Simple Strategy Safely Connects Transformerless-Supply Circuits

Ramkumar Ramaswamy

Electronic Design

Low-power circuits commonly use transformerless power supplies. However, the use of earth grounds in many of these circuits creates a serious problem that is often ignored.

The very popular Microchip Application Note [1] shows a circuit ground (0 V), but the reference is silent about whether it can be connected to the main power supply’s earth ground. Similarly, Apex Technology’s Application Note [2] describes circuits that cannot be interfaced safely and legitimately with any external test equipment.

The reasons for these problems are not difficult to see. In the former [1], the transformerless power supply’s circuit ground has high voltages with reference to neutral, and connecting it to earth would cause a dangerous situation. Therefore, connecting any external equipment whose circuit ground is connected to earth would be downright dangerous.

In the latter [2], the transformerless power supply’s circuit ground is connected to neutral, so connecting any external equipment to the circuit (such as an oscilloscope to troubleshoot the circuit) would violate wiring regulations in most countries and usually cause the circuit breaker to trip if the external equipment has its circuit ground connected to earth.

One solution might be to try to insert a small resistor between the circuit ground in question and the of the external equipment. This prevents the mains circuit breaker from tripping and technically avoids violating wiring regulations, but it causes a new problem – noise between the earth and neutral lines. This noise gets into the circuit, defeating the purpose of the earth connection. Usually, the result is that the earth ground is, quite simply, rendered unusable.

A simple workaround to this problem assumes that the transformerless circuit’s ground is tied to neutral, which is the much more popular and preferred strategy for transformerless power supplies (see the Figure 1). The circuit is nothing fancy. It’s a simple differential amplifier, but its use in this specific context is innovative. It illustrates how a signal from equipment A can safely and reliably be fed into equipment B irrespective of which one has the problematic circuit ground.

Figure 1.	A simple op-amp circuit placed between a transformerless circuit with a ground tied to neutral and a circuit with an earth ground will eliminate the safety and noise problems associated with such connections.

For example, A could be a signal generator feeding into a transformerless circuit, B, in which case G1 is earth, and G2 is neutral. If B is an oscilloscope being used to troubleshoot a transformerless circuit, A, G1 is neutral, and G2 is earth. Either way, G1 is A’s circuit ground and G2 is B’s circuit ground, as well as the op-amp circuit’s ground.

The gain at both op-amp inputs is equal and opposite, so the earth-neutral noise (which is the noise at G1 with respect to G2) cancels out completely and does not appear at the input of B. Effectively, the circuit allows for the safe use of both neutral and earth terminals by making the neutral-earth noise voltage appear as a common-mode signal at the op-amp input. This simple strategy overcomes the two problems described above.

Any general-purpose, unity-gain stable JFET-input op amp with a low offset voltage will work.

References