qucs_s VS ttyloopdriver

When talking about open source tools, here are some:

KiCad may be used as a schematic entry tool for ngspice, especially for discrete or PCB-based electronics. You may watch several simulation examples (including oscillators) here: https://forum.kicad.info/t/simulation-examples-for-kicad-ees.... KiCad integrates ngspice internally, but may also provide external ngspice with netlists for simulation.

Another GUI to ngspice is XSCHEM, especially useful for IC design work (see https://xschem.sourceforge.io/stefan/index.html). Device models are available by the Open Source PDKs from Google/Skywater, Google/GF, or IHP. A growing community is supporting digital, analog or mixed-signal design flows.

QUCS-S (https://ra3xdh.github.io/) is a GUI for ngspice or XyCE.

Indeed device models have to be added manually to the devices in the circuit schematic, when invoking ngspice via KiCad or QUCS-S, except for some basic devices with integrated models. Models are provided by device makers, distributors or web sites like this one: https://ngspice.sourceforge.io/modelparams.html .

https://ra3xdh.github.io/ (QUCS-S - seems to be active)

​https://ra3xdh.github.io/

https://ra3xdh.github.io

It's still missing a comprehensive component library like LTspice does, though.

KiCad can also simulate circuits through NGSpice, but it's missing a library and suffers from a lot of quirks.

Also see Qucs-S, which is a GUI for ngspice and a few other engines.

https://ra3xdh.github.io/

Try https://github.com/ra3xdh/qucs\s)

> Out of curiosity, wouldn't it be easier/cheaper/faster to use some existing off-the-shelf design and drill/machine some holes if you need it slightly modified?

For small production runs, definitely. There are standard aluminum extrusions for boxes with PC boards. You slide the PC board in, and provide custom flat end plates with holes for connectors and controls. Here's one of mine.[1] The end plates were cut on a laser cutter. Here's a supplier in China.[2] For small boxes, the aluminum extrusion alone should cost a few dollars.

Somewhere above a few thousand, custom injection molding becomes cheaper. Amusingly, these are better boxes than plastic injection molding, but don't look like consumer products.

[1] https://github.com/John-Nagle/ttyloopdriver/raw/master/board...

[2] https://www.alibaba.com/product-detail/CHANGHE-electronics-a...

You don't have to let the spikes from the switcher get very far.

Here's the schematic for a switcher I designed.[1] This is a strange application - USB power in, 120V out, to drive an antique Teletype machine. Without any filtering, there would be huge spikes in the DC across C1-C2. But it didn't take much filtering to fix that. There's a small ferrite bead at L2, and an RC filter at the snubber at R1-C7. The back to back Zeners are to absorb inductive kickback from the output electromagnet. That's the output side. On the input side, there's more noise suppression, to prevent injecting noise back into the USB power source, which is usually a laptop here. Note L1 and C12. Those are all tiny surface mount parts, total cost in quantity maybe US$0.20.

It's an exercise in LTSpice to get the values right and make the DC power smooth DC, in both voltage and current. This is well understood.

There are radio hams using this thing, and they report it's not blithering in the RF spectrum.

[1] https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...

I designed a DC-DC converter once.[1] This is an exotic application - providing 60mA Teletype signals at up to 120VDC, with power from a 5V USB port.

There are two main trouble spots in DC-DC converter design - protection and noise.

A switching power supply is a dead short across its input once the inductor has saturated. The switch, usually a power MOSFET, needs to turn off on every cycle before that happens. Otherwise, something will fail and probably burn out. Also, the failure mode of power MOSETS is usually "on". So protection circuitry is needed. Fuses, current limiters, etc. This is why UL approval for switchers connected to the power line is important.

Switchers work by generating big inductive spikes. Those spikes are supposed to be directed into capacitors and smoothed out into DC. Without suitable filtering, spikes will be pushed into the power source, the load, and the RF spectrum. A few ferrite beads, Zener diodes, and small capacitors in the right spots will fix this. LTSpice simulation is useful in picking the component values. You're not done until both the current and voltage curves are flat.

[1] https://github.com/John-Nagle/ttyloopdriver

It's extremely useful when used for its intended purpose - designing power supplies using Linear Technology components. Here's a design I did with it.[1] This is a little box for driving antique Teletype machines from a USB port. Those need a current loop running at 60mA constant current, up to 120VDC, driving an electromagnet with a 5H inductance. I wanted to generate that from the USB port's power, which required a moderately exotic switching power supply design.

The LTSpice simulation made it possible to get that right. Especially from a noise control perspective. Switching power supplies generate spikes, and so does turning off that electromagnet with its huge inductance. Those spikes have to be kept out of the USB port or its protection circuitry will shut it down. Spikes need to be suppressed in both the current and voltage dimensions. LTSpice lets you watch the switcher spikes, the elecromagnet spikes, and the inrush current. When you add a few surface mount capacitors and ferrite beads in the right places, the spikes can be almost totally suppressed. It just took a few cheap parts. The simulator lets you find values that work.

The SPICE models for Linear Technology components match reality well. That's what LTSpice is really about - a good model library.

[1] https://github.com/John-Nagle/ttyloopdriver/tree/master/circ...

The true differentiator in ES is practical experience rather than book knowledge once you're past a certain point and fluent in C/C++/Python syntax. Obviously spend the time to be able to read a schematic (for instance, look through Nagle's ttyloop driver, download Gustedt's Modern C, pick up K&R, and so on, but...

The LTSpice simulation shows that they're all needed. The simulation was used to choose the values.

This is why good switching power supplies have more parts than bad ones. You see that in teardown videos.

[1] https://github.com/John-Nagle/ttyloopdriver

[2] https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...