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| Home Brew Analog Computer System |
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| GK:
--- Quote from: T3sl4co1l on March 31, 2015, 12:00:10 am ---Note that shorting the output will also damage the input protection diodes. --- End quote --- No it doesn't. I have over 48 hours of continuous cycling on the 1st prototype and the input diodes survived unscathed. That is with the output railed to +105V and the output shorted to ground - 10s off/10s on. I can't see the unit getting that degree of punishment in actual use. This relay looks adequate: http://www.digikey.com.au/product-detail/en/EC2-12NU/399-11029-5-ND/4291095 |
| GK:
Worst case input diode surge current for an instantaneously shorted output. Note the 1uF instance exceeds the diodes surge current spec (1A for 1mS and 4A for 1uS), but they do survive. |
| T3sl4co1l:
Is the inductor actually rated for (read; doesn't saturate) at 10A? Tim |
| GK:
--- Quote from: T3sl4co1l on March 31, 2015, 12:00:10 am ---Under those transient ringing conditions, the peak current may also damage the input and output protection diodes. --- End quote --- Charge equalization between the caps will not force any current to flow through the protection diodes and as I stated previously, peak current through the input protection diodes when the 1uF is switched in is limited by the current that the op-amp output stage can provide - limited at 20mA. The dv/dt of the supply rail charge/collapse during power on/off is orders of magnitude too slow to cause any significant timing capacitor discharge currents to flow through the protection diodes. Yes, regarding that quick and hasty sim the inductor will be well and truly saturated at 11A, but the DCR is a couple of ohms. As a quick aside before lunch I managed to partially kill a 1n914 with 1uF capacitor discharge pulses. At a 30A peak limit the diode was unaffected even after dozens of discharges but just over 20 discharges with the peak current ramped up to 90A finally did it. The diode is still Hi-Z in the reverse direction but Vf has dropped to 380mV. The only situation in which the input protection diodes can be subjected to a current surge beyond the specified guaranteed safe maximum is if the integrator output is abused by abruptly shorting it to ground with a low impedance when the output voltage is either steadily or instantaneously at a high value. Accidently miswiring the integrator output when patching a program and then firing the computer up, resulting in the integrator op-amp current limiting, will not have the same effect, as at that time the timing capacitor has not acquired a charge. I honestly don't see it as a potential problem given normal use of the machine, and more so as my attempts at blowing up the initial prototype by shorting the output were wholly unsuccessful. |
| GK:
In solder rather than spice this time. I get a peak current of 24A. Yes the current sensing resistor is a wire wound type, but having such a low R value I can assure you that it has negligible inductance. Yes the 100uH inductor must be saturating to some degree but obviously the effect is not instantaneous and the pulse rise time and peak amplitude is still significantly effected. The initial negative spike I believe is due the inductors capacitance. If I remove the L the peak current drops to 10A and has an almost instantaneous rise time on the 5uS/div time scale (just as you'd expect from ohms law) but I have since zapped the diode with that 24A over 300 times now with no ill effects, which is in line with my prior testing on the initial prototype. Vf for the diode peaks at 5.2V. It therefore has a dynamic resistance of approximately 0.22 ohms. I have desoldered the diode(s) and tested for reverse leakage at significant V and forward voltage drop and it appears to be completely unaffected. There just doesn't appear to be enough energy in this discharge to kill the diode. I also found that the ability of the 1uF cap to deliver a high current pulse is less than you'd expect from a basic calculation from its basic ESR. In order to get that diode-killing 90A discharge that I mentioned in a previous post I had to ramp the supply up to 150V and delete all of the series resistance (except for the 0.1R). I've ordered the replacement relays. |
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