement and/or the device under test.

author: strawberry
author_rank: Super Contributor
timestamp: September 15, 2022, 03:23:58 pm
content: Quote from: macboy on September 15, 2022, 01:38:24 pmif you dont need that 7.5 digit , I will repair transformer or upgrade leakage spec

author: ecclectrics
author_rank: Contributor
timestamp: September 17, 2022, 12:05:17 pm
content: Thanks to everyone for all the explanations. Unfortunately they don't really address my questionsI am aware enough of the importance of the power supply for sensitive circuits to judge when the battery can be replaced or which measures need to be taken to make it work.Also saving time and money is not my primary goal when pursuing electronics as a hobbyWhat I am not so familiar with are the safety requirements for double insulated equipment and how they can be meat in a DIY project. So if anyone can provide information on these issues I'd be very grateful!

author: Someone
author_rank: Super Contributor
timestamp: September 17, 2022, 01:20:17 pm
content: Quote from: ecclectrics on September 17, 2022, 12:05:17 pmIt is because meeting class II safely is out of scope for DIY, too much experience, difficult supply chain, and expensive specialised test equipment. "state of the art" is not something you can realistically pursue, that's all high frequency III/V semiconductor stuff with custom magnetics.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 17, 2022, 03:13:12 pm
content: Quote from: ecclectrics on September 17, 2022, 12:05:17 pmThere are reasons why DMMs are battery-powered.That is why it is much easier to leave batteries at theirs place.If you work with DIY-electronix a lot then just buy a wall-powered multimeter. If not - buy several spare batteries. It is much more easier and effective.(I'm sorry if I seemed a bit rude).

author: strawberry
author_rank: Super Contributor
timestamp: September 17, 2022, 03:58:27 pm
content: engineers crate safety requirementsstrike your device with tesla coil , if it survive, it pass all requirementscopy* fluke 732AI guess for consumer products double isolated means transformer placed inside device (plastic or wood case)