# Input range of operation amplifier

Lets have a look at the following circuit:

(Input waveform is sinusoidal and Vpp would be ~32V)

The input waveform should represent an AC Source from a transformer (230VAC/12VAC).

The circuit should automatically disconnect the load (between the two open points in the schematic) if the load draws more than 500mA.

As you can see, there is a relay which does the job.

Now imagine a short circuit in the load. Let's say the resistance of the load is 1mOhm:

Since RL is practically a dead short, the voltage across the shunt resistor will be the full waveform. And since the shunt resistor voltage is amplified by an op-amp, there is a problem because the op-amp supply is +/-5V! (The input voltage should never exceed the opamp's supply range!)

But since I never allow the load to draw more than 500mA, the voltage across Rshunt should really never exceed 0.5V. But since the circuitry has a lag, there will be a short voltage spike on the op's input.

Now, how to solve this problem? I don't want to rely on my circuit behind the op.

One "quick and dirty" solution which came to my mind was using two diodes antiparallel like this:

The voltage across Rshunt should now never exceed +/- 0.7V which is fine by me, because 0.5V is the detection limit. (0.5V = 500mA through RL).

But is there a better way to "cut off" upper voltage limits? For example let's say I want to cut off an AC frequency to an upper limit of 5V and a under limit of -5V, is this possible with diodes too?

As said, 0.7V is okay but not perfect because it's very close to my 0.5V limit...

BTW: Here is a small illustration of what I meant:

(Left: input voltage, right: desired "clamped" voltage)

EDIT:

As two gave me also feedback on the "Overcurrent Protection" Subcircuit, I'll poste it here:

(UB+ of opamp here is 5V and UB- is 0V)

• Can you use Rshunt with a smaller value? – Nick Alexeev Jun 21 '15 at 21:34
• Let's say I can't. What do I do now? – d3L Jun 21 '15 at 22:10
• @d3l: divide voltages? – PlasmaHH Jun 21 '15 at 22:12
• How? RL is actually the load connected to my circuit, Rshunt is to determite the current flow through the load. – d3L Jun 21 '15 at 22:14
• Perhaps you could tell us more about your setup. What's the nature of the voltage source in your diagram? Why are you measuring the voltage drop? What are you trying to achieve? Without knowing that, I can only suggest to rethink this Rshunt of yours, and use smaller resistance there. – Nick Alexeev Jun 21 '15 at 22:33

As depicted, this could be a disaster (depending on how much current the AC source can deliver), because the 1 milli-Ohm resistor would cause the diodes to forward-conduct fully, so current is only limited by the AC source current and diode ratings. If the AC source is a tiny-current output (such as an oscillator or sensor) then this circuit should be ok, but the thing generating the signal could be damaged.

The solution as I see it is to omit the diodes and create a second voltage-divider off of Rshunt, such that a maximum (shorted RL) signal input would yield a maximum of +/-5v to the op-amp. You may want to make Rshunt a smaller value also, as the op-amp has gain already due to the 100k/25k. (If you need more gain, adjust these values.)

There is another issue, which the "Sense and Trip overcurrent-protection" may or may not address. That is that the op-amp output can vary from +5v to -5v (assuming it is a "true rail-to-rail" op amp. If an AC signal is applied to the op-amp, then an AC signal will come out - causing the relay to chatter on and off rapidly. Furthermore, an op-amp driving an inductive load directly (a solenoid) is probably not the most robust idea. So take that op-amp output, rectify it with a small signal diode, and charge up a small cap. Use this positive voltage to drive a darlington power transistor, MOSFET, or other suitable device to energize the relay. Make sure to use a fast diode anti-parallel to the coil, to prevent large inductive spikes when the coil is de-energized, from damaging the power switching device.

• I forgot to mention that there is a fuse in between the transformer and the load. Also, I attached the schematic for the "overcurrent protection" block. – d3L Jun 22 '15 at 21:18