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Using Resistor for Sensing Current for Shut Down Applications

In many sensitive electronic circuits or loads, limiting current beyond a set threshold could be one of the most crucial requirements for ensuring a fail proof safety for the various vulnerable devices and components. A simple method of implementing using current sensing resistors has been discussed in this article.

Integrating a Current Sensor Stage


The current from a power supply is mostly rated at much higher levels than the required safe value for a particular circuit under test or under operation.

Some high grade power supplies may include an adjustable feature for controlling current at their outputs, however normally we don't see this facility within ordinary or homemade units.

A simple current sensor configuration which could be built and used with sensitive circuits is shown below.

The resistor current sensor circuit utilizes just a single BJT and a few resistors.

Since most circuits would include an IC, or a configuration which would have a shutdown point somewhere within the circuit, this sensor module could be rigged with this "weak point" in order to shutdown or restrict the circuit from functioning in case the current intake rises above the danger mark.

How it Works


The functioning of the current sensing module can be understood as given under:

For circuits which may have a ground shutdown feature, the circuit using the NPN device could be employed as shown in figure on left.

Here Rx is selected such that a potential difference of about 0.6V is developed when the amp consumption at the output or by the load tends to go above the marked danger threshold.

The 0.6V is the optimal conduction voltage level for the shown BC547 or any low power general purpose BJT, thus as soon as this level is reached, the BJT conducts and grounds the available shutdown pin, switching of the supply to the load instantaneously, and the process keeps repeating at the threshold making sure the consumption is not allowed to exceed the set level.

Calculating the Sensing Resistor


Rx may be selected with the help of the following Ohms law, and as per the circuit requirement:

R = 0.6/I

The preset at the base may be used for fine-tuning the cut off region.

Circuit Diagram



For Positive Shut-Down Feed


In cases where the current sensor circuit facilitates a positive shutdown option, the current sensor circuit at the right could be used, which is exactly identical to its NPN counterpart with its functions, except the polarities which are configured to produce a positive drive for the shutdown pin connected across the collector of the BJT.

Now let's learn a few of the example circuits and ICs which facilitate a shut down feature within its allotted pinouts.

Integrating with IC 555


For IC 555, the reset pin#4, or the control pin#5 can be used as the shut down inputs, the NPN sensor above may be wired up here for the intended results.

Integrating with IC LM317/LM338/LM396


For LM317, LM338, LM350, LM396, the ADJ pin functions as the shutdown pin, again the NPN module would work with these devices for the desired current restrictions.

Integrating with IC 4017/IC4060


ICs such as 4060, 4017 may be stopped from functioning by applying a positive reset voltage at their pin#12 and pin#13 respectively. Thus for these examples the current sensor with the PNP device will suit perfectly for the required amp control.

Integrating with IC SG3525/IRS2453


Other examples include IC SG3524/5 = pin#10, positive trigger shutdown.

IRS2453 full bridge driver = pin#5, latched shutdown, positive trigger (2V)

For opamp protection circuits, the input pin which may be responsible for inhibiting the power device at its output may be targeted as the shut down pin and appropriately wired up with either of the above current sensing modules.

Need Help? Please leave a comment, I'll get back soon with a reply!




Comments

  1. Hello sir
    Pls how exactly will the preset fine tune the cut off region
    For example if using the PNP version
    At 220v 1500w we have 1500/220 = 6.8 approx..
    Now Rx = 0.6/6.8 = 0.08 ohm approx
    So assuming this is connected to the inverter and set up as per the calculations above then when this current exceeds 6.8A how does the preset known to sensitize the circuit.
    Basically how does the preset work.
    Also if I wanted a delay will just a capacitor at the base /emmiter of transistor work or do we use an RC time constant technique as I may want up to 5 secs max delay.

    ReplyDelete
  2. Hello sir
    Also one thing am bothered with is for the PNP version the power supply positive will be from Inverter output so won't this melt our resistor

    ReplyDelete
  3. hello sir,when i made the circuit pnp type i connected it to pin 10 in sg3524 and ground it with a 47k resistor, but it didnt work, at exceeded rate, it didnt shut down

    ReplyDelete
  4. atinuke, check the voltage across base/emitter of the transistor, it must reach 0.6V to conduct....and also measure the voltage at pin#10 of the IC, it must acquire 5V to trigger the shut down....

    ReplyDelete
  5. Hello sir Swagatam. I was looking for an overload protection circuit for an inverter circuit and you referred me to this post. It appears to be exactly what I need for the intended purpose which is to shut down an SG3525 i.c in an inverter circuit when overload is detected in the transfo output. I'll have to use the PNP version. Thank you very much sir. But I have some questions:
    1. Where should i connect the "to load positive" to? Is it to the inverter transfo 220V output?
    2. Where should i connect the "to power supply positive" to? Is it to the battery? If yes, can i also connect it to the output of a12V regulator incase I'm using one to power the driver session of the inverter?
    3. How can i choose the correct Rx value for a particular maximum load? Is the method used by mexzony in his comment correct?
    Anticipating your usual prompt response. Thank you sir.

    ReplyDelete
  6. Godson, the supply positive side must go the battery positive directly, and the other side to the inverter side positive, meaning the whole power to the inverter must pass through the sensing resistor, otherwise the circuit will fail to produce the intended results.

    the formula for the Rx is already provided in the article:

    0.6/shut down current

    ReplyDelete
  7. Alright sir. Thank you very much. I really appreciate you.

    ReplyDelete

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