Why does this diode/capacitor combination appear on every board in a vintage computer?

Question

I am working with a 1970s era computer system with multiple functional boards on its bus (similar to S100 but not S100.) Unlike S100, this system distributes power as already-regulated 5V (and 12V and -5/-12V) to each board in the system.

Every board, near its power connections, has a diode and a 100uF capacitor. I am not a trained engineer and want to understand the purpose of these components in this case:

Sometimes the diode is a 1N4001, sometimes it's a 1N914, but for all boards there's this sort of network around the power connection. (On boards that use multiple of the voltages, it's more complex, with multiple capacitors and diodes across the rails.)

I understand the frequent appearance of the ceramic disc 0.1uF decoupling (or is it bypass?) capacitors near each IC across Vcc and GND as a noise reduction function.

Is the larger (100uF) electrolytic near the power connection serving the same purpose but for the whole board, or is it doing something else?

Is the diode there as a kind of reverse current protection? What circumstances might it be trying to protect against?

Answer 1

The diodes are reverse voltage protection. They conduct if the voltage is reversed shorting out the supply enough to blow a fuse or trip a crowbar. It’s done this way rather than a series diode to avoid the voltage drop a series diode would introduce.

The large capacitors are reservoir caps. They are there to provide a reserve of current when there is a momentary high draw from the circuit that would cause a voltage drop in the resistance between the board and supply.

Answer 2

When you have a system with bipolar power supplies (+/-12, +/-5V), various start-up, shut-down and fault conditions can lead to reversed voltage on one of the supplies, which can destroy chips.

It's cheap insurance to include a reverse-biased diode. See, for example, this schematic from here:

The 100uF caps are just bulk storage.

A smaller diode such as the 200mA-ish glass diodes might be deemed adequate if the loads on the supplies are minimal.

Answer 3

It's good practice as all cables have inductance. Even if your source have ideal stable 5.00 V at all times, if it's say a meter away, you will have some tens of nH of unwanted inductance in series. If your downstream device have no steep load changes what so ever, you may get away with it from an over/undershoot standpoint and you only need your capacitor for EMI purposes.

Your average power supply will however happily try to respond to load steps as fast as it can and if you can't tolerate much overshoot, you need a capacitor to "terminate" your cable and once again make it appear as stable 5 V despite fast load changes. It will however not take away any resistive drop from the cable resistance.

When you say 1970 era computer, I would expect honking amount of current on the 5 V and any over or undershoot would suffer from ΔU=L*di/dt. L may be small, but di/dt can be large and the allowed ΔV overshoot very small.

As for EMI, capacitors on any constant voltage rail is almost always wanted and the input (or output) of any device is a prime place to put them. 100 uF electrolytic normally have too high ESL to reach high enough up in frequency to cover all your needs, so hopefully there are some MLCCs in parallel somewhere.

Green curve is 10 nF, blue is 100 uF:

Diode is as GodJihyo says reverse voltage protection. It requires a fuse or some other kind of current limitation in the system to function.

Answer 4

The diode provides low-energy reverse-polarity ESD protection during handling.

Answer 5

Worthy of mention is that tantalum capacitors are very sensitive to reverse voltages. With a multi-rail design (+12, +5, -5 etc) it's quite possible that a momentary reverse voltage could be experienced during power-down. Obviously, a signal diode would quickly die if the supply were connected incorrectly, but it would be able to protect against small residual currents.

Answer 6

A very nice blog post that I recently read explains the capacitor:

In the end, even seemingly minor digital switching can cause significant voltage fluctuations and electrical noise across the entire circuit.

[..]

unexplained and hard-to-diagnose issues can creep up with ease.

[..]

the purpose of decoupling capacitors: they are placed across the voltage supply lines and physically close to the offending chip to handle switching transients while preventing high currents and minimizing voltage fluctuations in other parts of the circuitry.

As you describe it, it sounds like the capacitor may be in a suboptimal place for this purpose, but:

[..] the understanding of decoupling caps continues to be hit-and-miss. Some folks skip them altogether and live to tell; others follow ancient lore of uncertain origins, producing monstrosities [..]