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The EasyEDA Precision Commodore 64 Computer Saver PSU overvoltage protection circuit

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For the Commodore 64 overvoltage protection applications, this design is superceded by Uberclamp:

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:

The difference between this TL431 based precision design and the zener based EasyEDA Commodore 64 Computer Saver PSU overvoltage protection circuit shown here:

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, offering faster and more accurate overvoltage protection than the Carlsen Electronics 'C64 Saver':

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

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:


The EasyEDA Precision C64 Overvoltage Protection Circuit Schematic

How to protect the C64 9V AC PSU outputs.

The EasyEDA Precision C64 Overvoltage Protection Circuit PCB


ID Name Designator Footprint Quantity BOM_Manufacturer Part BOM_Description
1 220n C2,C1 C_MLCC_RES_DIP_5.5X3MM_P5MM 2 Various 220nF 16V ceramic capacitor. 5mm pitch resin dipped MLCC or similar.
2 BC557B Q1 BC557_TO92_TRIPOD 1 BC557B BC557 PNP BJT in TO-92 package.
3 1k R5,R6,R3 RC07 3 Various 1k 250mW Metal Film 2% or better.
4 STP36NF06L M1 STP36NF06L_H 1 STP36NF06L Logic level MOSFET N-Channel 60V 30A
5 BZX55C5V6 D1 BZX55C5V6-TAP 1 BZX55C5V6 Zener Diode 500mW BZX55 series or similar. 10% tolerance
6 1k R1 RC07 1 Various 1k 250mW Metal Film 2% or better.
7 LED_RED DFAULT LED3MM 1 Any 3mm lead spacing red LED. (Almost any LED can be fitted if the leads are bent or wired to the PCB) Red 3mm LED with approx 2V forward drop.
8 LED_GREEN D5VC64OK LED3MM 1 Any 3mm lead spacing green LED. (Almost any LED can be fitted if the leads are bent or wired to the PCB) Green 3mm LED with approx 2V forward drop.
9 FUSE_IN_HOLDER_PTH_2A_Antisurge_5X20MM F1 FUSE_IN_HOLDER_PTH_2A_ANTISURGE_5X20MM 1 Various PCB Mount 20mm Fuseholder
10 SIP7 P1,P2 HDR1X7 2 n/a These can be 1x7x0.1" pitch headers or just wires soldered directly into the PCB.
11 unplated_M3_clearance_hole 3M5_H1,3M5_H2,3M5_H3,3M5_H4 HOLE_3MM5 4 n/a You don't have to buy these: they come free with the PCB.
12 TL431 U1 TL431AILP_TO92_TRIPOD 1 TL431 TL431 Voltage Reference
13 1.5k R2 RC07 1 Various 1.5k 250mW Metal Film 2% or better.
14 100 R4 RC07 1 Various 100R 250mW Metal Film 2% or better.
15 1n C3 C_MLCC_RES_DIP_5.5X3MM_P5MM 1 Various 1nF 16V ceramic capacitor. 5mm pitch resin dipped MLCC or similar.




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