I got tired one day of fumbling around for power for a small breadboard project I was working with.
I needed a 3.3VDC supply and a 5.0VDC supply and a 12VDC supply, and the mass of AC Cord,
alligator clippies and cabling was driving me nuts.
So here we describe a universal power supply capable of multiple simultaneous low-noise outputs,
with it's own self-contained rechargeable battery (actually, 3 batteries).
Using 3 series LiPo 3.7V 10AH Batteries, this PSU (Power Supply Unit) is completely shielded, and
can supply 8 DC Outputs, and 2 commons ("grounds") on a single-row 10-position barrier strip.
(1) COM (Common)
(2) 2.500 VDC Precision Reference (+/-2.5 mV, max 20 mA)
(3) 3.3 VDC Regulated
(4) 3.7 VDC Unregulated (Battery)
(5) 5 VDC Regulated
(6) 6 VDC Regulated (this output can be varied from 4.5 to 9.5 VDC)
(7) 7.4 VDC Unregulated (Battery)
(8) 9 VDC Regulated
(9) 11.1 VDC Unregulated (Battery)
(10) COM (Common)
- All regulated outputs are linear regulators, and therefore low-noise.
- On loads connected to (5) thru (9) multiple batteries (in series) are supplying power
- On all outputs except the 2.500 Vref (max 20mA), user must limit output to 0.5A (1 A peak)
- Batteries are individually fused at 1 A Slow, using MST-250 type "micro" fuses
NOTE ABOUT COMMON & CHASSIS & GROUNDS:
- During normal operation and charging, 'COM' and 'Common' connect internally to the Chassis
(box). While the chassis may be left "floating" during normal battery operation, its actual
potential will be determined by the load's ground (example:, the QRP transceiver ground).
However, because a failsafe thermostatic switch (TSW1) makes that connection internally, it is
best NOT to ground the chassis independently, NOR to use the chassis as a load
return. (to do so will defeat the failsafe)
SPECIAL NOTES:
- All loads must be connected on one side to "COM". Do NOT, for example, connect a load
between 3.3V and 9V. Such is unaccounted for in the design, and could cause problems.
- If using multiple outputs, consider the aggregate current - or else you will blow fuses.
- Example 1: If I am using 3.3V @250mA, and 5V @500mA, and 11V @300mA, aggregate is
(0.25 + 0.5 + 0.3) = 1.05A, and I will likely blow the fuse on Battery No 1.
- Example 2: If I power my QRP transceiver from 11.1V, and because of poor antenna match
(or even normal ineffiencies) it draws 1.1A during transmit, I will likely blow a fuse
on one (could be any one) of the batteries.
- Average currents greater than 0.5A could cause the PSU to overheat and shut down. Peak
currents greater than 1A may blow fuses. A typical QRP CW transceiver running at 50% duty
cycle is probably fine, as long as the transmit (key down) current does not exceed 1A.
- While rated at 10 Amp-Hours, practical battery capacity is about 4-5 Amp-Hours.
- Charging isolates the batteries from each other. Therefore charging can only be done
when PSU is OFF (i.e., not in use and not supplying load current).
- Charging requires an external power supply (2.1x5.5 plug) with 6-15VDC at minimum 2.0A.
Charging (for 25% battery) typically takes 12-16 hours.
Note: Some external power adapters may "hiccup" continuously if not rated for peak current.
I use a 6VDC 2.0A power apapter Mfgr p/n DK19-060-2000 (Amazon's ASIN B08T5XKMPB)
Let's look at the basic I/O (Input/Output) because a picture can say a thousand words:
(click on any picture to see larger version)
Above, we're seeing the output end of the power supply. At the top, we see the 10-position barrier
strip. 'COM' means common "ground" to all the other outputs, but it isn't necessarily "grounded"
to the diecast box. Check. Composited (top, for labeling purposes) is an image showing which terminal
is which. Of note is 'VAR-V' which is a dedicated terminal for variable voltage, settable by the user
to any voltage between 4.5 to 9.5 VDC. Normally, we set this to 6.0V, but just to the right of
the label 'VAR' is a trimpot adjustment (small slotted screwdriver) whereby you may adjust it to
the desired regulated voltage. Note, the adjustment only affects the output on terminal (6).
Also notice in the above image, the status LEDs 'G' 'Y' 'R' (Green, Yellow, Red). When illuminated,
they indicate the respective status 'OPERATE' 'CHARGING' and 'CHG-PWR'. The latter indicates
charging power is present (connected to the unit), but not necessarily that it can be charged. (A
switch must be thrown on the opposite end of the PSU to allow charging).
(click on any picture to see larger version)
In the above image, we see the opposite end of the PSU (the INPUT end). When charging is required,
that voltage is supplied to the 2.1x5.5mm jack shown. Normally, that voltage is only supplied
when needed for charging.
The left "ON-OFF" toggle switches the batteries in-or-out of the circuit, and also routes power to
the Charge-detector circuit (which drives the yellow LED we spoke of earlier). When OFF, no battery
is connected anywhere - "OFF" is OFF. To shut everything down, turn that switch OFF.
The right "OPER-CHG" toggle routes the batteries where they need to go - either configuring them in
series to supply the outputs and output regulators, or configuring them for charging. In the
"OPER" position, voltage applied at the Charger input jack goes only to the Charge Detector Circuit,
which is unconnected to the batteries, and therefore cannot charge. Nonetheless, the RED LED
will light, indicating the charger's input voltage is connected (the YELLOW LED will NOT Light).
When the right "OPER-CHG" toggle switch is placed in "CHG" the batteries are switched from a series-
configuration (as during operation with a load) to individual charger circuits having a common
ground.This disconnects them from the load and regulator circuits. The Charge Detector Circuit
will light the YELLOW "Charging" LED as long as the aggregate charge current exceeds 25 mA. Charge
cytcle is essentially complete when the YELLOW LED goes out.
NOTE about Charging Indicator: I often find that the YELLOW "Charging" LED stays on way
longer than I would expect. Usually, this is because the charger module is supplying a very
small amount of current to top off the weakest battery, when in fact that battery is 99% charged. It
seems the charger can spend 95% of its time charging the last 5% of the battery - HIHI - kind of
like my software projects - HIHI. You can always check the battery on the output terminals with a
voltmeter to see if this is the case, and check the chart to see if it is essentially "all
charged up". Often, you'll find the battery is at 4.20V and waiting to get to 4.25V. Not worth
the extra time. You can certainly leave it charging (and wait for the Yellow LED to extinguish),
or you can simply unplug the charger.
The schematic is too busy for depiction here. Download a PDF file - here - to follow this narrative.
The two toggle switches, SW1 and SW2, are key to this discussion. SW1 is a heavy-duty 4PST switch
(NKK S41T) and SW2 is a 6PDT switch (Copal p/n ET620N13). SW1 (with four circuits), switches
the NEG side of each of 3 LiPo 10AH rechargeable batteries, and routes them individually to separate
sets of contacts on SW2. The 4th circuit on SW1 routes Charge Input power to the Charge Detector
Circuit and the 6-Channel Charger Module U12. SW2 has the job of placing 3 batteries in series (and
connecting their POS terminal to the input side of their respective voltage regulators) -OR-
routing those same 3 batteries (individually) to ground (NEG) and their respective charger (POS).
In this way three clear but important functions are accomplished:
1. "ON/OFF" switch SW1 assures all power circuits and batteries can be turned OFF, absolutely. 2. with SW2 "OPER" selected, the PSU is now a battery-sourced power supply 3. with SW2 "CHG" selected, batteries can be charged, but power supply functions are disabled.
So now lets look at the remainder of the schematic. On the righ-hand side of the page, find five
linear (low noise) regulators:
- operating from B1 (Battery No. 1, 3.7V nominal, 3.6-4.25V actual):
- U7 (MCP1501T-25) 2.500V high-initial-accuracy voltage reference chip (limited to 20 mA out)
- U8 (LM1085T-3.3) 3.3V linear voltage regulator (TO-220 pkg mounts to chassis for heat sink)
- operating from B1 & B2 in series (7.4V nominal, 7.2-8.5V actual):
- U5 (LM1085IT-5) 5.0V linear voltage regulator (TO-220 pkg mounts to chassis for heat sink)
- operating from B1, B2, & B3 in series (11.1V nominal, 10.8-12.75V actual):
- U1/U3 (LM317T + TL431) 9.0V linear Regulator (TO-220 pkg mounts to chassis for heat sink)
- U2/U4 (LM317T + TL431 Vref) 4.5-9.5V Adjustible linear Regulator (TO-220 pkg mounts to chas)
In addition to the linear regulated outputs, the raw battery outputs (3.7, 7.4, 11.1V nom) are
As a last chance fail-safe (i.e., should all else fail...), a thermostatic switch TSW1 is mounted
to the chassis. This is an "O.O.R - Open On Rise". During normal operation, and should the chassis
temperature rise above 55 deg C, this thermostatic switch serves to open the return on the
batteries, thus shutting the outputs down. This assures that the batteries and electronics cannot
overheat. [ Did I say that out loud? ]
Lastly, there are two charger-related circuits. At the very bottom is the Charge Detector Circuit.
This is comprised of Current Sensor U11 (LT6107) which provides an accurate amplified sample of
current through R18 (coming from the external charging supply and going to the charger module U12).
The current sensor U11 is relatively immune to common mode voltages that may occur as a result of
circuit operation or changing input voltage. Trimpot R13 allows the detection threshold to be
changed anywhere from 15-45mA, to energize the gate at power MOSFET Q1 to turn on the YELLOW
"Charging" LED.
Module U12 is a (chinese) 6-channel LiPo charging module with LED readouts. Info on this module can
be found - here - as Amazon's ASIN B07FD741J7. Charging and status monitoring is managed by its
on-board controller. Responding to the presence of input voltage and battery voltages, it manages
the charge to each of our 3 batteries, terminating the charge at the prescribed 4.20V(+/-0.015V)
with constant-current charge for each battery of 0.6A max. Only 3-channels are utilized (for 3
batteries). User may view the LED status of each channel through cutouts in the main cover.
Chan 2, 4, 6 represent the current voltage on each battery. Charge may be terminated automatically,
or by the user (by removing charge supply or turning off). Anything above 4.10 V will generally
represent a fully-charged battery (90%+)
Everything is housed in a 6.8"L x 4.8"W x 2.2"H die-cast box (Bud p/n CN-5709). Much of the wiring is
point-to-point, and must be kept neat and tucked away so that batteries and electronics may
be mounted in remaining internal space. It's a tight fit and not for the faint-hearted.
The major assemblies are the chassis itself, the regulator board, the fuse board, and the 6-chan
(chinese) charger module. And of course, the battery stack (of 3 each 10 AH LiPo batteries).
The regulator and fuse boards are built on good-quality perf board ( plated-through holes and 0.1" pitch).
Below is an image of the completed regulator board (top & bottom):
(click on picture above to see larger annotated version)
(click on picture above to see larger version)
The four TO-220 regulators (along the back edge) all will mount to the chassis side, using an
insulator hardware set.
Below, is a picture of the box internals, with the regulator board mounted, and most of the
wiring done.
(click on any picture to see larger version)
The regulator board, with the four TO-220 regulators bolted to the chassis, can be seen along the
top. On the right (topmost) is the 6PDT "OPER-CHG" toggle switch, while just south of it is the
4PST "ON-OFF" toggle switch. Then, just left of that switch is the fuse board, mounting (using
sockets) 3 ea fuses (Schurter p/n 0034.6615, 1A Slow, one for each battery). A detail of the fuse board
can be seen - here -
Also noteworthy above, is the battery fit (showing the first of 3 batteries being fitted). They are
separated by a hand-cut silicone-foam gasket (4 are required, which cushions the battery on each
side, and provides for a little air circulation. Each battery has its own integral BMS Safety board
with short circuit, thermal, overvoltage, and undervoltage protection, so an additional
BMS
(Battery Management System) board is not required. Note also that the battery is captured
in-place by 6 tall nylon standoffs. When the lid is ultimately installed, the foam cushioning
and standoffs serve to secure and stabilize the stack of 3 LiPo batteries. When procuring
batteries, it is important to confirm that there is an integrated BMS, and that the aforementioned
safety features are all included. The batteries I purchased are MakerFocus brand (Amazon's ASIN
B093WS6C66).
After drilling the cover so that peep-holes are located where the displays and LEDs can be viewed,
the charger module is mounted with 4 small spacers. The spacer length is selected so that the
asssembly helps to capture the batteries in place, but without putting pressure on the batteries
or the charger. To that end, a soft foam pad on the mating plate of the subassembly helps. Here's
a picture of that subassy mounted, just before the cover is installed.
(click on any picture to see larger version)
Below is a picture of the cover installed. The Charger is mounted inside, roughly in the center of
the cover. Peep holes allow the user to see the 6-channel Charger Module's status. 4 numerical
LEDs scroll information continually (when charging is underway). Most of that info is not of
interest, but about once/minute, it will display the current battery voltage, by channel number.
For example, it might display 'ch-1' then next '4.25'. The channels of interest to us (as wired)
are Channel 2, 4, and 6. You'll also see Vin (the charger input voltage) displayed periodically.
An blue LED (see holes labeled 1-2-3-4-5-6) also shows the more conventional single-LED-
per-channel (i.e., slow-flashing when charging).
The three insulated holes on the left are for voltmeter probes during development. Those are of
little use, and can be eliminated. The chart at the upper right can help the user determine the
battery status by simple voltmeter measurements at the output barrier strip (during operation).
When any one battery drops to 30%, operation should be terminated shortly, and a charge cycle
should be initiated (by connecting the charge adapter and flipping the switch to CHG).
(click on any picture to see larger version)
I keep certain common cables mounted to the output barrier strip, like 1.35x3.5mm plug & jack for 5V,
and a 2.1x 5.5mm plug & jack for 11.1V. If I need to check something quickly (a new project,
or an accessory), I can do that quickly and easily.
I can also just grab the box and go portable, knowing I have 12-16 hours of operational battery for
my QRP rig.
I keep a sticky tag on the side, and write down when I charge the unit. This morning, I noticed
that I had not charged it in 11 months! Oops! I checked the batteries, and they were still 95%
charged. Not bad , especially since I know I've used it on the bench a few times for quick-checks
and tests.
73,
Monty N5ESE
dit dididit dit
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