# 24 Volt AND gates?

Discussion in 'Electronic Basics' started by [email protected], Apr 23, 2007.

1. ### Guest

In college, I was used to using 5 volt signals for logic, but does
anyone know if they make IC's that support 24 volt (DC) signals as an
AND gate, or even a family of 24 volt logic gates?

Jeff

2. ### John PopelishGuest

18 volts (4000 series CMOS) is the highest voltage logic I
am aware of. You may have to make your gates with diodes
and comparators.

4. ### Doug MillerGuest

Or relays, if easy availability is more important than size or power
consumption ...

5. ### Guest

Becareful with that first example. If you applt +24 to the bottom
transistor and ground to the upper one, you reverse bias the base
emitter junction of the top one beyond it zener voltage, which will
not only make the gate malfunction (the output high will be weak) the
reverse base current will slowly reduce the gain of the upper
transistor.

7. ### Guest

Does that circuit require that the inputs be allowed to source
current? I would be driving it with "high side" drivers that can't
source current.

8. ### Guest

Looking at the datasheet:
which value would I be concerned about to find out what the limit is
on the zener voltage? Would it be the Base-Collector breakdown
voltage?

Thanks.
Jeff

10. ### Guest

The diode transistor logicshown sources current out through the inputs
that must be sunk to ground to pull the input low. And the schematic
shown is a NAND, not an AND.

But the circuit can be rearranged for pull up inputs and converted to
and. What must the output drive?

11. ### feeboGuest

not just trannies... use a sprinkling of diodes too and it all becomes
very easy and a very real proposition... if you start looking at
circuits in this light, you'll be surprised how many include the logic
primitives in this form. Sometimes I will use a DTL configuration
rather than chuck a chip in and leave 3/4 of it unused (although I do
weigh the costs in such a decision)

12. ### Guest

A motor controller that is expecting 24 volts signal to activate the
drive. There's no serious current draw.

13. ### John PopelishGuest

What does the input voltage go to when nothing is connected?
In other words, must the gate output pull positive or
negative?

14. ### MartinGuest

While doing it in discretes is certainly possible, unless your logic
is very minimal
(less than 4 or 5 gates worth) it may be worth just throwing a
regulator into the circuit and using standard gates.

I think that for anyone here to give an answer that really is the best
solution (however you may chose to define best: (least cost, smallest,
simplest etc...)) you need to give MUCH more detail about what you are
trying to do.

Tell us the end result you are looking for, what you have already
aquired hardware wise
or are committed to not changing, where your inputs come from, etc.

I strongly suspect you should not be doing your logic at 24 volts, but
should be changing another part of your circuit to take logic level (5
or 12 volt) inputs.

Martin