convergence said:
Hi,
I have to measure the voltages of hundreds of batteries and display
them on a PC. For the start the number is 200. 20 batteries in series
form a group and we have 10 groups in total.
I need 400 differential Inputs. The old technology we are replacing
uses Relays. If the user selects the Relay Group, the analog voltages
of this group can be read out.
I want to replace the Relays with ICs. Any suggestions? Should I use
multiplexers?
Thanks
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Because of the high voltages which are probably involved, it might be
better to stick with the old technology. Reliability of most relay
multiplexers of this type can be improved with simple inexpensive
additions to the ATE, and setting up a preventative maintenance
schedule to replace the relays on a regular basis.
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Hi. I'm assuming you're not satisfied with the reliability of your
relay multiplexer, and you're looking at replacing it with a solid
state system (based on analog switches) in order to improve
reliability.
Before you walk away from the relay concept, you should be aware of its
advantages. Open is really open (100s of megohms or gigohms between
contacts). If you've got a good relay contact, closed is really closed
(milliohms). You're not going to get that ratio between open and
closed with any analog multiplexer IC. Not only that, but an analog
switch also has leakage currents between switches on the same IC, which
isn't a problem at all with relays. And as long as the relay contacts
are rated to withstand voltages greater than that seen in your battery
system, you don't have to worry about finding high voltage multiplexer
ICs. You didn't mention the voltages you're measuring, but that could
be a serious problem.
Another big advantage is that relay contacts are immune to ESD
(electrostatic discharge). People who switch to solid state
multiplexers have to deal with the fact that they're now working in an
anti-static environment. Static can easily kill many analog
multiplexers. Of course, many analog switches are made with ESD
protection, but you always pay for that with a fairly dramatic increase
in leakage current and usually with a decrease in the ratio between off
resistance and on resistance.
If reliability is your only issue, you might be able to improve that
quite easily and keep a working system running without the hassle of
redesign.
First, you should look at the type of relay you're using. Since you're
switching into a multimeter, you should choose a relay that's made for
"dry switching", meaning that there's not enough current or voltage to
wet the contacts on make or break (the "cut" on the data sheet is
usually 5 V at less than either 1 mA or 5mA). Standard relays won't
work well for test. You didn't mention the type of relay you're using,
so that's one thing you might want to look at. If the minimum
switching current isn't specified on the data sheet, you can call the
relay manufacturer to find out. Reed relays are generally OK for dry
switching, but you should check. If you don't have reed relays, ones
with bifurcated contacts are frequently capable of doing this job.
If you have reed relays, you should also check the withstand voltage
specified on the data sheet. Many reed relays are only specified to
switch 100VDC or less. Again, something to check, since you didn't
mention which relays you're using or the voltage you're switching.
Another issue which might be a problem is the battery environment,
which sometimes has a very acidic and corrosive atmosphere. This
affects relay contacts as well as any other metal. If this is an
issue, it also affects electrical connections and such, and should be
addressed first. This will be a killer no matter what you do. Sealed
relays might give you better results, but it's better to improve the
environment. Another option would be to place the test equipment in a
remote location with good ventilation. These are basically DC
measurements, so a couple of hundred feet of cable shouldn't affect
your measurements too much unless you're measuring individual voltages
in a millisecond time frame.
Every reed relay manufacturer specifies an open contact capacitance, as
well as a capacitance between contact and coil. Usually this is only
several pF, which isn't much of a problem if you're just using just
one. But if you're using hundreds of relays in parallel, 4pF can be
multiplied to the point that it's a big issue. You haven't described
the layout of your test setup, but your description implies that you've
got 10 banks of 40 relay contacts, each of which go to another set of
20 DPST contacts. You might want to look at the schematic of your test
layout, and imagine a several pF cap in place of each N.O. contact.
You'll see that, in fact, you're switching hundreds of pF of
capacitance when you're switching voltages. That changes things quite
a bit, because now you're talking about surge currents that are limited
only be the resistance of the wire and the relay contacts. That can
easily reduce the life of the relay contacts and make a very unreliable
system. This is valid no matter what the input impedance of the meter,
which is generally a DC current and a capacitance added to the above.
Looking again at the data sheet of the relay, you'll see a maximum
specified current, along with a specified number of electrical
operations at that current. Generally, you'll also see a specified
number of mechanical operations (at essentially no electrical current).
The first number for reed relays can be as low as 50,000 to 100,000
operations, and the second is usually in the millions. At a bare-bones
minimum, you should add enough series resistance to the test line so
the peak switching current for the capacitive load is less than the
specified maximum relay current. Usually that means adding a small
series resistor between each voltage and its relay contact. The less
current you switch, the better. I like to start with a series resistor
that limits current to 1/10th of specified maximum or less, if I can.
I'll then somewhat arbitarily call the expected life at 1/2 to 1/5 of
mechanical maximum (depending on balance between maintenance costs and
cost of bad readings).
You have to take a good look at the load impedance of the meter here.
If you've got a very high (e.g. 10 Megohm) input impedance meter,
adding a 2K series resistor (0.5 amp max. reed relay contact current,
set resistor for 50mA pk. at 100V) shouldn't cause you any problem.
Your voltage readings will only change by 0.2%, which isn't a problem
for most ATE applications. If it is, you can compensate with software.
But if you've got a load resistor at the meter (which may affect the
above calculations of switching current a bit), or have a low input
impedance meter (many DVMs have different input resistances for
different ranges), you may have a voltage divider between the series
resistor and the meter. Your best bet here is to obtain 0.1% resistors
(they're cheaper by the hundred) to minimize the resistance
uncertainty, and compensate in software for the voltage divider.
Now it's time for a little math. Take a realistic look at how many
cycles you're putting the relays through per unit of time, and
extrapolate that to find out how frequently you have to replace your
relays as preventative maintenance. Your inside set of 20 relays will
cycle 20 times for every time the outside 400 cycle, so you'll have to
replace the inside set of relays 20 times as frequently. This number
will give you a good idea of the maintenance costs of your relay
multiplexer system per test cycle and per year.
I found it useful in systems like this to get a small Omron counter
with LCD display, voltage input and internal 5-year battery, and place
the increment input of the counter across one of the outer ring coils
(they're on longer, so there's no problem with activation time). I
would then document that the counter be periodically checked, and the
relays be replaced on a regular schedule.
If the existing relay system is older or has been well used, the
contact resistance of the relay sockets may be a bit of a problem.
This is especially true if people have been swapping out relays at
random in frustration. By putting together a test program to measure
resistance of closed contacts with new relays, you can get a good
handle on the status of the relay sockets with several
insertion/extraction cycles (reseat relay, measure short circuit
resistance, repeat). If some show up bad, it might be best to replace
all of them. The good thing is that, if you have a rational
preventative maintenance cycle, you limit the number of socket
insertion/extraction cycles to a minimum.
By this point, you've got a good handle on how to make your automated
test setup very reliable, as well as getting a handle on how much
maintenance of the wear parts (relays) will cost to keep it reliable.
Having that, you can make a judgment on whether to go with analog
multiplexer ICs and the failure mechanisms associated with them.
If you want more follow-up information, please include the following:
* Maximum voltages switched
* Floating or grounded batteries or meter
* Type of meter used
* Load resistance at meter and load resistor (if any)
* Type of relay used
* Measurement frequency (once per millisecond, second, minute, hour?)
Moral of the story -- old ain't necessarily bad. ;-)
Good luck
Chris
