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DC protection relay.



LR02 is DC protection relay for low level signals like line-signals and headphone-signals.
The load is disconnected in case there is DC on the amplifier or if one of the supplies drops below a pre-set limit.
DC detection threshold can be set down to 25 mV and supply voltage range is ±3 V to ±15 V.


The component values shown on the following schematics are for LR02AAA that is designed for a headphone amplifier running on ±15 V supplies.
A complete schematic and a spread-sheet to calculate component values are in the download section.

Reference voltage.

Reference voltage schematic.
Fig.1: Reference voltage schematic.

The reference voltage for the circuit is a simple shunt regulator.
For supply voltages of ±10..15 V, D12 is a 5 V zener diode and R14 is selected so the current through D12 is around 1 mA.
For lower supply voltages, D12 is a lower-voltage zener diode or a signal diode.
If you, for any reason, should need a more accurate reference voltage, the PCB will accept a 3-terminal IC regulator like TL431 ( 2.5 V ) or TLV431 ( 1.24 V ).
The reference voltage must be selected at least 1.5 V below the minimum supply voltage (the power supply detector threshold voltage) in order to maintain the comparators within their input common-mode range.

DC detector.

DC detector schematic.
Fig.2: DC detector schematic, one of two channels shown.

The DC detector is a low-pass filter followed by a window comparator.
The low-pass filter is R11-C11 or R11-C12-C13. C11 is a film capacitor that will be suitable for most applications, but if a higher capacitance is needed, C12 and C13 can be used instead.
C10 is a ceramic capacitor that prevents false triggering due to HF pick-up from the AMP wiring.
U30B and U30C are two open-collector comparators that will pull DC# low if the voltage on C11 goes above V1 or below V2.
The sensitivity is selected by the value of the reference voltages and the values of R12, R13 and R22, R23.
The values of V1 and V2 can be set as low as 25 mV, but for most practical purposes it will be 0.5..1.5 V.

Power supply detector.

Power supply detector schematic.
Fig.3: Power supply detector schematic.

Open-collector comparators U31C and U31D pulls V5 low if V3 goes below VR+ or V4 goes above VR-.
R30 limits the current through U31,s outputs during the discharge of C30. R32 limits the maximum voltage across C30, so it is possible to use a capacitor with a lower voltage rating than the total supply voltage.
When V7 goes above V6, U31B's output goes low, turning on Q50 and energizing the relay E50.
Terminals RE1 and RE2 are for use with an external relay.
Zener diode D51 reduce the relay's turn-off time. The following table shows the release time with 2 different relays and with/without D51. The release time is measured from the time where ON# goes high to the end of the relay's bounce time.

Table 1: Measured relay release time.
Relay typeCoil voltageCoil resistanceR50D51Release time
NEC MR62-24SR24 V1050 Ω220 Ω0 V8 ms
NEC MR62-24SR24 V1050 Ω220 Ω24 V4 ms
NEC MR602-24SR24 V2880 Ω620 Ω0 V3 ms
NEC MR602-24SR24 V2880 Ω620 Ω24 V2 ms

LED driver.

LED driver schematic.
Fig.4: LED driver schematic.

When the ON# signal is high (relay de-energized) the astable multivibrator circuit around U31E will flash the LED at around 3 Hz.
When ON# goes low, the LED is on.

Signal switch.

Signal switch schematic.
Fig.5: Signal switch schematic.

The terminals AMPL, AG and AMPR goes to the amplifier and the terminals HPL, HPG and HPR goes to the output connector(s).
The circuit GND and the relay contact grounds are not connected on the PCB and must be connected externally.
Resistors R10 and R20 gives a slightly softer turn-on than just by putting the relay contacts in series with the load. When the relay is off (as shown), HPL is shorted to GND. During relay change-over, the load is powered through R10 and as the relay turns on directly from the amplifier.
The minimum value of the resistors are:
R = VCC ² / P, where R is the resistor value, VCC is the supply voltage and P is the resistor's power rating.

Additional components.

Additional components schematic.
Fig.6: Additional components.

H91..H94 are the PCB mounting holes. C91..C94 are ceramic HF decoupling for between the PCB ground and chassis. These are normally not required.
C31..C34 are decoupling for the comparators. These must be ceramic types.
D13, D23 are reverse voltage protection. These are optional.


Specification are shown for the first prototype of the LR02.

Table 2: Specification for LR02A prototype.
Supply voltage±15 V
Supply current (relay and LED on)20 mA
Power supply detector threshold±12 V
DC detector threshold±1.2 V
DC detector trig level at 10 Hz6 V ( 8.5 V-p )
DC detector trig level at 20 Hz12 V ( 17 V-p )
DC detector release time at 15 V DC input12 ms
Board size (length / width / height)96.5 mm / 50.8 mm / 13 mm


LR02 top-side photo.
Fig.7: LR02A.

Download LR02A design files.

I have boards available for this project. See the PCBs page.

Known Issues / updates.

No known issues.

Copyright and disclaimer.

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Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: diy@poulpetersen.dk

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