The Power Supply Modding Guide (or TL494 -> Anything)

[Th3_uN1Qu3]

Re: The Power Supply Modding Guide (or TL494 -> Anything)

Is it Christmas already? Looky what i built... a protection board! Undervoltage/short circuit, overvoltage, and two channels of overtemperature protection.

A few issues were found and fixed in the prototype, hence rev 1.1. Most notably, the LEDs have been spaced properly. I found out after i did the board that 5mm leds wouldn't fit because they were too close together... so i had to use 3mm for mine. 5mm ones will fit with the new layout.

Connect the "out" of this circuit to pin 4 (DTC) of the TL494. If the comparator detects any problem, the TL494 is shut down. Note that the circuit isn't latched: overvoltage and overtemp protections will allow the 494 to restart once the fault condition clears. This may, or may not be desirable for your use.

Use this voltage divider calculator to find out the resistor values for your needs. Output voltage must be 2.5v (as this board uses a TL431 2.5v reference), and select resistors so that you have 1-2mA of current. You do remember Ohm's Law, don't you?

For the nitpickers:

1. You'll have to mount one electrolytic cap on the bottom of the board. You need a cap over the top resistor of the undervoltage protection to create a time constant long enough for the main caps of the supply to go past the undervoltage protection threshold (that's for yourself to figure out), otherwise the protection circuit won't allow the power supply to start up at all.
2. Yes, the screw holes are too small and too close to the corners. You fix it.
3. I only installed one channel of overtemperature protection on my board, don't point that one out. If you don't need one of the protections just take out the LED. And yes, the LEDs are a working part of the circuit. If you don't want them use 1N4148 diodes instead and increase the 5k1 resistors to 10k.

[Th3_uN1Qu3]

Re: The Power Supply Modding Guide (or TL494 -> Anything)

Fixed my router box a few days ago btw... Found its original PII-300 which seated snugly and fired right up. I'll have to buy it a slightly better chip but for the time being it does fine.

So, i give you the first TL494 daughterboard layout! It is the standard halfbridge mode with collector output and 5v reference voltage. Assumes that driver transistors and pull-up resistors for the TL494 are on the power supply PCB. Brings out both error amps and the deadtime control input for external protection support. There is space for 2nd order compensation on both EAs (if 3rd order is necessary, then the other RC group can simply be installed on the main power supply PCB, since the voltage divider resistors are going to be there anyway), and error amp 2 is configured with a variable reference, to be used for current limiting. You can see pics of the actual board here. Use ExpressPCB to open and print the layout for toner transfer.

Next up, the accompanying LM339 board, supporting overvoltage, undervoltage (which also acts at short circuit), and overtemperature protection! Complete with LED indicators for each.

[Th3_uN1Qu3]

Re: The Power Supply Modding Guide (or TL494 -> Anything)

I forgot to mention that at least one of said multimeter(s) will need to be able to measure inductance as well, if you want to wind your own transformers or inductors.

Stay tuned, coming soon... I have LOTS of stuff to do including a couple exams coming up that i failed in the summer so imma have to take them again, and my router box just crapped out (was a Celeron 566 in a Slot1 board, the CPU slot was wiggly, and when the box was moved for cleaning under that desk, it failed altogether) so currently my only source of internet is leeching wifi from my neighbors. Yay.

[TheLaw]

Re: The Power Supply Modding Guide (or TL494 -> Anything)

Unique you're a boss. Thanks for sharing all of this.

Your theme-song: http://www.youtube.com/watch?v=nW4Acfqzpz4

[newtoCaps101]

Re: The Power Supply Modding Guide (or TL494 -> Anything)

Thank You uN1Qu3! I will keep an eye on this thread, I have plenty of power supply's laying around. Some broken, some dont work, Others I used up for parts

[everell]

Re: The Power Supply Modding Guide (or TL494 -> Anything)

Another tool I find quite handy is a Dremel drill and a number 60 drill bit. Seems like I am always needing to drill another hole so that I can add something. The number 60 drill bit is the smallest I have found at the hardware store, and is small enough to add a hole without demolishing several circuit traces at once.

[Th3_uN1Qu3]

This topic is a WIP, and for now it will mostly be a placeholder for things to come. Here i will try to cover the basics of power supply modding, ranging from the simplest to the extreme. I will also present the TL494 in detail.

First we will start with Tools of the Trade. This is a bare minimum of what you need to survive as a PSU modder. You will need:

• At least one multimeter with frequency and duty cycle measurement. Two are better.
• Another two cheap meters, but make sure one of them can measure AC current. So you can measure input voltage, input current, output voltage, output current at the same time and determine efficiency. Working only with line powered PSUs? Then you can substitute a Kill-A-Watt or similar for two of the meters. Still, having a bunch of meters handy is always good. I have six.
• Oscilloscope, 10MHz at the very least. A x10 probe with compensation is also mandatory.
• Two soldering irons. One low power ~25W for general work, the other 60W or more, chisel tip. For removing big things like transformers and inductors.
• Desoldering braid and desoldering pump.
• Enameled copper wire for making your own transformers and inductors.
• The Mega SMPS Design Cheat Sheet
• Screwdrivers, screws, boxes, things like that.

I will not bore you with basic theory and i assume you have some knowledge of that. Instead, we will explore hands-on how to modify a power supply and what to look for, and whether what you want is achievable with your particular supply.

A lot of us want to turn old PC PSUs into proper all-round bench supplies. I will show you how to achieve that, even if the PSU uses a custom controller. I have several "universal" TL494 daughterboard PCB designs which i will make available, which you can use to run *any* PWM-based power supply.

Questions? Feel free to ask.