The objective here is to make a low-cost replacement power supply. We’d like to stay close to the original design.
Picking the Transformer
Below is the schematic for the original power supply. More info on the original can be found in this article. We’ll need a transformer with two center-tapped coils on the secondary side.
The original transformer had two 17v AC secondary taps, but that is not something that can be readily bought.
The AC output is used for the +5v and -5v rails, anything between 13VAC~22VAC would work. The DC output is used to generate the 12v DC rail, we want to rectify it to about 13VDC~24VDC. Therefore each secondary will need to be about 14VAC~26VAC.
A perfect match for this is the Triad Magnetics F-241U. It is rated at 36VA and has two 18V secondaries. The current price on Mouser is $24 USD.
An alternative is the Bel Signal Transformer 36-1, which is currently priced at $70 USD. For folks in 220V~240V regions of the world, there a 230V option available, the Bel Signal Transformer DL-36-1.
You can often find these transformers on used market as well (e.g., eBay).
Features and Design
There are a lot of features we can incorporate into our design such as power switches, LEDs, accessible fuses, etc. However, I would like to keep it as close to the original design as possible.
I do want to add an LED so that we know when we forget to unplug the power supply. This is a linear power supply, so it should be unplugged when not in use to save energy, and having an indicator helps with this. For this, I ordered 5mm 12V DC green LEDs which include a plastic holder to be mounted to our power supply box.
We’ll use the same 1A fuse as the original. I plan to insert it internally just as the original to keep a simple, clean design on the outside. I’ll solder it directly to the wire, as there’s not enough room on the PCB that we’re going to make.
I won’t be including a power switch on the power supply, the TRS-80 has one on the back. You can add one if you’d like.
Instead of having a power cable connector on the back for the power cord, I rather just run the cable out just like the original. I don’t want to look for a spare power cable every time I want to use this power supply.
We want to make our power supply compact with a minimum enclosure. We can cross-reference the transformer dimensions with the enclosure to find a good fit.
The Bud Industries PN-1328-DG is the best widely available enclosure at $11. The “DG” in the part name means this is the higher quality made of high-impact ABS material with a dark gray color. The color will match the TRS-80, so we won’t need to worry about the heavy transformer inside if it gets dropped from a low hight.
The transformer will fit perfectly in there and we can mount it to a custom-made PCB.
For the DIN 5 connector, instead of making my own cable, I bought a high-quality pre-made cable and just cut it in half.
Hands-on Testing
Before I drew/ordered a PCB, I wanted to verify the setup works as intended. Below is my first setup to test the power supply with the TRS-80.
My new power supply outputs 17.3AC and 15.5DC, but note I’m in Japan where the input voltage is 100v instead of 120v, so my number here is slightly lower. My original TRS-80 power supply output was 21VDC and 23VAC. One concern I had was whether the change in input voltage would affect the 5v and 12v rails and if those needed to be adjusted via the pot on the main board. I confirmed that the 5v and 12v voltage stayed precisely the same when switching between the original and new power supply, which is excellent!
The lower voltage output compared to the original power supply resulted in less heat generated on the TRS-80. The main Q4 2N6594 power transistor on the TRS-80 used for the 5V rail runs at a much cooler temperature of 58°C instead of 69°C. The zener diode at CR2 which generates the -5V also ran cooler at 50°C instead of 60°C. I would consider this a win.
The DC output powers the LED via a resistor. I initially planned to use a 10k resistor for the LED, but it was too bright. A 50K resistor works much better.
I noticed that the wires inside the DIN cable I used were very thin as they are meant for carrying data, not power. When I used a thermal gun, I noticed the cable does warm up as seen in the above image! The cable gauge is about 26 AWG and the temperature rises to 31°C. According to this calculator, 26 AWG is appropriate for 1A at 2 feet. I went back and checked the cable on an original power supply, which also went up to 31°C, so this should be fine.
PCB Board
Now that we are confident this setup works, we can proceed with designing and ordering a PCB board. Below is my final design.
I’ve added some additional holes in case someone wants to use a slightly larger transformer (supports 2″, 2.25″, and 2.5″ width). I trimmed the board’s max length down to 10cm so that it can be bought at a lower price on PCBWay or JLCPCB. T3 and T8 refer to the tap number on the transformer. They will be the two outer taps.
Gerber can be downloaded here, you can upload it directly onto PCBWay or JLCPCB for ordering.
Final Assembly
The PCB arrived and everything fit perfectly in the box.
After putting it together, it looks great.
Alternatives
There is a design available online that uses two transformers. It provides additional smoothing and fits inside the TRS-80 Expansion Interface. It, however, seems relatively inefficient because now you have two linear power supply transformers running. It would’ve been better to use one 18V AC transformer and a 15V switching mode power supply on the DC side. The suggested transformed (70060K) is only rated for 25VAC. This seems a bit low, especially if someone tries to run the Model 1 and the Expansion Interface. My TRS-80 alone uses 29VA!
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