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Author Topic: Do 7400 (original series) chips always run hot?  (Read 462 times)
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Danger Boy
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I have a circuit with 4 74284 and 4 74285 chips in it, the rest of the chips being 74LS.  The 7400 chips run conspicuously warm to hot.  Warmer than just warm, enough to be uncomfortable if you press your finger on them and hold for 5 seconds, but not to the point of burning or anything.  Is this normal?  I've put the logic probe on all the pins and the chip is getting power and ground on the right pins, 0 or 1 on all inputs (no floaters) and is producing correct values on the outputs. 
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Yes Certainly those '285 are binary multipliers and depending on clock speed some ran hotter. Remember those are "Real" TTL ic's... some ran warm enough that they were placed in Ceramic cases. even today real high speed chips run HOT not just warm. Stop the fan of your computer and see how long it takes to shut down/melt down. My CPU and GPU run about 10deg above the ambient it is 77deg in my room now and both are @ approx. 90 Deg F I have a 6 core AMD 6100 3.3 Ghz Cpu and the of the "Bridge" chips is also the GPU and I am not running a gamers box... just a reasonably fast "normal" box. Silicon will work well to 70 - 80 deg C buut for many other considerations should be kept below 50C which is about what you are measuring with your "digital" thermometer.

Doc
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Yes Certainly those '285 are binary multipliers and depending on clock speed some ran hotter. Remember those are "Real" TTL ic's... some ran warm enough that they were placed in Ceramic cases. even today real high speed chips run HOT not just warm. Stop the fan of your computer and see how long it takes to shut down/melt down. My CPU and GPU run about 10deg above the ambient it is 77deg in my room now and both are @ approx. 90 Deg F I have a 6 core AMD 6100 3.3 Ghz Cpu and the of the "Bridge" chips is also the GPU and I am not running a gamers box... just a reasonably fast "normal" box. Silicon will work well to 70 - 80 deg C buut for many other considerations should be kept below 50C which is about what you are measuring with your "digital" thermometer.

Doc

There is no clock on this circuit.  It's a straight combinatorial circuit.  I am just doing it because I felt like implementing a neat circuit I saw in the TI 1988 TTL Logic Data Book (page 2-789) , the 8x8 multiplier using 4 284/5 pairs with 3 ALUs, 4 dual full adders, and a lookahead carry generator.  The inputs are not fast-changing, only as fast as I can change the switches.  It is still hot.
« Last Edit: August 05, 2012, 01:26:26 am by JoeN » Logged

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I know the circuit well I did several of them Many years ago... The "Clock is the rate at which the data moves through those logic IC's. They weren't static. Your '285 takes 2 4 bit binary words and "Adds them together... although it is a parallel process it Still was "Clocked Logic"... There is a specific rate at which the data moves through the adder... That is the clock rate. TTL was also famous for it's quiescent current and when you produce a chip of that complexity there's a lot of gates drawing power... They get even hotter when you run them fast... BTW LS means "Low Power Schottky" which was about as fast as "regular" 7400 series logic and drew about 1/10th the power and there were ECL logic chips that could ruin your whole day... If you touched them... kind of a circle because ECL and things like USB are Very similar in theory/technology as ECL was the fore runner of what is called LVDS or "Low Voltage Differential Signalling". The theory is that it doesn't take as much power (heat) to move a pair of wires about a common LOW center voltage as it does to take one conductor and slam it between Vcc and ground as fast as you can as for one.. the parasitic capacitance's aren't as big so it doesn't take as much energy (heat) to charge and discharge them. Does this help?

Doc
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Danger Boy
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I know the circuit well I did several of them Many years ago... The "Clock is the rate at which the data moves through those logic IC's. They weren't static. Your '285 takes 2 4 bit binary words and "Adds them together... although it is a parallel process it Still was "Clocked Logic"... There is a specific rate at which the data moves through the adder... That is the clock rate. TTL was also famous for it's quiescent current and when you produce a chip of that complexity there's a lot of gates drawing power... They get even hotter when you run them fast... BTW LS means "Low Power Schottky" which was about as fast as "regular" 7400 series logic and drew about 1/10th the power and there were ECL logic chips that could ruin your whole day... If you touched them... kind of a circle because ECL and things like USB are Very similar in theory/technology as ECL was the fore runner of what is called LVDS or "Low Voltage Differential Signalling". The theory is that it doesn't take as much power (heat) to move a pair of wires about a common LOW center voltage as it does to take one conductor and slam it between Vcc and ground as fast as you can as for one.. the parasitic capacitance's aren't as big so it doesn't take as much energy (heat) to charge and discharge them. Does this help?

Doc

I am not sure that it helps but it is very interesting and useful information in a general way.  As far as my circuit goes, I guess I should just expect those 8 ICs to run hot.  I can live with that as long as it is not damaging them and so far, no problems.  Not like I wanted to run that circuit 24/7 inside a computer anyway, I just wanted to wire it to prove to myself that I could do it and it works.  What it does is takes two 8 bit values and multiplies them a nybble at a time.  Byte 1 is nybbles A,B, Byte 2 is nybbles C,D.  The four multiplier pairs multiple AC, AD, BC, and BD and the other chips basically sum those partial products (utilizing carry lookahead to keep it as fast as possible), just like you would on paper but in octal rather than decimal.  It's a cool circuit.
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The original 74xx series of chips had a long run with many devices made my many manufactures. The series spanned SSI (small scale integration) functions like simple and, nand, or, nor and other simple devices. Then came MSI (medium scale integration) functions like shift registers, counters, etc. Finally a few LSI (large scale integration) devices showed up like in complete 4 bit ALU devices, etc.

So the heat dissipation for 7400 series chips (or any series really) is directly proportional to the number of transistors used in the chip as well as the speed the chip is being operated at.

So yes, some 7400 series chips ran pretty hot and consumed quite a bit of current, so your observations are probably quite normal.

Lefty
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