Protection circuit for 125VA transformer and how to use it with a PWM load - Page 1
Forum section: electronics / beginners

author: rssd
author_rank: Newbie
timestamp: February 24, 2020, 06:16:41 pm
content: Dear members,I am developing a soldering iron circuit and I was looking into a proper power supply design. I discovered that most 24V switching power supplies are not complete potential free but rather have their output GND connected to protective earth. So now I am here with my isolation transformer and I am thinking about how to properly use it.The Transformer we are talking about:https://www.tme.eu/en/details/tts120_z230_24-24v/toroidal-transformers/breve-tufvassons/125VA, 2x 24V Output, 230V InputWhat I think I need for a resistive but switched load. I am not thinking about certification yet but I still would like to do a proper design!Input side:Short Circuit FuseInrush protection, nor sureMOV or similar, not surePower SwitchOutput side:Over Voltage Protection TVSFuseIf the transformer is ideal, I think it would not be acting as an inductor and thus no further protection would be needed if the load is regulated with PWM switching. But as far as is know they still have some parasitic inductance which I tried to simulate with LTSpice, adjusting the K value. But even a K value of about 0.99 seems to create a massive inductive behavior and creates huge voltage spikes on the secondary side. Adding a TVS Diode, just converts those spikes into strong current spikes. The inductance value I used is obtained with a cheap component tester, maybe someone knows some real-world values for those type of transformer. Yes I know, with AC Systems like this the switch off should be done with zero current (voltage with resistive) load and for regulation the switch on can occur somewhere on the have wave, but i still would like to make sure the system can handle it if switching occurs under load.Attached is the schematic and a simulation result plotting the voltage and the current on the output side TVS Diode.So the questions are:Am I missing some circuit elements?How to do a proper circuit simulation? Or protection for the secondary sideThank you!

author: ArthurDent
author_rank: Super Contributor
timestamp: February 24, 2020, 07:42:57 pm
content: In the first place the 1N4148 diodes you are using are normally rated at 150Ma.

author: rssd
author_rank: Newbie
timestamp: February 24, 2020, 08:06:28 pm
content: Quote from: ArthurDent on February 24, 2020, 07:42:57 pmAhh, you got me. I was just throwing those in for simulation, the rectifier will be active with 4 N-Mos to increase efficiency.

author: Benta
author_rank: Super Contributor
timestamp: February 24, 2020, 09:07:16 pm
content: You're overthinking this totally.1: the input fuse is not there for short circuit protection, but for fire protection.2: inrush protection: normally not necessary, the 230 V mains line is stiff enough to tolerate this.3: MOV, sure, if it makes you feel good,4: power switch: yes, of course.5: TVS? Depends on your circuit.6: secondary fuse? Certainly, and this is where your overload protection should be.But frankly: for a resistive load such as a soldering iron, that has a relatively long thermal time constant, switching it on or off every couple of seconds is fully sufficient. A relay is enough.