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| Home Brew Analog Computer System |
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| TimFox:
Keep up the good work on this project! In college, we had the Heath EC-1 analog computer with 9 opamps and +/- 60 V output range. I believe that each amplifier used a single 6AN8 pentode/triode for an unbalanced inverter followed by a cathode follower, open-loop gain roughly 1000. A few years ago, I was visiting a Boeing laboratory. They received the Boeing house organ magazine, which was always interesting for its historical articles showing cool aircraft. One month, they had a picture and short article about an analog computer, circa 1950, in the engineering lab. Interesting details: 1. The engineer was properly dressed, with white shirt and tie. 2. To his right, there was a large blackboard with the differential equations and cross-reference to the controls on the analog computer. 3. To his left, there was a 19" rack with about 6 feet of analog computer panels. 4. The article, presumably written by a young person, said something like "The analog computer generated so much heat that 200 vacuum tubes were required to remove the heat." 5. Next month's issue had quite a few corrections from senior Boeing engineers. |
| SeanB:
Can you not put a resistor of around 10R in series with the contact? It looks like the killer is the current spike when the relay switches the capacitor and there is a voltage stored on one set of capacitors that then equalises. High peak current, and with up to 100V of drive that will be a pretty hefty pulse, especially for a small low power relay to handle. |
| TimFox:
I just remembered another article, where the editor must have been a young person. This was in EDN, maybe 10 years ago. It was on the history of the vacuum-tube operational amplifier. First, it described the design by Julie during WWII, which used a 6SL7 (octal) dual triode as the differential input stage. Then, it described the postwar commercial design from Philbrick, a very similar circuit using a 12AX7 (9-pin miniature) dual triode. Both devices were referred to as "high-micron" triodes in the article, as a misunderstanding of "high-{mu}". |
| GK:
--- Quote from: SeanB on March 30, 2015, 04:19:58 pm ---Can you not put a resistor of around 10R in series with the contact? It looks like the killer is the current spike when the relay switches the capacitor and there is a voltage stored on one set of capacitors that then equalises. High peak current, and with up to 100V of drive that will be a pretty hefty pulse, especially for a small low power relay to handle. --- End quote --- Yes, I didn't mention that, but the other cap is only 10nF. Shorting a 10nF charged to 200V on the bench gives me the tiniest spark. Though I guess now too much of a spark for the selected reed relay. In hindsight a better design would have used a SPDT relay to select either one capacitor or the other. I can't tolerate any resistance approaching an ohm in series with the 990nF as that gives too much of a departure from an ideal transfer characteristic for a computing integrator. I just need a more robust signals relay. |
| T3sl4co1l:
Don't forget that, that cute little capacitor offers a peak current of 200A with a pulse width or cycle time around 10ns (frequency ~16MHz). At that voltage, it will arc over before the contacts touch, probably ablating a microscopic crater in the contacts and leaving a molten peak, which welds imperceptibly when it closes. Under those transient ringing conditions, the peak current may also damage the input and output protection diodes. Can you arrange a follower so the capacitor is maintained at input voltage (at one end) without loading the input or output? Note that shorting the output will also damage the input protection diodes. Tim |
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