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Project by Stefan Keller -Tuberg
This device will disconnect a load from its
power supply if the voltage is reversed or too
high. It also disconnects the load if it draws
current above the adjustable trip level. Its
dual-rail support means it can work with
devices like audio amplifiers with a split
(positive and negative) supply.
I
Dual-Rail
Load Protector
n June 2024, we published three DC
Supply Protectors that guard against
reversed or excessive supply voltages,
but they could only handle a single-rail
supply (siliconchip.au/Article/16292).
This design provides even more functions, extends the reverse/overvoltage protection to split rails and adds
adjustable current limits with automatic or manual resetting.
If you’ve built something that uses
flying power leads, you may already
have had a close call mixing up polarities. Or have you ever forgotten to
check that you’re using the right supply to power a device? If any of these
ring true, this design might help avoid
a catastrophe by introducing power
supply protection. It’s so versatile
that you’ll think of many applications for it.
The overvoltage cut-out levels can
be set between ±5V and ±19V (or 5-38V
for the single-rail version). If the supply overshoots the protection level,
this device will rapidly interrupt it.
The overcurrent thresholds are set
by a current sense resistor and trimpot. The sense resistance is chosen
so the voltage drop is approximately
50mV at the nominal protection level.
The trimpot range permits adjustment
from zero up to twice the nominal current level.
When the current limit is reached,
it interrupts the offending power rail
72
Silicon Chip
by turning it off completely, similar
to a fuse blowing. This minimises the
chance of damage due to a fault compared to simply holding the current
at the threshold by reducing the voltage, as a current-limited bench supply would do.
Also, if the device is unattended
when it fails, interrupting rather than
limiting the power delivered could
help avoid an even larger disaster.
It can be set so that when the overcurrent circuit trips, it will automatically reconnect after a two-second
delay or require manual intervention (a button press). If your dual-rail
application is asymmetric, you can set
different overvoltage and overcurrent
thresholds for each rail.
Depending on the Mosfets used, it
can handle up to 4-7A per rail without heatsinking. Adding heatsinks to
the Mosfets will allow them to handle
more, up to 10A for the higher-current
Mosfets specified.
Due to the design’s modularity, you
only need to populate the required
features. To start with, you can equip
it to suit single or dual supply rails.
Configuring it for a single rail saves a
few components and doubles the single supply maximum voltage to 36V.
If your device to be protected has
more than one positive or more than
Fig.1: some of the different
ways the Supply Protector
can be used. If the maximum
voltage of ±18V for the dualrail version is not enough for
your application, you can
stack two boards to double
that, as shown on the far
right.
Australia's electronics magazine
siliconchip.com.au
Features & Specifications
● Voltage range: 4-36V DC or
±4-18V DC (±4-36V DC with two
boards)
● Over-voltage cut-out: 5-38V DC
or ±5-19V DC (±5-38V DC with
two boards)
● Voltage withstand: up to ±60V
at either input or across both
inputs
● Current capability: 7A+
without heatsinking (more with
heatsinks)
● Voltage insertion loss: typically
<300mV <at> 10A
● Over-current protection:
disconnects rails independently
if current draw exceeds a set
threshold
● Over-current reset: automatically
after two seconds or manually
via pushbutton
🔹
🔹
🔹
🔹 exact values depend on parts used
one negative rail, you can common the
grounds and use two or more of these
boards to protect them all, including
dual-rail applications operating up to
±36V, as shown in Fig.1.
You can leave some components off
if you don’t need overcurrent protection. You can also leave off the overvoltage sections if you don’t want that
feature. The circuit is arranged in three
sections, each supporting one or two
power rails.
Reverse polarity protection
The first section of the circuit, shown
in Fig.2, uses Mosfets Q1 & Q2 like
‘ideal diodes’. They have a very low
voltage drop when forward-biased but
a high impedance when reverse-biased.
If you accidentally connect the input
voltages with the wrong polarity, the
internal body diodes of Mosfets Q1 and
Q2 will be reverse-biased, and no current can flow. Q1 and Q2 remain off,
protecting all the downstream components from the abnormal condition.
The specified Mosfets have reverse
voltage ratings up to 60V, offering
plenty of protection against accidental power supply reversal.
However, without protection, the
Mosfets could be damaged by gate-tosource voltages exceeding 20V. Zener
diodes ZD1 and ZD2 ensure that the
voltage between the gate and source
of each Mosfet cannot exceed 15V.
Other Mosfets in the design have similar protections.
When the input voltage polarities
are correct, the internal diodes of Q1
& Q2 are forward-biased. As current
starts to flow, 47kW resistors pull the
Mosfet gates to ground, so they switch
on. As the gate bias exceeds 2-4V and
the Mosfet channel resistance drops,
the internal protection diodes will be
shunted, so very little voltage will be
lost across the Mosfets.
The Supply Protector’s minimum
voltage rating of 4V is because that
is the minimum voltage at which the
Mosfets used are guaranteed to switch
on and conduct sufficient current.
Over-voltage protection
The following section deals with
over-voltages. Zener diodes ZD3 and
ZD6 set a fixed value for each rail’s
protection threshold. The knee voltage for 1W zener diodes rated above
5.6V occurs around 3.5-5% below the
nominal zener voltage. As the supply
voltage reaches this level, they will
start to break down. Lower voltage
zener diodes have a more rounded
knee, so the difference from nominal
can be larger.
When enough current flows to
develop 0.6V at the gate of the associated SCR, it will trigger and switch
off either Mosfet Q4, in series with the
positive rail, or Mosfet Q5 in the negative rail, disconnecting and protecting
the downstream circuitry and the load.
The SCR will remain latched until
the supply voltage is removed. Providing the applied voltage remains below
the Mosfet specification (55V or 60V),
the unit will tolerate the condition
indefinitely, and the device you’re
protecting will stay safe.
Most applications won’t require
fine overvoltage threshold adjustment,
so you can simply set it by selecting
the nearest zener. Two extra diodes
labelled D4 and D5, in series with
the zeners, allow the threshold to be
tweaked. Usually, they are replaced
with wire links, but if required, regular or schottky diodes can be fitted to
increase the overvoltage trip thresholds by 0.3V (SB140/1N5819) or 0.6V
(1N4004).
Op amp IC1 has an absolute maximum limit of 40V, the highest overvoltage threshold supported. In practice, the trip points should be no more
than ±19V for one dual-rail device or
38V for a single-rail version, giving a
small safety margin.
The 220μF and 3300μF electrolytic capacitors are to counteract the
effects of power source inductance.
At switch-on, many devices cause a
momentary current surge as the supply
Dual-Rail Load Protector hard-to-get parts (SC7366, $35): includes the PCB and all semis except the optional/varying diodes.
siliconchip.com.au
Australia's electronics magazine
October 2024 73
Fig.2: the Supply Protector circuit has mostly independent positive and negative sections with three stages each. The
first is reverse polarity protection (using Mosfets Q1 & Q2), followed by overvoltage protection (Mosfets Q4 & Q5), then
overcurrent protection (Mosfets Q10 & Q11). The only sections shared between the positive and negative rails are the
half-supply generator (IC1d), reset oscillator (IC1c) and reset switch.
bypass capacitors charge. This high
current pulse can interact with inductances in the wiring etc, causing ‘ringing’ (oscillation), which causes an overshoot voltage to appear on the affected
power rail, sometimes a significant one.
One of my test supplies caused
damped oscillations with a frequency
of around 2MHz, resulting in a peak
overshoot voltage of around 50%
above the nominal supply. This persistently tripped the overvoltage protection at power-on.
The electrolytic capacitors dampen
power-on overshoot to avoid false
overvoltage trips. In severe cases, you
may need to increase the value of the
220μF parts, although they should
be sufficient for most cases. It is usually more severe with a longer input
power cable.
Overcurrent protection
The third section of the circuit
74
Silicon Chip
provides overcurrent protection.
The load current is monitored by the
voltage drop across the ‘+sense’ and
‘-sense’ resistors. For the positive rail,
LED15, VR1 and one ‘+bias’ resistor
set an adjustable reference voltage at
pin 3 of IC1a that is a couple of volts
below the +ve rail voltage.
LED17 and the other ‘+bias’ resistor create another voltage at IC1’s pin
2 that varies with the ‘+sense’ voltage
drop. IC1a compares these voltages; its
output is low when the sensed current
is below the setpoint, so Mosfet Q10
is usually on. If the current setpoint
is exceeded, IC1a’s output goes high,
switching off Q10 and disconnecting
the load, while also lighting overcurrent indicator LED21. Op amp IC1b,
Mosfet Q11 and LED22 function similarly for the negative rail.
The purposes of LED15 and LED17
aren’t to emit light; they provide
consistent voltage drops so the op
Australia's electronics magazine
amp inputs remain within the chip’s
common-
mode range, which does
not go up to the positive rail. The fact
that LEDs have a higher voltage drop
than a regular silicon diode (around
1.8V rather than 0.7V) is useful in this
application.
The voltage across the ‘+sense’ resistors is approximately 50mV at the
nominal overcurrent trip point. VR1 is
for fine-tuning; its 100W value means
that with 1mA flowing through it, a
full trimpot rotation will cover twice
the nominal voltage range expected
across ‘+sense’.
Note that the ‘+set’ LED (LED15)
usually goes out when the overcurrent
LED (LED21) lights. However, there
are cases where the current is near the
overcurrent set point where both could
light. So if you notice that, it’s normal.
The overcurrent protection only
interrupts the rail experiencing the
overload. When that happens for the
siliconchip.com.au
positive rail, D19 provides a feedback
path to latch the state even after Q10
interrupts the current and the ‘+sense’
voltage drop falls back to zero. It will
remain off until the condition is reset
by NPN transistor Q8 switching on
and pulling pin 3 of IC1a below the
pin 2 level.
When the output of IC1a goes high,
another NPN transistor (Q9) inverts
the transition to create a falling edge.
This is combined with any falling edge
from IC1b by diode D24. These are
AC-coupled to IC1c by a 1nF capacitor and diode D29, which works as
an overcurrent reset monostable. A
two-second delay is provided by the
1μF capacitor and 2.2MW resistor. The
100kΩ resistor at pin 9 of IC1c prevents
damaging input currents when pins 9
and 10 differ by more than 5.5V.
When the monostable times out (if
enabled) or the reset pushbutton is
pressed, Q6, Q7 and Q8 temporarily
siliconchip.com.au
shift the voltage levels at the inputs of
IC1a and IC1b. This forces them out of
their latched states, re-enabling Mosfets Q10 and Q11.
If the auto-reset feature is enabled,
voltage to the load will be restored
two seconds after it trips. If the overcurrent condition persists, the trigger-
delay-restore process will repeat indefinitely every two seconds (or until the
fault clears).
This monostable arrangement
requires a reference voltage at the
midpoint of the IC’s power supply.
We could have used the GND line for
this reference, but that would mean
the circuit would only work with symmetrical dual rails, reducing its flexibility. So IC1d synthesises a mid-rail
voltage (halfway between +ve and -ve)
that self-adjusts without needing different configurations.
The 100nF capacitor and 1MW resistor connected to header CON3 ensure
Australia's electronics magazine
that resets are only momentary, even
if the pushbutton is held down. This
quickly rearms the overcurrent protection while preventing the output
from being held on continuously if
excessive current continues to flow.
The 100nF capacitor at IC1c’s output works similarly for monostable-
initiated resets.
We don’t care about the brightness
of LED15-LED18 since, as mentioned,
they are not indicators. However, it
does matter for LED7, LED8, LED21
and LED22. The circuit shows 22kW
current limiting resistors for them,
suiting high-brightness LEDs. If using
regular LEDs, reduce the values to
around 5.6kW for more current.
Alternatively, if they’re too bright
(which may happen with higher-
voltage single-rail applications),
increase the series resistances.
Diodes D26 and D27 across the
output terminals are normally
October 2024 75
reverse-biased. These protect the circuit from inductive loads or long output power leads. Any inductance in
these can cause a reverse voltage spike
when the load current is interrupted
(as can certain capacitor configurations in the load). These diodes will
safely dissipate that energy.
Component selection
The PCB is designed for miniature
1/8W resistors, 3.5mm long. You can
get them from element14, DigiKey or
Mouser. You can use more common
1/4W resistors, but you will need to
stand them up (at least partially).
The current sense resistors will
typically be below 0.1W. element14,
DigiKey and Mouser have wide ranges
of low-value ‘current sense’ resistors,
many of which will be suitable, even
if their power ratings are higher than
necessary. Alternatively, parallel two
or more resistors to create a lower resistance. The PCB holes are large enough
for the leads of current-sense resistors
or multiple regular resistors.
The parts list gives information
on supported capacitor lead pitches
(although you can bend them if you
have to) and suggested Mosfet types.
However, many more suitable ones
will be available.
If substituting other Mosfets, pay
particular attention to their maximum
on-resistance, Vgs threshold voltage and reverse breakdown voltage.
Particularly for P-channel Mosfets,
cost-effective options with a low on-
resistance aren’t common. The higher
the maximum on-resistance, the hotter
the Mosfets will run.
Heatsinking
Using, say, IRF1018E and IRF4905
Mosfets, at 4A current draw and 10V
or higher, they will dissipate 135mW
and 320mW each, respectively. The
temperature of a TO-220 package in
free air rises by around 70°C/W, so
without heatsinking, they will rise to
9°C and 22°C above ambient.
Note that the ambient temperature is
the air temperature within the enclosure, which could be significantly
higher than room temperature.
The Mosfet on-resistances could be
a little higher when using rail voltages
significantly below 10V, so for lower
operating voltages, pay close attention
to the Mosfet temperatures during testing. If they become too warm to touch
comfortably, they require heatsinks.
If you know or suspect you’ll need
heatsinks in advance, it will be easiest
to fabricate and mount the transistors
onto them before soldering the transistors to the PCB.
You can fashion heatsinks from
3mm aluminium using three separate
bars or angles for the three rows of
Mosfets. Cut the material to fit comfortably within the component footprint.
For a dual rail application, mount two
Mosfets per angle with their centre
holes spaced 18mm apart. Use insulators and Nylon bushes and/or screws;
insulating each Mosfet from its heatsink is the best practice.
You won’t necessarily require large
heatsinks; it depends on how much
power needs to be dissipated. For maximum heat dissipation, bridge the tops
of the three aluminium angles with a
commercial heatsink.
Construction
Figs.3 & 4: the dual-rail version of the Supply Protector uses all the parts on
the PCB, although some sections can be omitted. Parts that can be left off if you
don’t need over-current protection are shown, in Figs.7 & 8. Soldering heavyduty 1mm2 wires to the underside of the board, as shown here, will reduce
the resistance of the current-carrying tracks. That will lower the voltage drop
between the input and output and allow the PCB to handle more than 5A.
76
Silicon Chip
Australia's electronics magazine
Start by selecting the values of the
current sense and bias resistors, and
over-voltage threshold zeners, using
either the panel or Tables 1 & 2 overleaf.
The Supply Protector is built on a
double-sided 96 × 69mm PCB coded
18109241. If you’re building the full
dual-rail version, fit all the components shown in Fig.3, while if you
want to make the single-rail version,
fit just the components shown in Fig.5
or Fig.6. Figs.7 & 8 show further variations, which experienced constructors
siliconchip.com.au
could combine with one of the single-
rail variants if desired.
The two adjustment diodes, D4 and
D5, are generally not required and can
be replaced with wire links. If you
need to fine-tune the trip voltage, you
can fit diodes instead, as explained
earlier.
We suggest constructing in two
stages: build and test the reverse-
polarity and overvoltage protection
sections before adding the remaining
components. Roughly speaking, the
reverse and overvoltage protection
components are below and to the left
of the 3300μF electrolytic capacitor in
the middle of the board, not including
the two 3300μF capacitors (use Fig.7
as a guide).
Pay close attention to the orientation of the transistors, diodes, LEDs
and electrolytic capacitors. The two
SCRs should face in opposite direc- The fully assembled Dual Rail Supply Protector PCB with all features available.
tions. Start by fitting the lower-profile Note the use of smaller-than-usual resistors to keep it compact.
components like the diodes and resistors that will lie flat on the board (see
Figs.5 & 6:
Table 3), then the capacitors, then the
these overlay
taller components like the Mosfets.
diagrams
shows the
If you want to minimise the voltboard fitted
age drop across the device, or will be
with just the
using it at high currents, you can solder
components
extra wires to the underside as shown
needed for
in Fig.4. That should not be necessary
a single
for applications up to around 5A per
rail Supply
rail, though.
Protector.
You need
to add wire
links where
shown in
red. When
building
any of these
versions,
watch the
orientation of
the IC, diodes
and Mosfets,
as they must
all be correct.
Initial testing
The easiest way to verify the correct
operation of the reverse polarity protection is with a variable power supply. Apply power to the input connector in reverse but starting at 0V. Ramp
the voltage slowly up to -1V and monitor the “+rail” and “–rail” test points
with a multimeter to verify the absence
of any voltage. It is working if the supply reaches -1V and there is no voltage
on those test points.
You could use one AA or AAA cell
if you don’t have a variable power
supply.
Note that if you previously applied
power in the correct direction, your
multimeter may read the residual
charge on the 220μF capacitors.
Next, verify the overvoltage protection threshold by switching off the
variable power supply and reconnecting the supply with the correct polarity. As you ramp the variable power
supply up, by the time it reaches 1V,
a voltage will be detectable on both
test points.
siliconchip.com.au
Australia's electronics magazine
October 2024 77
Pluggable terminal blocks for the inputs
and outputs make connecting the wires easy.
The board can be mounted using a tapped
spacer in each corner.
Figs.7 & 8:
these
overlays
shows which
components
you can
leave off (or
link out) if
you don’t
want either
the overcurrent or
over-voltage
protection
feature. If
building a
single-rail
version, you
will need to
refer to Figs.5
& 6 as well,
and figure
out which
components
to leave off or
link out.
78
Silicon Chip
Australia's electronics magazine
The readings will initially be around
0.7V below the variable power supply level. Once you reach 3-4V, the
test point voltages should rise to the
input voltage.
There are two additional test points
labelled “+rail prot” and “–rail prot”,
which you can now monitor. Continue
increasing the variable supply towards
the protection threshold. As you pass
the threshold, each overvoltage protection LED should illuminate and
then, at a fractionally higher input
voltage, the “+rail prot” and “-rail
prot” test points should start falling
back to zero.
The actual tripping thresholds may
differ from the calculated value due to
component tolerances and the zener
knees.
Remember not to ramp the input
voltage past the ratings of the 220μF
capacitors, and be very careful not to
ramp your variable supply past 40V (or
±20V) until you are sure the overvoltage protection in both rails is working
correctly, or you risk damaging IC1 (if
fitted). If either rail’s protection hasn’t
kicked in by 1V beyond the calculated
trip point, it’s either not working, or
the zeners are wrong.
To reset after the overvoltage protection has tripped, return the variable supply to 0V or temporarily disconnect it.
With the overvoltage protection section working, you can finish fitting
components to the PCB, starting with
IC1; ensure its pin 1 indicator goes at
upper right as shown in the overlay
diagram. Don’t mount the two trimpots yet.
If your board needs cleaning because
it’s covered in flux residue, submerge
it under isopropyl alcohol or methylated spirits and gently rub it with an
old toothbrush. Wait for it to dry, then
mount and solder the trimpots.
Check that Mosfets are electrically
isolated from any heatsink metal.
To verify the overcurrent trip circuit,
connect the power rail (or rails) to a
variable supply but don’t yet connect
a load. Adjust the power supply to the
same voltage you used to calculate
the bias resistances. If either overcurrent trip LED is already illuminated,
slowly wind the trimpots clockwise
until they extinguish automatically,
use the pushbutton reset or short the
upper two pins of CON3.
If either of the overcurrent trip LEDs
fails to extinguish, there is a problem
siliconchip.com.au
with the reset or overcurrent circuit.
If you’re using auto-reset, check the
output of IC1d at the “VG” (virtual
ground) test point is near ground level
for a symmetrical dual-rail version,
or otherwise approximately midway
between the power rails.
Assuming both overcurrent LEDs
are off and the set LEDs are on,
wind each trimpot anti-clockwise
until either the trimpot is fully anti-
clockwise or the corresponding set
LED switches off and the overcurrent
LED comes on. When you’ve reached
this point, nudge the trimpot clockwise (and press the reset button if
equipped) so both the overcurrent
LEDs are again off.
The overcurrent protection circuits
will now be armed at a very small current threshold. Use a low-value resistor (say 100W or less) between ground
and each output rail to verify that
the overcurrent protection triggers.
The corresponding overcurrent LED
should illuminate instantaneously,
and the associated set LED should
extinguish.
If your overcurrent trip point
exceeds the normal operating current by an amp or more, connect a
dummy load that will draw current
just below the overcurrent tripping
point and adjust VR1 and VR2 so the
load remains turned on. Otherwise,
use your intended load to set the overcurrent trip point.
Finally, run the device for ten minutes while monitoring the temperature
of the Mosfets. If the Mosfets become
too warm to touch comfortably, turn
off the power and fit heatsinks before
using it. Wire it up, and you can sit
back and relax, knowing your load
device is protected!
Parts List – DC Supply Protectors (all features)
1 double-sided PCB coded 18109241, 96 × 69mm
2 100W miniature top-adjust trimpots (VR1, VR2)
2 3-way 5.08mm pitch pluggable terminal blocks (CON1, CON2)
[Jaycar HM3113+HM3123, Altronics P2873+P2813]
1 3-way pin header and jumper shunt (CON3)
1 NO momentary pushbutton (optional)
4 M3 × 6mm panhead machine screws and matching spacers
Semiconductors
1 NCS20074 quad rail-to-rail output op amp, SOIC-14 (IC1)
3 high-current P-channel Mosfets, TO-220 (Q1, Q4, Q10) ★
3 high-current N-channel Mosfets, TO-220 (Q2, Q5, Q11) ★
2 BC556 45V 100mA PNP transistors, TO-92 (Q3, Q7)
3 BC546 45V 100mA NPN transistors, TO-92 (Q6, Q8, Q9)
2 C106 SCRs, TO-126 (Q12, Q13)
8 high-brightness 3mm LEDs (LED7-8, LED15-18, LED21-22)
[Vishay TLLK4401]
6 15V 1A zener diodes, DO-41 (ZD1, ZD2, ZD11, ZD12, ZD23, ZD25)
2 1A zener diodes, DO-41, values to suit application – see Table 1 (ZD3, ZD6)
2 1N5819 or SB140 40V 1A schottky diodes (D13 & D14)
6 1N4148 75V 200mA diodes, DO-35 (D19-D20, D24, D28-D30)
2 1N4004 400V 1A diodes, DO-41 (D26, D27)
2 extra diodes to fine-tune over-voltage thresholds (D4, D5; optional, see text)
★ suitable types include IRF4905 (up to ±55V & 4A),
IPP80P03P4L-07 (±30V & 7A) and SUP90P06-09L-E3 (±60V & 7A)
★ suitable types include IRF1018E (7A), CSD18534KCS (7A), DIT050N06 (4A),
STP60NF06 (5A) & IPP80N06S4L (8A; all can handle up to ±60V,
ratings are without heatsinks and are only a guide)
Capacitors
2 3300μF 50V electrolytic (5mm or 7.5mm lead pitch) [Altronics R5217]
2 220μF 63V low-ESR electrolytic (3.5mm or 5mm lead pitch)
1 1μF 50V ceramic or MKT
9 100nF 50V ceramic or MKT
2 10nF 50V ceramic or MKT
1 1nF 50V ceramic or MKT
Resistors (all ⅛W 5% miniature axial unless noted) ♦
1 2.2MW
9 47kW
4 3.3kW 1W 5%
3 1MW
9 22kW
2 910W
1 330kW
1 15kW
4 Rbias resistors – see Table 1
2 150kW
6 10kW
2 Rsense resistors – see Table 2
3 100kW
♦ regular ¼W resistors can be used but they won’t sit flat on the PCB
Table 1 – zener diode values
Table 2 – current sense resistors
Trip
ZD3/ZD6 Bias resistors
Adjustment range
Sense resistor
~5V
5.1V
3.0kW
0-1A
100mW 1/8W
~5.5V
5.6V
3.3kW
0-2A
50mW 1/4W
~7.25V
6.8V
5.1kW
0-3A
33mW 1/4W
~10.3V
10V
8.2kW
0-4A
25mW 1/2W
~13V
13V
11kW
0-6A
16mW 1/2W
~15.1V
15V
13kW
0-8A
12mW 2/3W
~18V
18V
16kW
0-10A
10mW 1W
~20V
20V
18kW
~23.8V
24V
22kW
~29.5V
30V
27kW
Tables 1, 2 and the panel on the next
page are used to determine the best
values of various components to suit
your needs.
~38V
39V
36kW
siliconchip.com.au
Australia's electronics magazine
Table 3 – resistor colour codes
October 2024 79
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Calculating component values
Several component values should be selected to suit your application as follows.
Overvoltage trip point
First, you must determine the highest voltage that’s safe to apply to the load. If
unsure, measure the output of the existing power supply and add a safety margin.
Zener diodes ZD3 (‘+OVtrip’) and ZD6 (-OVtrip’) set the overvoltage trip point for
each rail in combination with the 3.3kW resistors. The SCRs will trip when their trigger input reaches approximately 0.6V. Allowing for a voltage drop of about 100mV
across the resistors, the required zener voltage is (Trip – 0.7V) × 1.05.
As mentioned earlier, low-voltage zeners may trigger at lower voltages than
expected. Also, typical zeners diodes have 5% tolerances.
In the middle of the voltage range (eg, around ±15V), you can generally get away
with a zener diode that has a voltage rating close to the desired trip point, as the
0.7V and 5% factors cancel out.
Because the expected overvoltage trip point lies within a range, and zeners are
only available in certain preferred values, you may need to use adjustment diodes
if you require high precision. Adding a schottky diode for D4 or D5 (like a BAT85,
SB140 or 1N5819) will increase that rail’s trip point by around 0.3V, while adding a
silicon diode (like a 1N4148 or 1N4004) will increase it by around 0.6V.
Don’t use zeners below 4.3V or above 19V (for a dual-rail configuration) or 39V
(for single-rail operation). You can use different values for the two zeners for asymmetric applications.
Ensure that the 3300μF output capacitors have voltage ratings above the trip
points. For example, if you have ±18.1V overvoltage protection thresholds, select
25V capacitors.
Because the 220μF capacitors after Mosfets Q1 & Q2 are on the unprotected
side of the overvoltage protection circuit, they will experience any overvoltage, so
their voltage ratings should exceed the highest expected input voltage. We recommend using 50V or 63V rated capacitors there, although you might get away
with 35V caps in some cases.
Overcurrent trip point
The ‘+sense’ and ‘-sense’ resistors are used to monitor the current in each rail.
The overcurrent trip is calculated for a sense resistor voltage drop of about 50mV,
although the trimpots let you set it up to 100mV.
Use Ohm’s law, R = V/I, and the power formula, P = VI, to calculate the required
resistances and power ratings. Let’s use 2A as an example. For a 50mV drop, the
formulas give R = 25mW (0.05V ÷ 2A). If you can’t find a resistor with the calculated value, round the resistance to the closest available value. A 0.022W, 0.025W
or 0.033W resistor would be suitable in this case.
We calculate the power at 100mV as we don’t want the resistor to overheat if the
trimpot is set to maximum, so P = 200mW (0.1V × 2A). Ideally, the resistor should
have close to twice the power rating (to account for elevated ambient temperatures etc), so in this case, use a ½W or 0.6W resistor.
If your application has asymmetric current requirements, you can choose different values for the two resistors.
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Bias resistances
Four resistors are labelled ‘+bias’ or ‘-bias’. The bias resistors are selected so
that about 1mA flows through them when the supply is at its nominal (not overvoltage trip) level.
The series LEDs have a forward voltage drop of around 1.8-2V, so consider that
when calculating the resistor values. The exact drop doesn’t matter as long as the
four LEDs (LED15-LED18) are the same type, so the voltage drops are similar. Red,
orange or yellow LEDs with a forward voltage drop below 2.3V will work.
You can measure the LED’s forward-biased drop using a digital multimeter’s
diode testing function.
Say the nominal power supply is ±12V and you have red LEDs with a 1.6V forward voltage. The required resistance will be R = (12V – 1.6V) ÷ 0.001A = 10.4kW.
Choose the nearest available resistance, 10kW in this case.
If you have an application with asymmetric voltage rails, the ‘+bias’ and ‘-bias’
SC
resistances may differ.
80
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