Gosh, this made my day! A very amusing read, thanks to have taken the time to write it instead of just saying "yeah, just put in those 1k5 and stfu" ;-)
On 17/02/14 19:50, Vince Mulhollon wrote: > On Monday, February 17, 2014 11:38:46 AM UTC-6, Fabio Battaglia wrote: > > I don't have 1K/1.2K SIP-10 resistors, but I do have enough 1.5K, > do you think those would be OK too? > > > I obtained a copy of the Wilcox book to go with my 68000 board build > and this kind of thing is discussed at length (pages and pages of > equations) early in the book, from memory less than 500 ohms could hit > the overcurrent limit for his design and more than 8500 ohms would > undercurrent as a pullup depending how thirsty the inputs were for > current. Lower switches faster but wastes power, higher uses less > power but switches slower. > > I ran the numbers myself using a real live Motorola 74LS682 datasheet > and you've got two criteria: > > 1) Too low of a resistance to Vcc and a TTL L output will overcurrent > and catch fire. > So figure the data sheet Vol = 0.5V and Vcc at 5 V, thats 4.5 volts > across the resistor. And the 74LS682 (or most any LS) will catch fire > around 24 mA so 4.5V/0.024A = 187 so less than 190 ohms to Vcc could > burn out a LS output. Thats assuming there's no load on the TTL > output other than the resistor, which would be pretty pointless. So > I'd keep it WELL above 200 ohms. > > 2) Too high a resistance to Vcc and the input current of 0.4 mA will > pull a false low. So figure 5 volts Vcc, half a volt for ground > bounce and half a volt for noise, that leaves 4 volts, then Vih for > 74LS is 2.0 volts, so the resistor must drop less than 4-2 = 2 volts. > At the previously defined 0.4 mA. So 2/.0004 = 5000 ohms. Thats > assuming it only drives one input. So I'd keep it well under 5000 ohms. > > So, well over 190 ohms and well under 5000 ohms. How bout a geometric > mean? (5000*190)**0.5 = 975 ohms so ideally it would be around 1K > which is probably where the designers are getting 1K from. > > How bad is 1.5 K? Well, lets get a percentage of the range, from at > the low end where it catches fire and the high end where it eternally > outputs a false Low level. and call that full range 100%. That 1500 > ohm pack is (1500-975)/(5000-190) = about 11 percent high of center of > the safe range. Now you were probably building stuff with 10% > tolerance resistors so the range of plus or minus 5 percent higher or > lower means the possible ranges just barely don't overlap by about a > percent, so there should be a measurable effect. > > But will the measurable effect matter? Well, worst case is your 10% > resistor could be 5% high (or 5% low, which doesn't matter) and you've > decided to use something that centers 11% high, for a theoretical peak > of 16% high of perfectly centered on a 100% scale. 16 outta 100 is a > long, long way to go until it fails in eternal false L level. So > you're all good, by far. > > What if you got the idea of throwing two 1500 resistor packs in > parallel? Well that would be 750 ohms. (750-975)/(5000-190) is 5 > percent low, so its technically better, however, look at the failure > mode... Too low means it burns out permanently, but too high doesn't > hurt anything. So you're closer to parallel two, but in a dangerous > direction, and further away but in a safer direction to just use one. > Combined with two of them costing twice as much, I'm thinking, don't > parallel them. Also the component you never install can never fail or > have a soldering issue or add to capacitive load, so its twice as > reliable, sorta, not to install parallel resistors. > > TLDR don't worry about 1500 ohms, on the other hand, I wouldn't go > trying 4700 ohm packs.. > -- > You received this message because you are subscribed to the Google > Groups "N8VEM-S100" group. > To unsubscribe from this group and stop receiving emails from it, send > an email to [email protected]. > For more options, visit https://groups.google.com/groups/opt_out. -- Dott. Fabio Battaglia -- You received this message because you are subscribed to the Google Groups "N8VEM-S100" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
