This is my entry to the 7400 contest.
I have 2 pcs of HD74LS00 ICs and (mis)use it as a full wave rectifier. I was challenged by one of the criteria which is the part that says “Show us what 7400 logic was (never) meant to do”. I was also checking out other 74xx IC and I was figuring out what can I do with a 7400 that other people might not have thought of before. Some perhaps will use it as the usual logic gates and build something out of it. But I was thinking of something simple and yet defies “LOGIC” in a sense.
I want to have an entry that it simple enough that other people can try out with their breadboards. Something that is easy to do, and yet feature a mis(use) of a typical 74LS00 IC.
Watch This Video
The 7400 Based Full Wave Rectifier Schematic
Now here’s something that defies logic. I am using the 7400 chip without even powering it on. Check it out, there is no power coming in at Pin14 which is the VCC. Moreover, if you look at the circuit closely, the GND pin of both 7400ICs which is pin 7 is not connected to the circuit GND. It is actually connected to the + side of the transformer.
The transformer I used has this specs (220v primary /6v-0-6v secondary). When I checked using my oscilloscope, I’m actually seeing a 23v peak to peak sine wave. Since I’ll be mis(using) the 74LS00 as diodes, one chip will be connected to one of the 6v line and the other chip will be connected to the 2nd one. Each chip will rectify half of that AC signal. I’ve placed a 50v/100uf electrolytic capacitor to smoothen it out and produce a decent DC signal.
Here’s how the signal looks at Pin7 of the 74LS00.
This is the signal at Pin5 of the 74LS00.
Notice how the negative part of the ac signal is gone.
This is how it looks when I combine the output of the 2 74LS00.
The above image is how it looks without the capacitor.
Here’s how the signal look when I put in the capacitor.
I’m getting an average of 10.4v DC (without any load).
Here’s another thing that defies logic.
If you check out the datasheet Absolute maximum ratings. The maximum voltage that can be handled is 7v
But in the image I got from my oscilloscope, I was getting 10.4v DC. Again, it might seem to defy logic. I guess that absolute maximum voltages would only apply if I’m using it the way it was designed. Fortunately, I’m not using it the way they designed the chip.
I’ve put in a simple load. I have a flashing LED lying around so I’ve connected that in series with a 10k resistor to limit the current. My plan was to start with a big value resistor to keep the current low then work my way down. Fortunately it works when I used 10k.
I measured the current going through the circuit and it was around .70 to .80 mA. So that’s less than 1mA but that’s enough to power the flashing LED.
Why it works?
The reason why it works is that, unknown to many, inside the chip, there’s a zener diode that runs between the chip’s GND pin and to all of it’s input pins. So that means I don’t have to “power on” the IC to use that diode which is why you don’t see anything going into the VCC pin.
Note: During experimentation, I’ve blown up some of the inputs 😀 Luckily the 7400 is a QUAD NAND gate which gave me 8 inputs to play with.