Hello all,
I need a current sensing circuit solution for my application. I have a power board which has 5v line and 24v line. Each lines are going to two different loads. In load 1 which has 5v input i have an internal thermal switch which shuts it off when temperature increases. So, I need some hardware solution(current sensing switch / transistor/op-amp etc) to sense the current on 5v line when load 1 shuts off (which would be 0 Amp) and accordingly turn off load 2. I need some cheap solution by which i mean which doesn't require to ad a new additional board.

 

What is the normal 5v load current?
Max.

 

Welcome to AAC!
How much current do Loads 1 and 2 draw when on?
How much space available on your present board for additional circuitry?

 

What is the normal 5v load current?
Max.

Thanks for the reply Max, it is around 1.5 Amp.

 

Welcome to AAC!
How much current do Loads 1 and 2 draw when on?
How much space available on your present board for additional circuitry?

Load 1 draws around 1.5 Amp and load 2 is around 2 Amps.
Considering a square the area would be around 16 cm^2.

 

I just looked at a current sensor being used in another project. Might be useful to you as well?
https://www.sparkfun.com/datasheets/BreakoutBoards/0712.pdf

 

Would a relay work for you?
Relay coil in series with the 5volt load. No load, the relay drops out, 24v line connected to relay contacts, contacts open, 24v isolated.

 

You could experiment with the Honeywell SS400 sensors, using a simple hand wound low resistance electromagnet in series with the 5v load to trigger the SS400 sensor.
Max..

 

I would use an opto-coupler across the 5 volt load and connect the output transistor either to a relay or use it to turn off another transistor in series with the second load.

And by transistors…I mean MOSFETs.

 

I would use an opto-coupler across the 5 volt load and connect the output transistor either to a relay or use it to turn off another transistor in series with the second load.

And by transistors…I mean MOSFETs.

I was thinking along those lines but the OP mentions the thermal switch is internal to the load.
Max.

 

Considering the thermal switch is in the load I like the ACS 712 suggestion but the ACS 712 is being phased out making way for the ACS 723 series.

"These parts are in production but have been determined to be NOT FOR NEW DESIGN. This classification indicates that sale of this device is currently restricted to existing customer applications. The device should not be purchased for new design applications because obsolescence in the near future is probable. Samples are no longer available. Date of status change: June 5, 2017

Recommended Substitutions: For existing customer transition, and for new customers or new applications, use ACS723".

The ACS 723 has a few perks over it's predecessor. Like the ACS 712 the new chip is available on a breakout board from SparkFun and other distributors. The ACS723LLCTR-10AU-T2 is a 0 to 10 amp version. That gives a sensitivity of about 400 mV/Amp.

Ron

 

There is an ic just for this, can't remember the part name.. but you put a really low value resistor in series, maybe 0.1 or 0.25 ohm, and the ic detects the current based on voltage across the resistor. You could do the same with discreet parts, but this was purpose made for it.

 

You could experiment with the Honeywell SS400 sensors, using a simple hand wound low resistance electromagnet in series with the 5v load to trigger the SS400 sensor.
Max..

Or wrap a few turns of the wire feeding the load around a reed switch.

 

This is the IC I was thinking of, thee Texas Instruments INA195A current monitor:

https://www.digikey.com/product-detail/en/texas-instruments/INA195AIDBVT/296-17165-1-ND/665962

 

Or wrap a few turns of the wire feeding the load around a reed switch.

That is a slick trick which works well and I always manage to forget suggesting it.

Ron

 

Reloadron, yeah, thats works well. I used that trick to detect a o/c stop light on my car. Lamp current coil around reed switch, "calibrated " to switch on for initial cold lamp current then went out. If it didn't light up on brake application one globe was faulty.Worked a treat.