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I need to detect when current being drawn exceeds 7.5a and then operate a relay to switch another circuit. Originally this was done using a large relay (1930's technology ) which is missing from the control box. I do not want to use microprocessors etc, just something nice and simple. Can anyone advise?

Cheers Chris

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  • \$\begingroup\$ You mean like with a shunt, amp, and comparator? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 15 '15 at 8:24
  • \$\begingroup\$ To give meanigful advice we need more information. For example what voltage level are we talking about? 24Vdc? 220Vac? What are the characteristics of the circuit to be switched (voltage? current? power?). Is the machinery in an industrial environment? Is it subject to vibration, dust, moisture? A schematic, if available, would be welcome too. \$\endgroup\$ – Lorenzo Donati supports Monica Aug 15 '15 at 10:26
  • \$\begingroup\$ The unit is a 32v dc generator. A relay was used to ddetermine when the line load exceeded 7.5 a, it closed contacts which initiated a start sequence for the motor. As components capable of dealing with 32v seem scarce, I may have to reduce the 'monitoring' system voltage down to either 12 or 24v dc and have a separate conversion to switch the 32v to start the engine. I don't want to monitor voltage alone as it's is connected to a sizeable battery bank of 1000ah cells. \$\endgroup\$ – Chris Aug 16 '15 at 8:33
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You can build a comparator and measure the current with shunt, it needs low voltage power supply, a voltage reference (zener diode) and comparator to compare voltage on shunt and reference voltage. You didn't mention if the current is AC or DC. I have seen in old machinery to have a current limit shutdown with relay and shunt. The resistance of relay's coil and shunt's resitance were choosen to activate a relay at certain current, not precisely but working. You can also buy a ready made product, look for Weidmueller monitoring module PAS CMR 4,5...10 A DC or similar

I would change the control logic to 24VDC as it is an industrial standard. Then you have the option to use many industrial components. For example Weidmuller MCZ SC 0..10V has two treshold levels, you need only a shunt that can be also used to hook up an ammeter for current monitoring, or you can leave the old shunt.

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  • \$\begingroup\$ Thanks. What you have seen on old machinery is what I need to replicate and the old coils are gone. It was made in 1930 and all records were destroyed in WW2.So other than some photos, I have no schematic. \$\endgroup\$ – Chris Aug 16 '15 at 8:36
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You can try the following simple configuration using BJTs: enter image description here

The current sensing resistor could be calculated using simple Ohm's law:

R = V/I

= 0.3 / 7.5 = 0.04 ohms

wattage will be = 0.3 x 7.5 = 2.25 watts

I have assumed 0.3V as the triggering voltage for the NPN in this example, please correct me if I am wrong.

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