Ok, it may be related to the adhesive I'm using to glue the toroids on the 3D prints.. I'll wait till tomorrow for it to cure and test again..

I'm trying to build a portable/battery powered core rope ROM after successfully having built a "desktop" one.

This is the device I built:

enter image description here

I'm having trouble however with the waveforms of the transformers when the 2 AA batteries start to drain, with just one or two transformers it works fine until 2.5V and even less, but in the case of the following set of transformers at 3V already the one with the blue line does not trigger the latch (ch 4 on the scope).

enter image description here

I've been empirically swapping toroids with more or less windings to see how well I can get it to work at lower voltages but now I'm just plainly confused because the three transformers shown on the scope shot are made from the same toroids(datasheet), with the same amount of windings. I'd understand if there was a bit of variation (i.e. maybe I counted the windings wrong or there is some fabrication tolerance) but this waveform is just way too different. I wound a new one and it had the same lower waveform, however if I swap a "good" toroid with a "bad" one in position, the waveform follows the toroid. So I guess it is something related to the winding or material of the toroid.

  • Is this something design related or fabrication related? (i.e. the permeability is way lower on this one and I have to test a lot of them?)
  • Is there something that maybe I'm not taking into consideration? Do the order of the toroids matter? Does the distribution of the windings around the toroids matter? (although they are identically wound as far as I can tell)

These are the 3D prints that the toroids are glued on:

enter image description here

Here follows a block diagram of the system that produced these waveforms:


simulate this circuit – Schematic created using CircuitLab

(Sorry I could not find a better way of drawing transformers with multiple primaries)

p.s.: this just wasn't a problem on the desktop version because I could drive them a lot harder since parts count and power weren't issues

  • \$\begingroup\$ Why don't you just put a boost converter on the board to get a stable voltage power source? \$\endgroup\$
    – sx107
    Commented Nov 2, 2018 at 2:36
  • \$\begingroup\$ @sx107 -- parts count and cost, as this is intended to be a project for possible workshops, etc (I guess if I were to do that I'd prefer to just use 3 batteries instead of 2) \$\endgroup\$
    – Wesley Lee
    Commented Nov 2, 2018 at 2:43
  • 1
    \$\begingroup\$ I know little about this sort of memory - born too late - but I thought they used a ferrite material with a very square BH loop. \$\endgroup\$ Commented Nov 2, 2018 at 3:43
  • \$\begingroup\$ @SteveHubbard - I think those are core RAMs, that relied on the histeresys of the material \$\endgroup\$
    – Wesley Lee
    Commented Nov 2, 2018 at 3:44
  • \$\begingroup\$ Thanks. I clearly know even less than I thought :) . I'll read up on them sometime. Keep us updated. \$\endgroup\$ Commented Nov 2, 2018 at 4:11

1 Answer 1


(I think answering it myself is better than deleting the question or leaving it unanswered, so here it goes. If anyone has a better explanation I'll gladly accept the answer.)

TLDR: I had just glued that toroid in particular and the uncured glue changed the characteristics of the transformer.

About 18hrs later, this is how the waveforms look like:

enter image description here

Turns out, as I am testing different toroids, I had just wound and glued one to a 3D print with UHU all purpose glue. As it still had not cured and was still quite liquid, I believe it changed the overall characteristics of the transformer.

I realized this could be the case when I was moving the board around and when my hand got close to one of the "good" transformers, its waveform had changed. Then I remembered that the glue was still "liquid" on the "bad" transformer. Facepalm.

  • \$\begingroup\$ This is quite an interesting project! I can't understand the scope display. There are several bits of information about time, but can you mention the standard time per division on the horizontal axis? I'm wondering if the glue or movement affecting the shape of the pulse is capacitive, and not really affecting the core's properties at all, unless the glue was really hot. Capacitive coupling and parasitic effects however can happen anytime. \$\endgroup\$
    – uhoh
    Commented Mar 25, 2019 at 23:58
  • 1
    \$\begingroup\$ Time per division on the image is 500nS. Maybe it could be capacitive but I find it unlikely since being just close to the wire doesn't change the signal. I don't understand permeability very well but I suspect that holding the coil/ferrite changes the overall permeability of it so the field/current gets affected? \$\endgroup\$
    – Wesley Lee
    Commented Mar 26, 2019 at 10:49

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