Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!zephyr!tektronix!tekig5!tekig4!tekig3!jime
From: jime@tekig3.LEN.TEK.COM (James W. Edwards)
Newsgroups: sci.electronics
Subject: additional info on switch problem
Message-ID: <4100@tekig4.LEN.TEK.COM>
Date: 17 Jun 89 22:37:42 GMT
Sender: root@tekig4.LEN.TEK.COM
Lines: 76

I posted a description of a switching control problem a few days ago.
I have gotten quite a few responses.  Several people have asked for
additional info.  Also, I have been informed that without this additional
detail that I would probably get overdesigned solutions.  To refresh
your memory I have included the original request along with the additional
detail.


>I have a problem for you electronics wizards.  I would like to control
>the DC power being supplied to two loads as shown in the following diagram:
>
>  +--[Load 1]-------------[CP1]-------------+-------+
>  |                                         |       |
>  |                                         |       DC
>  +--[Load 2]-------------[CP2]-------------+      POWER
>  |                                                 |
>  |                                                 |
>  |                                                 |
>  +-------------------------------------------------+
>						     |
>      					           GND
>
>CP1/2 = Control Points 1/2
>
>I would like to be able to turn on/off the power to either of
>the loads through CP1/CP2.  CP1/CP2 are simply whatever electrical
>component(s) that you feel would be necessary to achieve this objective.
>The switching action of CP1/2 must be controllable through a digital
>signal.  CP1/2 must each be able to switch 15A @ 12/24 V to their loads.
>The switching action is actually more complicated than just switching
>on/off:  In the off position, the associated load must still
>recieve some power (say 10%), and in the on position it will 
>recieve full power.  Furthermore, when switching from off to on or
>from on to off, the rise or fall in current/voltage to a load must 
>be controlled (for example, it could rise/fall exponentially to the
>final voltage/current).  The intended circuit must be very
>reliable and operate in a harsh environment (-70F to 150F and
>high, non-condensing humidity).  If it matters, the power source
>is regulated, but probably fairly noisy due to the surrounding
>environment.  Finally, it is very desirable that the most 
>likely failure mode will result in both loads recieving full 
>power (i.e. CP1/2 switched on).  Thanks in advance!
>
Additional Detail:
=================
1). CP1 and CP2 will both use the same circuit design.  I will be
    sequencing them on/off.  I only showed both CP1 and CP2 for
    completeness.
2). It makes no difference if CP1/2 operate between the DC power
    source and the load, or the load and ground.  Apparently,
    due to the popularity of N-type mosfets, the load and ground
    configuration is more desirable.
3). The load will be switched on/off at a regular rate (~1Hz)
    for extended periods of time. 
4). A fairly large voltage drop across CPx is tolerable - for
    the intended purpose of this circuit I don't see any problem
    with up to a 1V drop.
5). Even though CPx is switching a regulated DC power source to
    the load, the load is not particularly sensitive to slow
    variations in the voltage.  I'm just trying to avoid 
    transients which could affect the load life.  Thus,
    the ramp up/down spec in the voltage during switch on/off
    is not particularly tight - a resistor/cap network to slow
    down the edges is sufficient.
6). Reliability, simplicity, and low cost are all very
    desirable (aren't they always).  Highest priority is
    for reliability.   Think of this circuit in terms 
    of controlling the headlights on your car (this is
    not the actual application) - if the 
    circuit were to fail in the wrong way while you were
    driving at night, you might be in potentially dangerous
    situation.

Thanks!

Jim Edwards