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**For the Commodore 64 overvoltage protection applications, this design is superceded by Uberclamp:**
**https://easyeda.com/example/Uberclamp_Schematic_PCB_and_BoM-r4YgysK2k**
This project contains a non-simulation schematic, Bill of Materials and PCB layout for the EasyEDA Precision Commodore 64
Computer Saver PSU overvoltage protection circuit simulated in:
https://easyeda.com/example/Commodore_64_Computer_Saver_overvoltage_protection_circuit_simulations-0ht3pLJ42
The difference between this TL431 based precision design and the zener based EasyEDA Commodore 64 Computer Saver PSU
overvoltage protection circuit shown here:
https://easyeda.com/example/The_EasyEDA_Commodore_64_Computer_Saver_PSU_overvoltage_protection_circuit-bS4ECcMXV
is that the TL431 based clamp circuit used in this design is considerably more accurate than the zener based clamp.
The TL431 is available in 0.5% (B grade), 5% (A grade) and 2% (standard) initial accuracies whereas the breakdown
voltages of zener diodes are generally specified to no better than 5% and more typically 10% and are quite current
and temperature dependent.
This project shows a circuit, developed for EasyEDA by signality.co.uk, offering faster and more accurate
overvoltage protection than the Carlsen Electronics 'C64 Saver':
http://personalpages.tds.net/rcarlsen/cbm/c64/SAVER/saver%20schematic.jpg
The original C64 PSU tends to fail such that the output pulls up to about 9V. In the event of such a PSU fault,
the circuit described here protects the C64 motherboard components against exposure to a voltage that is above
their Absolute Maximum ratings.
The EasyEDA design uses a high power shunt voltage regulator based on an amplified zener diode to, in the event
of an overvoltage fault from the C64 PSU, clamp the 5V supply to the Commodore C64 to a safe maximum voltage of
6.225V and in so doing, to blow a fuse to prevent accidental reconnection to a faulty PSU.
The design has two LEDs, a green one to indicate the the power to the C64 is OK and a red one to indicate that an
overvoltage fault has occurred.
With adequate heatsinking the design could operate simply as an unfused shunt clamp but it is considered safer to
blow a fuse than to encourage people to rely on the clamp to then regulate what is, by that time, a broken PSU. Because
the clamp only dissipates any power during the brief period between when the overvoltage fault occurs and the
fuse blows, this then removes the need for any heatsinking other than the PCB on which the power MOSFET used in
the clamp is mounted.
Note also that the use of an Anti-surge or time-delay fuse means that any short, transient voltage overshoots
(in the region of tens of microseconds) from the PSU output will be clamped by the shunt regulator but will not
blow the fuse.
The TL431 based shunt clamp therefore operates to completely prevent any rise of the 5V supply to the C64 above
the clamp level and, within a few milliseconds after the fault event, to blow a fuse and so permanently
disconnect the 5V supply to the C64.
**How it works:**
with a nominal 5V supply at V5V and hence V5VC64, the 5.6V zener diodes D1 and the TL431, U1, do not draw any
cathode current. Therefore, there is no current flow in R1 or R4 so the red DFAULT LED and PNP transistor Q1
are both off.
If Q1 is off then so is the MOSFET M1 because the gate voltage is at ground.
Assuming that both red and green LEDs are chosen to have a forward drop of approximately 2V, there is roughly 3mA
flowing through R4 and the green D5VC64OK LED is on.
As the voltage across R2 and R3 increases such that the voltage at REF reaches the TL431 reference voltage value
of 2.49V, then current starts to flow into the cathode of U1. The same current flows through R2. Negligible
current flows through the base emitter junction of Q1 until the voltage drop across R2 reaches about 0.5V.
At this base-emitter voltage (Vbe), Q1 starts to turn on, pulling the voltage across R5 up towards the V5VC64 rail.
This turns on M1 which then draws a large current from the V5VC64 rail.
Due to the gain of the TL431, the exponential relationship between Vbe and base current in a bipolar transistor and
current gain of Q1, only a small increase in the voltage drop across R2 will cause a large increase in the gate
voltage applied to M1. Therefore, once the voltage across R2 in series with R3 exceeds 6.225V - which causes the
voltage on REF to exceeds 2.49V - the current drawn by M1 increases rapidly.
The relationship between the clamp voltage and the TL431 reference voltage is given by:
>Vclamp = 2.49*(1+R2/R3)
Hence, Vclamp can be adjusted by suitable selection of the values of R2 and R3.
If the fuse F1 was replaced by a short circuit, the V5VC64 rail would therefore be clamped to a maximum of 6.225V by
the shunt regulator action of the circuit. M1 would sink any difference in current between that of the load and that
available from the source as the source tried to raise the voltage on the V5VC64 rail and so M1 would dissipate
significant power and heat up.
However the presence of the fuse, F1, means that as soon as the total current through the fuse reaches 2 Amps for
more than a few tens of milliseconds, then the fuse blows open circuit. The pulsed power handling of the
STP36NF06L chosen for M1 is >70W for 10ms so, given the limited current available from the C64 PSU source, M1 can
safely handle the power dissipation for the short time before the fuse blows without needing any heatsinking in
addition to that already provided by the copper area under the device on the PCB.
Once the fuse has blown, the V5VC64 supply to the C64 motherboard drops from the momentary clamped voltage of
6.225V to zero with a decay limited by the internal decoupling capacitance and load current presented by the C64.
Once the fuse has blown, the green D5VC64OK LED extinguishes.
Once the voltage on V5V exceeds about 5.6V, D1 starts to conduct and so current flows through R1 and the red DFAULT
LED. Once the C64 PSU has failed and the protection circuit blown the fuse, the output of the C64 PSU pulls up to
about 9V. Allowing for a drop of 5.6V across D1 and 2V across the DFAULT LED this gives about 2.4V across R1 and so
approximately 24mA in the DFAULT LED.
Note that the body diode of the MOSFET, M1, in combination with the fuse also provides inherent input supply
reverse polarity protection. A reverse connected 5V input supply will cause the body diode to conduct, limiting the
voltage applied to the C64 mother board to approximately -1V only until the fuse blows a few milliseconds after the
fault is applied.
**Constructional notes.**
The copper floods under the the tab of M1 on both sides of the PCB are at V5VC64 so no insulating washer is needed
but a smear of heatsink compound or a silicone heat transfer pad under M1 is recommended to improve heat transfer
into the PCB.
Also take care that any bolt or rivet used to fix the tab of M1 to the PCB does not short to the V9VAC6 or V9VAC7
tracks on the underside of the board. The solder mask covering these tracks is easily scratched or perforated by
star washers etc. so do not rely on it to provide sufficient electrical insulation.
Note that the two of the four M3 clearance mounting holes are surrounded by copper ground flood so conductive
fixings may be at ground if the solder mask on the bottom surface of the PCB is damaged.
**All parts except the PCB are available from Bitsbox.co.uk**
**The PCB can be purchased from EasyEDA.**
* **Please note that this project is subject to the CC-BY-NC-SA 3.0 license.** For more information please see:
>https://creativecommons.org/licenses/by-nc-sa/3.0/
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The EasyEDA Precision C64 Overvoltage Protection Circuit Schematic
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