AMMETER PUT INTO CIRCUIT TO MEASURE ACTUAL CURRENT DRAW
I bought a 20A ammeter through Amazon.com from Uxcell, claimed to have a proper internal shunt and accurate to a reasonable degree. With the ammeter in the circuit, even with 44 bulbs installed it barely registered 1/2 amp draw. However, I was suspicious about the correctness of this given the dimming that happens with that many bulbs plugged in. I am returning it to Amazon since it doesn't work properly.
CROSS CHECKING AGAINST MY VOM AC AMP MEASUREMENT
My VOM has an AC current measurement but a max range of 200ma, not very useful for measuring more than a handful of bulbs, but it did give me a baseline for comparison. With four bulbs inserted, I measured almost 7V from the supply and 172ma of draw.
The defective ammeter on the other hand is off by a factor of 4 to 5x. As a second check, I grabbed a second VOM, one with a higher range of AC current measurement, and put it into the test circuit with 10 bulbs installed. I should see 400+ ma of current draw if my other measurements are correct. I recorded 440ma with the unloaded voltage of 7.06 dropping to 6.89VAC when the lamps were lit.
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| Current consumed by 10 bulbs lit by Lamp Test |
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| Populated the board with just 10 bulbs to do the measurement |
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| Voltage while supplying 440ma to light the lamps |
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| Unloaded voltage of the AC supply |
Gate draw is going to be pretty negligible by comparison to the lamp current. The resistor voltage divider to each gate consumes about 0.2ma of current, 34ma for all 164 circuits simultaneously conducting. The thyristor adds some microamperes to the draw, which we can ignore.
ACTUAL BULB CONSUMPTION DETERMINED
That suggests that at full supply voltage in the IBM 1130 these will draw about 40-50ma apiece when lit. All 164 positions, if the same type bulb is used, would only draw around 7 amperes, well within the capability of the new beefier transformer that will arrive Tuesday.
Some T 1/4 size bulbs have a spec of 200ma, others 50ma, and it is possible that I have a mix of both types in my supply. I have been measuring their cold resistance (10 ohms approximately) to verify lack of shorts, but I decided to set up a jig to test all bulbs for current draw and sort them into groups.
The consumption of the bulbs directly wired to the AC feed is different from their draw in circuit, in part because of the approximately 1V drop across the thyristor while it is conducting. I think that IBM specified 7.25VAC for its lighting circuit with that drop in mind, yielding about the nominal 6.3V at the bulb that matches the bulb specifications.
The measurements confirmed the expectation that the entire board will consume between 7 and 8 amperes depending on the exact voltage inside the IBM 1130, with all lights illuminated simultaneously.
NEW BULBS ON ORDER ARE HIGHER DRAW - NET EFFECT
If I determine that I have about 150 bulbs that draw 50ma and add 14 that draw 200ma, that would bump the board consumption up over 9 amperes but still within the short burst capability of the transformer that will arrive soon. I should look for a source of bulbs at the proper current, since each burned out bulb will force me to bump up the board load by 150ma, which would scale up the draw quite a bit over time.
As an alternative, if I could find 2114 bulbs, which are the type I already bought, that would be a preferable component to use with my console. In the end, I did manage to buy a couple dozen of the desired bulbs and thus can do a full up test of the board with Lamp Test once everything arrives here.
7-9 amps seems like a bodacious amount of current. Makes me curious, what fraction is that of the 1130's main power supply?
ReplyDeleteAnother thought that occurs to me: those triacs are switching rapidly all the time the machine is running, making the lights twinkle as fast as the register contents change. Do they make any electrical noise? Is the display panel emitting RF hash?
Third notion: Given they display binary values, the lamp duty cycles must average out around 50%. Some will be on less or more because some registers spend a lot of time with a high byte or nybble of zeros (or ones), but across all lamps the duty cycle must be in the 40%-60% range. So will the display panel be averaging a constant draw of 2-4A while running?
Hi David
ReplyDeleteThe 7-9 amperes is on the 7.25VAC lighting circuit, so that would only be around 65W of power, still under 300W even if I used the high current 8610 bulbs.
The 1130 consumes roughly 5KW of power served from a 220V circuit, thus the lighting is a sliver of the total consumption.
As far as duty cycles, probably even a bit less than half because the registers are just 96 of the 164 locations, others are status that may only light 1 of a group, such as the T clock and X clock displays. Still others have less than half average usage for example the Interrupt Levels are off far more than on.
I had to engineer for the use of Lamp Test, which forces all lamps on, even if the average is less than half of that.