The 74HC14N Chip

[AlainB]

Hi,

What is the benefit of this chip. I see it often on shematics. On a unipolar driver, I suppose in a way that it is there to clean the output of the parallel port. But probably there is more than that to its usage. Does it improve the performance?

A 7805 is often used to feed the 5 volts needed by the chip. Is the 5 volts output from a computer power supply good enough to replace the 7805.

Thanks!!

Alain

[Al_The_Man]

Which way is it being used? to input to the port or condition the output?
If the latter then I always use the ULN2803, this way you can use 24vdc control circuitry, using 5v out on the machine is not a good idea.
It is darlington driver, open collector and has free-wheeling diodes built in.
Al.

[dtman2]

The 74hc14 is very similar to the 74hc04 hex inverter but the inputs have schmitt trigger circuits with hysteresis. This makes the inverter output less likely to oscillate when the input changes slowly near threshold or is otherwise noisy (like if it is coming in over a long cable). The voltage supply is rated 2-6v usually so it could be supplied by a computer power supply - it will draw very little current. I agree though that sending the 5v from a running computer out to a peripheral can be a problem - any shorts or noise or whatever will cause the computer to screw up. The ULN2803 does sound like a better choice.

[neilw20]

The input current rating in the input clamp diodes is +/-20mA
So if you put a 15K resistor in series with an input it can withstand 300 volts at the loose end of the resistor. Input capacitance can cause a slight delay through the resistor, but usually this is of no consequence.
Or use 1.5K will protect against getting zapped by 24 volt machine parts.
But, you should use between 5K and 10K because transients from inductive devices (including wiring, wehich also has inductance) always make matters worse than ideal.

[Al_The_Man]

Also if you just need to condition couple of outputs, the 2N7000 is a good option, also the ULN2003 has one less driver than the 2803.
Al.

[AlainB]

Hi,

Thanks!

My question was only about the benefit of the 74HC14N. To be more specific, what does it do, i mean when used for instance in a (unipolar) stepper motor driver. I can imagine that one usage of this chip it is to "condition" the output of the parallel port but what is wrong with the output of a parallel port? Wy does it need to be conditionned.

Sorry! The computer power supply I was talking about is an external power, not the computer's power supply.

Many thanks!!

Alain

[Al_The_Man]

This particular IC is known a schmitt trigger inverter, this means that as soon as the input level reaches a logic-one level signal, the turn on is very fast (snaps on if you will), the output goes low.
Same with turn off, if you look at the hysterisis curve on the data sheet it shows it, although it will buffer the output of a printer port, you still restricted to TTL logic.
With buffers like the ULN2803 you can go up to 60vdc loads, to operate relays at 24v for e.g.
The printer port is not intended to drive any appreciable loads, so it is usually recommended to be buffered in some way.
If using external circuitry, the PC 5v or 12v supply can be used, as long as the load is withing reason and does not overtax the PC supply.
Al.

[pminmo]

Originally Posted by AlainB Hi, Thanks! My question was only about the benefit of the 74HC14N. To be more specific, what does it do, i mean when used for instance in a (unipolar) stepper motor driver. I can imagine that one usage of this chip it is to "condition" the output of the parallel port but what is wrong with the output of a parallel port? Wy does it need to be conditionned. Sorry! The computer power supply I was talking about is an external power, not the computer's power supply. Many thanks!! Alain

The problem isn't specifically with the parallel port, but the cabling from a parallel port to something. Using a 74HC14 helps clean up the signal transitions that can be influenced by the cable.

[tpworks]

I've used this circuit utilizing the 7414 to clean the signal from 1.25A 5804 to tip120's for 5A.
http://www.cnczone.com/forums/attach...4&d=1077306800
For more info on this go to this link.
Upping the capability of a 5804 IC

[AlainB]

Hi,

As I can see in the drawing, the 7414 is used after the 5804. On other schematics, the 7414 is often used before the chip (or chips) that are doing the sequences for the Mosfets (4030 and 4013 for instance).

What would be the best way to use th4 7414? Or is it better after for the 5804 and before for some other chips?

Alain

[NC Cams]

It depends on how much power you're swithing and whether you're switching a fet or a NPN or PNP transistor, as in where you place the 5804 versus the 7414.

Depending on how much drive you need, that would determine what sequence I'd put the chips. Afterall, the 7414 is only inverting and squaring up the signal logic so it can be rather forgiving where it is placed. I"d be inclined to have the 7414 properly trigger an adequate driver as opposed to driving a transistor directly, espcially if you're looking for speed.

Driving a fet with only 5v will not switch it on fast or hard enough. Will it work? Yes. Will it work better/cooler with a proper driver? Yes.

[neilw20]

Originally Posted by NC Cams It depends on how much power you're swithing and whether you're switching a fet or a NPN or PNP transistor, as in where you place the 5804 versus the 7414. Depending on how much drive you need, that would determine what sequence I'd put the chips. Afterall, the 7414 is only inverting and squaring up the signal logic so it can be rather forgiving where it is placed. I"d be inclined to have the 7414 properly trigger an adequate driver as opposed to driving a transistor directly, espcially if you're looking for speed. Driving a fet with only 5v will not switch it on fast or hard enough. Will it work? Yes. Will it work better/cooler with a proper driver? Yes.

Ther are some MOSFETS that only need 5v on the gate.
Check out manufacurers Master Selection Guides.