current relay idea

i have an idea of how to build a current relay, which closes a contact after current falls below a preset measurement/reading/rating. i have another thread about current clamps, https://www.electro-tech-online.com...asuring-device.35634/?highlight=current+clamp

which i can use with a under voltage indicator (like the one with a dual colour led used in cars to tell you if voltage falls below a certain level).

i want to use this as a headlight fault detection unit.
a 100w headlamp globe draws 8.3 amps at 12v. the supply to the headlamp is monitored by the ferrite clamp, which sends a constant signal to the voltage indicator. when the current flowing in the wire drops below 8.3 amps, the clamp, output voltage from the clamp drops, and the voltage indicator reads this as under voltage, and applies voltage to a relay, which controls an internal indicator, turning it on when the current falls below 8.3 amps, telling you there is a headlamp fault.

is this is an ok circuit/idea i could use? i just thought about it and it seemed possible.

it's a expensive and a unlogic way to see if your headlamp is broken (which you would notice anyway because it gives less or no light)

It is sort of like a "wind screen wiper not moving" indicator.

bunghole,

I'm not sure I understand...if you are thinking of using "ferrite clamp" as a current transformer, it won't work. Current transformers require AC...headlights are DC...at least in a car, that is. The clamp-on ammeters that read DC use a Hall-effect sensor.

One technique I saw in the long-ago past was to wrap several turns of heavy gauge wire around a glass reed switch, and put the wire in series with the headlight. When current was high the reed contacts were held closed...no current and the reeds opened. So...a resistor between +12v and one end of the reed, the other end of the reed to chassis ground, and an LED across the reed. Headlight-good...high current...reed closed and LED is off (shorted-out). Headlight-bad...no current...reed open and LED is on (in series with dropping resistor).

I haven't tried this, but remember it from an electronics magazine.

Ken

To measure the headlight current try measuring the voltage lost in the wires going to the headlights. I think there may be ¼ of a volt drop in the wires.

I have a current meter in the dash that measures the charging current by monitoring the voltage drop in the battery cable.

Current Relays

Years ago I used current relays in my big broadcast transmitters. They monitored current in vitals places.

If you have a relay who’s coil has 3000 turns of wire and closes with 10mA then I believe if it had only 3 turns of wire it will take 10amps to close the relay. So 6 turns will give you a 5 amp coil.

Relays have a pull-in and a release current. If you had a 5 amp coil that will close indicating your 8.3 amp headlight is good then the current will have to drop to 2 amps for the coil to open up.

You should use a hall-effect sensor.

if i put it simpler, it's a form of relay that measures current flowing through a wire. when the current falls below the custom-set level, a contact closes. this method is used in real-life. In my case, the contact is open at 8.3 amps and closes at less than 8.3 amps, illuminating a fault light. the diagram should help explain.

I made a current relay a few years ago to measure the current taken by the fog light on a trailer. That turned off the fog lights on the car, so that the driver didn't get dazzled by reflection from the trailer. Also doubled as a blown bulb detector for the trailer foglight.

I unwound a standard automotive relay, having measured its resistance. I then rewound the relay with the same number of amp-turns. It worked fine.

However, modern high side Mosfet switches like these:-

https://www.electro-tech-online.com/custompdfs/2008/02/ips521.pdf

have short circuit, and open load detection, so it will tell you if the bulb is blown.

There are current relays.

Relays work by current not voltage. (amp turns) I know some one will argue they work by voltage. “12 volt coil” It is amp turns.

The current (voltage) it takes to close a relay is not the same as the current (voltage) to open the relay. If a relay is rated to pull-in at 8 volts it will probably hold closed down below 4 volts. I know you want a relay to close at 8.3 and open when there is less than 8.3. A relay is not going to work like that. Lets say you found (made) a relay that trips at exactly 8.3. If you start out with 0 current and slowly ramp upwards the relay will close at 8.3 and remain closed above that. When you start reducing the current from 10 amps downward the relay will be closed at 10,9,8,7,6 5,and 4 amps and may open up at 3.5 amps and below.

I do not know of any mechanical relay that will be open below 8.3 and closed above 8.3. There are relays that will be open below 3.5. Closed above 8.3. Between 3.5 and 8.3 could be open or closed. (will not change state, will hold where it is)

It doesn't matter much for this application if there is a big gap between the turn on and the turn off voltage.

Headlights take lots of current on turn-on, enough to operate a current relay even if the relay needs a lot more than 8.3 A to turn on.

When a bulb blows, it almost always drops to no current at all, so any relay will drop out.

If there is some reason that exactly 8.3 A is important, then a relay would be useless. However, the current in a 100 W 12 V bulb will fluctuate around 8.3 A depending on temperature, age of the bulb, and supply voltage. The voltage on a car battery varies a lot, so the current could be as low as 4 or 5A and the bulb could still be OK.

i'll make it 'contact open when current flowing through, contact closed when no or 0 current flows through' yes?