# TTL Question

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#### Gween-Fwaker

##### New Member
I'm pretty new to electronics, and this question is probably pretty simple, I'd imagine.

On any TTL chip, I believe the supply voltage for the chip itself is 5V and the supply current is 20mA. Also, the supply voltage for a "high" is 5V and 0-.8V for a "low", I think. How much current is too much for TTL inputs, as in, how much until it fries? How much is too little?

Like I said I'm pretty new to all this, so thanks for your patience.

#### audioguru

##### Well-Known Member
A lousy old TTL IC is a few hundred (a few thousand?) years old. Everybody (except school kids) have used Cmos instead for many years.

Anyway, a TTL input low is 0.8V or less. The output of a TTL typically goes low to about 0.4V.
A TTL input high is 2.4V or more. I think a TTL output typically goes high to about 2.8V.

A TTL input draws a max low current of 1.6mA.
A TTL high input draws almost no current.

An input current of only 1.6mA won't fry anything. If the input voltage or power supply voltage is much higher than 5V then an old TTL IC will fry.

#### Gween-Fwaker

##### New Member
Sweet

Helpful thanks, but something still bothering me is this. If I have a power source at, say, 5v and 20mA, (a power supply I made from radioshack stuff) would that suffice for a whole TTL setup? Or can I only have a max current of 1.6 mA? My problem isn't the voltage being applied, I'm just afraid of putting too much current into it. And since we're on the subject, how come a high, which is, y'know, power coming from the output, draw such a small current where a low draws more?

Forgive me if these questions are stupid, just trying to learn

#### audioguru

##### Well-Known Member
The datasheet for each IC shows its power supply current. 20mA is an extremely low amount of current from a power supply. Some old TTL ICs draw more.

You don't put current into a circuit. The circuit draws only as much current as it needs.

An old TTL input draws a max 1.6mA amount of current when the input is low.
A Cmos input draws no current.
That is how they are made.

#### rezer

##### New Member
I'm pretty new to electronics, and this question is probably pretty simple, I'd imagine.

On any TTL chip, I believe the supply voltage for the chip itself is 5V and the supply current is 20mA. Also, the supply voltage for a "high" is 5V and 0-.8V for a "low", I think. How much current is too much for TTL inputs, as in, how much until it fries? How much is too little?

Like I said I'm pretty new to all this, so thanks for your patience.
There is 5V and low voltage TTL. But concerning 5V TTL, a logice low is = 0.8V and below and a logic high is around 2.4V and up. The current draw will vary depending on the device. This is the max. amount of current the chip will draw from the power supply. Not all chips are the same and they will vary in their ratings. The chip is not using this much current, if fact, most of the time it is a bit smaller and this depends on the load that the device is driving, the number of active outputs and so on.

You don't need to worry about frying anything by hooking up a 5V supply rated at a higher current than the device. The current rating of the power supply is the total amount of current the power supply can supply to it's load (i.e. TTL devices). If the device only requires 20mA, than that is all the supply will deliver.
A logic high output sources the current and a logic low sinks the current. Typically, the devices that are connected to the outputs have a high Z input that draws less current than when the outputs are driven low (sinking current) through the output transistor in the device. TTL uses a totem-pole topology, also called push-pull in that a PNP transistor provides the logic high and an NPN provides the logic low at the outputs. I hope this helps.

#### Gween-Fwaker

##### New Member
Fantastic

Actually helps bunches. Thanks a lot, that clears up a lot of my confusion.
Thanks to both of you

#### Willbe

##### New Member
.....A TTL output puts out 0.4v to 2.4v and a TTL input accepts 0.8v to 2.0v and the 400 mV difference is the Noise Margin; it allow 400 mV of noise on the ground or signal lines.
.....The advantage of any logic family is that these levels are guaranteed over all spec'ed variations of supply voltage and temperature.
.....That's what makes digital circuitry so reliable. The engineering has already been done for you.

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