Electric/electronic Circuit Breaker

[Stinky]

Can anyone tell me of a source for an electronicly controlled circuit breaker? I would like to have it for 120 volts alternating current, adjustable from approx. 100 mA to 15 amps. It should open at the preset current -- not like a fuse... which can operate (typically) at a significantly greater current than its so-called rating. I am thinking of building one and using a GFCI as part of the project. Any answers on this and/or advice will be greatly appreciated.

[MrAl]

Hi there,

How fast do you need it to open?
If you dont need speed, perhaps a triac and comparator circuit.

[Robert Reed]

Start with a current sense resistor of low value and moderate wattage. Don't have time now to compute, but its value, wattage will be based on adequate developed voltage at 100 ma and wattage at 15 amps. These are commonly available in the milliohm range (10 to 1000) and 1 watt thru 10 watts any where from 1% -3% tolerance and they are not terribly expensive. feed this output into an Op-Amp differential input. Give it suitable gain, then rectify and filter for peak DC.From there input this into a comparator, the out put of which will drive an opto coupled relay for tripping the circuit. The trip setting can be a pot placed almost any where in the circuit. This can be calibrated while the project is still on the test bench. Also you could use opto isolators in the sensing circuit for isolation. Oh-Oh - the more I type, the more circuit schemes keep popping into my mind -- better quit for now!

[Chris Smith]

For he experimenter.... the FET and feed back work really fine.

[dyarker]

Repeating MrAl's question, "How fast do you need it to open?"

That is important because many loads (like motors, power supplies with large capacitors, even incandescent lights) have a larger startup current than their max running current. Do you want a small time delay (1/10 to 10 Seconds) to allow for startup?

If the trip point is to be higher than startup, "how fast" is still a question. Triacs/SCRs won't turn off till zero crossing of the current (up to 8.3mS later). Fast enough?

Faster still rules out Triacs/SCRs. It can be done with transistors (bipolar or FET), but tripping mid-cycle means taking care of kick back voltage from inductive loads.

Do you have a particular load you're trying to protect? That will influence the answers you get.

Cheers,

[L. Daniel Rosa]

That's a moderately wide range. A current sense resistor may give you some grief at the extremes. You can probably get a sample hall effect current sensor from http://www.allegromicro.com/ .

[haklesup]

For AC current, A current transformer around the wire and a rectifier are easy and require no direct contact to the output. Though the wide range might justify 2 sense transformers and something to switch ranges. The voltage you sense at 100mA will be far smaller than at 15A. Using 1V/A as an example, you would need a comparitor capable of switching from 100mV to 15V. It's possible but you'll sacrifice precision if you do it all in one range. You also need a seperate 20VDC supply. To do this with a line powered device, you would need to sap the power before the trip switch so that it stays powered up.

[Chris Smith]

They make small signal fets and large power fets, all of which are capable of sensing the over all current as small or as large as you require.

Using a single low to high amperage out put sensetivity with out a selector nob to set all of your thresholds would be silly, as you don’t want to make something extremely sensitive at 100 MA’s, as well as heavy duty at 15 amps. Too big of a swing.

A selector switch with separate ranges and different trigger fets, set at these GENERAL presets is the way to go.

A tiny signal fet for the Milii amps, a small fet for the one amp range, a larger one for the heavier amps and so on, all set in motion using a simple rotary switch.

To get a infinite adjustability you add in a pot to the input [s] of the fets, mark it, and the added load will vary the out put trip load.

From memory, N&Vs had the rest on the Fet as a trip breaker.

[L. Daniel Rosa]

Okay, the PN is ACS712ELCTR-20A-T . It's good to 20A, with a sensitivity of 100mV/A, isolation pretty darn good.

It'll give you quick response and a range that a 10 bit ADC will take care of (assuming you'll be using a microcontroller).

The suggestion of a current transformer reminds me that I have a few spares kicking around- 1mA/A.

[Stinky]

Okay, the PN is ACS712ELCTR-20A-T . It's good to 20A, with a sensitivity of 100mV/A, isolation pretty darn good.

It'll give you quick response and a range that a 10 bit ADC will take care of (assuming you'll be using a microcontroller).

The suggestion of a current transformer reminds me that I have a few spares kicking around- 1mA/A.

Daniel -- Thanks for the reply. This is my first time in a site like this so my ignorance will probably show. You gave me a PN # but what company and/or catalog? I have been looking for a device (already built) like this for some time. I do build, in fact lots in the past.... but in the last few years I don't have the time. I am thrilled with the answers, though! I don't understand the "emoticons" .. HA HA!

[jimandy]

Actually Daniel gave the company in an earlier post in this thread -
http://www.allegromicro.com/

[danbob]

radio-electronics Dec 1991 has just what you want. it is a article about building a 1/10 to 10 amp ele. fuse. with schematic and printed circuit board lay out. most libraries subscribe to places that have these magazines in pdf format so you should be able to get a copy through your library. I started and got as far as winding the transformer ( about 90 turns on a toroidal core) and have as yet to get back to it. I would send you a copy of the file but I can not find it on the computer.

[jimandy]

Per following the Bham AL Library should have it and I can copy if wanted.

Record 2 of 2
Record: Prev Next
Title Radio-electronics.
Publisher New York : Gernsback Publications, 1948-1992.

Identity MICROFORM
Location Cen BST Micro
LIB. HAS MICROFILM:v.48 (1977)-v.49 (1978)MICROFICHE:v.50 (1979)-v.63 (1992)