The battery control system uses relays to ultimately control the
connect/disconnect operations. The front panel battery control switch
selects the external, off, or internal battery mode, mutually exclusive
(although they don't necessarily have to be).
The internal battery operation will be fully automatic. When AC power is
on, the relay connects the internal battery and provides the charging.
If the AC drops out, the relay is held on by the battery power, and the
blocking diode provides Vs from the battery. During discharge, the
charge control circuit monitors the battery voltage and the balance
between the two SLAs. If the total voltage drops to the minimum level
(about 22 V), or the balance goes out of range (about 0.6 V difference),
the tripper shuts it all down, totally disconnecting the battery. The
battery won't reconnect until AC power is restored, starting the process
again.
Normally, line dropouts will be short in duration, so the battery just
keeps everything going until normal operation is resumed. With full
charge, it should back up for several hours. When AC is restored, a
front panel indicator lights to show that line has been lost at some
point. If power is off beyond the backup capacity, and the battery trips
out, everything is off until power is restored. Then, another warning
light comes on, indicating that the battery has disconnected, and
everything had to restart from scratch - the Z3801A and the clock,
mostly. The warning lights stay on until manually reset, to show what
happened.
In this cold start condition, the Z3801A needs extra power for a while,
to get the ovens warmed up. At the same time, the battery would be fully
depleted, needing maximum recharging current available. The circuit
automatically reduces the current limit available for charging, and
gradually allows more as the Z3801A gets through its warm up process,
and demand decreases.
The charger will initially be set up for temperature compensated float
mode. Unfortunately, it could take up to a week to recharge a fully
spent battery. It may be possible to add a "fast" mode with higher
voltage and tempco, but there's only so much current available, so it
may not be worth it. After I get everything fully working, I'll be able
to assess this option.
I built the external battery/source control system to go with the
original DC-DC converter scenario, but it will work essentially the same
with the new boost converter. The new converter offers some possible
other options, to be considered later.
It is manually operated, with no provision for charging. It's also
independent of the AC power. It's main purpose is control and
protection, while taking as little current as possible, until activated.
The "hookup" indicator on the back panel, and one on the front, light
red if the external source polarity is reversed, and no operation is
allowed. If the polarity is right, the back indicator goes green, then
shuts off after about 20 sec. Another LED on the front lights to show
the external source is available. It's a high efficiency type, showing
nice and bright at less than 1 mA, which is most of the current draw
until activation. The idea is to keep the quiescent current very low, to
not run down an external battery - it's typically much less than the
self discharge rate, so relatively insignificant.
If the source voltage is above 15 V, no operation is allowed. If less,
then it's OK to go. When the battery control switch enables the external
mode, the "start" push button can force the relay on, connecting the
source to the load - the boost converter, in this case. If the source
voltage holds up or recovers to over 11 V, the circuit will latch on,
and continue operation, and an indicator shows that it is running. If
the source drops below 11 V, from discharge or insufficient current,
then it won't latch, or it will trip out if it was running. It can only
be activated by the button. If the source voltage rises above 15 V, it
will trip out, and allow no operation. The whole thing can be turned
off, of course, by the battery control switch.
Once the button is pushed, and latch-on is a success, the external
supply is committed, and will be loaded to somewhere around 50-60 mA,
(mostly for the relay coil) plus the load current. If AC is on, the
boost converter will be idling, around maybe 10 mA, and ready for
action. If AC drops out, the Vs line falls until it reaches the setpoint
of the converter. For now, I have it set for a little over 24 V. The
feedback for the converter is actually Vs, DC-wise, so when Vs is lifted
above the setpoint, the converter goes to zero duty cycle, essentially
off except for idling current. As Vs falls, the converter smoothly takes
over and holds it.
If the AC is off when the button is pushed, this is the worst case
loading, and possibly a cold start. It takes a stout source to get
things moving. The LT1070 activates at around 2.5 V, and tries like hell
to charge everything up and make 24 V from it. The first thing needed
was under-voltage lockout, and then soft start. The UVLO is set for
about 10 V, after which the soft start is allowed up, maybe about 50
mSec duration.
The moment the relay connects, the source has to charge about 20 mF
total Vs bus capacitance (not to worry, there's plenty of ESR and wiring
R to soften it up) to the UVLO level, where the soft start is enabled,
letting the converter run it up the rest of the way.
It gets more complicated when running the actual loads. There are three
(four if you count the boost converter itself) DC-DC converters in the
system - two in the Z3801A (the main load), and the one I added for the
12 V supply (fairly small load). Each has its own UVLO point and
negative resistance characteristics that will come into play. I found
the data sheets for them, and will have some more to say about this
after some study.
That's all for now. More later.
Ed
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