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Home Brew Analog Computer System

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GK:

--- Quote from: GK on January 10, 2013, 09:53:35 am ---Attached is the complete schematic for my logarithmic amplifier module. Due to the utility of log amps in a analog computer, I've decided to build 10 of these instead of only five. A custom 2U 19" chassis will house 10 of these along with their power supply. Another 2U chassis will house 10 exponential amplifier modules.

The transfer function is simply:

Eout = K * ln (Ein).

K = a fixed scaling factor of 20. The operating input voltage range is 0.01V to 100V, producing an output voltage in the range of +/- 92.1V.
The output amplifier clips at approximately +/-105V, so the dynamic range is a smidge over 4 decades.

The actual log converter based on the SSM2212 is a "fast" design I came up with on my own. Over the 4 decades it is very accurate (overall significantly better than 1% when trimmed).

A conversion to a logarithm of any base is simply a matter of external scaling by the programmer.

--- End quote ---


Two years and ~one month (wow) later and I've made a number of improvements to this original design. Attached is the final schematic for the board finally about (waiting for the end of Chinese New Year holidays.....) to be manufactured in a quantity of ten. For the record I've stumbled across prior art and my unique logarithmic converter circuit turns out to be not very unique after all.

TimNJ:

--- Quote from: GK on February 03, 2015, 05:31:33 am ---
Ancient analog computing books that I've tracked down on Abebooks listed here:

http://www.glensstuff.com/analogbookshelf/analogbookshelf.htm

--- End quote ---

Hi Glen,

I've been learning about analog computation in my signals & systems class. I'm finding it very interesting and I want to get some literature on it. Out of the books that you own, is there one that you think would be a good book to start with and give a good overview for an analog computer beginner?

GK:
Hi Tim,

I'd recommend Basic Analog Computation by Gerald R. Peterson.

TimNJ:

--- Quote from: GK on March 02, 2015, 07:50:14 am ---Hi Tim,

I'd recommend Basic Analog Computation by Gerald R. Peterson.

--- End quote ---

Thanks! Looking forward to more of your great work.

GK:
Tim - thanks for the interest,
Wilfred - great article.



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Well I have the first of the 3 integrator chassis' up completed electrically and fully tested and all works absolutely perfectly except for one annoyance. The reed relay model I selected for switching the integration capacitance doesn't seem to be up to the task. I am getting (about 5% of the time) contacts intermittently sticking on after switching in the ~1uF integration capacitor.

The reed relay I am using is HE721A1210, "form A" here:

http://www.farnell.com/datasheets/16797.pdf

The contact ratings are:

Voltage switching : 200Vdc
Current switching : 500mA
Current carrying : 1.2A
Power switching : 10W

When the ~1uF capacitor is switched in by the relay, the potential difference between the contacts can be as high as 210V. The integrator op-amp output V saturates at +/-105V. For example, if the 1uF was switched out while the integrator was railed out at -105V, the 1uF will be floating with a charge of 105V. If the 1uF is then switched back in when the op-amp output is railed out at +105V, then at that instant the relay contacts potential difference will be at that worst case 210V.

The op-amp power output stage is short-circuit proof and current limited to +/- 20mA, so that sets the maximum peak surge current through the capacitor when it is initially switched in. That is well below the relays maximum rated "switching" current of 500mA.

The relay datasheet makes not mention of switching current de-rating with switching voltage, but I figured that 20mA should be pretty safe. Well, unfortunately, not so. I'm getting intermittent contact sticking/fusing with that 20mA peak surge with less that 100V across the contacts. Giving the stuck relay a gentle tap reliably opens the contacts.

I guess that I am going to have to source a replacement relay, quite possibly not of the fragile reed type. I doubt that I'll find one with the same footprint so it will most likely have to be a fudge to the PCB.
 
Oh well, I guess you can't build something this complicated with out at least one thing going wrong.
 


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