The question of the relay quality has resurfaced and I want to characterize my stock.

What is their endurance ? The only way to try is to make them work, fast and hard, and see how long they last. The best circuit for this is a ring oscillator.

I have had bad experiences already (see the #SPDT16: 16-bits arithmetic unit with relays logs) but this time I know these relays better and I shouldn't make some blunders. In particular, the PBRL reduces wear on the contacts so a string of relays will do a good delay line.

Another interesting side is that the relays' speed can be measured, simply by the ferquency of a ring, depending on the number of relays.

The ring did not work as easily as expected, I had tried one or two relays in series but not 4 or 5.

Something I learned : decoupling is even more critical ! 470µF for 4 relays seems to work... Otherwise, I start to increase the PSU's voltage. With enough capacity, the voltage doesn't need to increase. They don't tell you that in the books !!!

OK, with the decoupling solved, the voltage becomes important. It drifts in a few minutes, and this can actually be seen on the 'scope, as the duty cycle is affected. The sweet spot seems to be around 3.8V..4V but it might change. It's important to find a voltage that works in cold and warm conditions.

Because of the crazy quantity of bouncing, it's hard to measure a precise frequency. But I estimated the following:

• 2 relays: Tcycle=15..17ms
• 3 relays: Tcycle=23ms
• 4 relays: Tcycle=30ms

It seems that each relay adds about 7ms to the cycle time, which includes the high and low states. A relay seems to increase the chain's delay by 7/2=3.5ms, which is consistent with my previous measurements.

From this, I can estimate the cycle time of the circuits. If I can keep the critical datapath short (less than 7 relays), I could reach maybe 40 to 50 instructions per second :-D

I have left the oscillator circuit working for the night at about 43Hz under 3.8V (@0.21A), and I will see tomorrow how it has aged... That's about 150K cycles per hour, or a million cycles in 7 hours. I know it's not signifiant because I should test more than 4 relays at once (20 ?) but it's a start.

And yes, those prototyping breadboards are not reliable... But my first try with a soldered board didn't work, I suppose by lack of decoupling.

Bad contacts stalled the circuit 2× in 1h already... I must solder that circuit.

3h later: the circuit stopped several times already. I must change the voltage to 3.4V because the working range was reduced to 3.21..3.75V (below the 3.8V I had initially set). When I started, it worked up to 4.1V...

For the next systems, should I "burn-in" the relays for a few hours ?

Almost 12h later: the working voltage range is now about 3.3..3.8V. I've set it to 3.5V. Just like I computed in the first PBRL log. I don't know if the ambiant temperature has affected it...

If I'm ever doing a computer with this, I'll have to add a diagnostic and tuning mode to set the POL DC/DC.

18h later: the range has been reduced to 3.25..3.65V so I set to 3.4V.

I should probably increase the bias resistor and halve the series resistor. The control current will be doubled but the working range will be greatly extended.

25h later: still ringning. The voltage range is now 3.4..3.7V => set to 3.5V.

Ambient temperature : 15°C.

I can't make sense of the variations of the range. Is my lab PSU faulty ?

However the current remains stable: 0.20 to 0.21A.

Yes, relays should be considered as current-driven devices.

28h later: it stopped ringing. The relays work but the range (3.45..3.65V) has shrunk too much for a reliable system. I'll see how long it works at 3.55V.

I've never seen such an erratic-looking behaviour, the range variations are hard to explain or to model. I must design a better set of resistor values.

H+36: Still ringing. The range didn't change much this time, did I reach a plateau ?

H+42: still clicking. That should now make about 6 millions of on-off cycles.