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**For the Commodore 64 overvoltage protection applications, this design is superceded by Uberclamp:**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**
* **For a design with a more accurate overvoltage setting, please see:**
>**https://easyeda.com/example/The_EasyEDA_Precision_Commodore_64_Computer_Saver_PSU_overvoltage_protection_circuit-G3pLJ5Paw**
This project contains a non-simulation schematic, Bill of Materials and PCB layout for the EasyEDA Commodore 64
Computer Saver PSU overvoltage protection circuit simulated in:
https://easyeda.com/example/Commodore_64_Computer_Saver_overvoltage_protection_circuit_simulations-0ht3pLJ42
The 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
approximately 6.2V 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 already 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.
The zener based shunt clamp 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, 5.6V zener diodes D1 and D2 do not conduct. Therefore, there is
no current flow in R1 or R2 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 D2 increases to about 5.6V, current starts to flow through it. 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 R3 up
towards the V5VC64 rail. This turns on M1 which then draws a large current from the V5VC64 rail.
Due to 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 total voltage across D1 and R2 exceeds about 6.1V (5.6V+0.5V) the current drawn by M1 increases
rapidly.
If the fuse F1 was replaced by a short circuit, the V5VC64 rail would therefore be clamped to a maximum of about
6.2V 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 about
6.2V 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 C64 Overvoltage Protection Circuit Schematic
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The EasyEDA Commodore 64 Computer Saver PSU overvoltage protection circuit
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Creation time: 2015-09-28 12:41:54
Published time:
2022-01-25 07:40:37
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