Can I run two or three CMOS inverters (74HC14=92s) in parallel to get more current?
I=92m using them as a monitor output with 500 ohms in series and then
50 ohms to ground. (Driving a cable to a =91scope) That's a bit more than the 5mA spec.Should I add a bit of series resistance before the input to each inverter? or before tying the outputs together?
George H.
I have often done so, with no ill effects. Up to 8 gates, ttl as well as cmos. Making the input slower, would not be a good idea, because the period in which the gates disagree, becomes longer. Which also means that the gates should come from one single chip, because they would have a better chance of being of equal quality.
Sure. We do 6 at a time sometimes. But HC14s are kinda wimpy; the more modern TinyLogic parts can drive harder.
At those levels, a single HC14 should work fine. With a 5 volt supply, figure around 40 ohms equivalent drive impedance, so cut your first resistor to around 400 ohms to get a volt out.
Nope, just strap them together, inputs and outputs.
John
If they're on a single chip watch the maximum ratings on the Vcc/Gnd leads. There will be a chip maximum current rating that often limits the number that can paralleled.
^ ^^^^
--
No.
Since:
E1 * R3
E2 = --------------,
R1 + R2 + R3
then:
. 5V E1
. |
. [40R]R1
. |
. [400R]R2
. |
. +---->0.51V E2
. |
. [50R]R3
. |
. GND
>>Should I add a bit of series resistance before the input to each
>>inverter? or before tying the outputs together?
>
>Nope, just strap them together, inputs and outputs.
n
ore
Hmm, I must be missing something. I'm driving something near 500 ohms with 5 volts... that's 10mA. Don't I want at least two? (Though I started with only one and it worked fine.)
George H.
Oh thanks for the responses guys...
OK now I'm confused. I was looking at the spec sheet and it lists maximum current as +/- 25mA. Where did the 5mA number come from? It's listed on the digikey web site but I don't see it in the spec sheet...
e
^^^^
R] =A00.25V
| =A0 / =A0
+--+-->[SCOPE] | [5R] || =A0 =A0 =A0 =A0 =A0
GND
Your first picture is what I'm using. It works fine for driving a coax to a 'scope. (something like a 3ns rise time and no ringy dingies.)
No. The HC gate won't look like 40 ohms if you load it that hard; it is a mosfet after all, not a resistor. And the 40 ohm value is only approximate anyhow.
It's easier and more precise to divide before the coax.
John
If you look at the actual HC fet drain curves (in the databooks, but not on individual datasheets), at Vcc=5, the initial e/i slope is around 30-40 ohms up to a volt or so, which would correspond to a 4 volt pullup at 25-35 mA roughly. They are still sorta ohmic there. But sure, use a couple in parallel... they're cheap enough.
John
-- If "precise" is what matters, then your: "around 40 ohms" and your: "cut your first resistor to around 400 ohms" are certainly out of place. As a matter of fact, there's no good reason why the 440 ohm resistor should have been substituted for the 500 ohm one, except perhaps that you thought a 10:1 divider would yield a 5:1 voltage difference between input and output voltages. In any case, since reflections don't seem to be a problem (probably because of back-terminating the cable) and if precision _is_ important, I'd suggest that George use a single-turn carbon or cermet pot like this: . 5V . | . [40R] . | . +---E1 . | . [1k]
en
ore
=A0^ ^^^^
45R] =A00.25V=A0 | =A0 / =A0
=A0 +--+-->[SCOPE]
=A0 |
=A0[5R]
=A0 |
=A0 | =A0 =A0 =A0 =A0 =A0
=A0GND
Thanks John, I don't need the precission at all... (who really cares if the pulse is 5 V or 4.5 volts.) I just wanted a monitor to see when things are triggering.
George H.
-- Then your original divider with a a single gate driving it will be fine. :-)
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