Variable circuit breaker

[YaranAyr]

I am looking to build or buy a variable circuit breaker. What I would like is a circuit breaker box that plugs into the wall socket and then a devise under test could be plugged into the box. I'd like the break open amperage to vary from, say, 0.5 Amps to 40 Amps - as wide of range as practically possible. DC as well as AC would be great too.

The idea is that while working on/troubleshooting a device, a breaker or relay would open rather than eating up a bunch of fuses.

Maybe just a bank of different sized breakers would be a good "farmer" solution. But a cool box with a knob and a relay sounds so much more exotic.

Thank you

 

[crutschow]

I am looking to build or buy a variable circuit breaker. What I would like is a circuit breaker box that plugs into the wall socket and then a devise under test could be plugged into the box. I'd like the break open amperage to vary from, say, 0.5 Amps to 40 Amps - as wide of range as practically possible. DC as well as AC would be great too.

The idea is that while working on/troubleshooting a device, a breaker or relay would open rather than eating up a bunch of fuses.

Maybe just a bank of different sized breakers would be a good "farmer" solution. But a cool box with a knob and a relay sounds so much more exotic.

Thank you

So is there a question there?

Sounds like you just build it as described in you last sentence, if you like the exotic. Just requires a switch, some relays to select the desired breaker value.

But I prefer the simple "farmer" approach is as described in you second to last sentence. Just wire all the breakers in parallel and turn on the one with the current value you need.

In either case you want to use standard circuit breakers. You don't want to try to build your own.

 

[Reloadron]

What I would like is a circuit breaker box that plugs into the wall socket and then a devise under test could be plugged into the box. I'd like the break open amperage to vary from, say, 0.5 Amps to 40 Amps - as wide of range as practically possible. DC as well as AC would be great too.

Where are you located that you have 40 amp wall outlet service?

Personally I would use the "farmer" approach myself. Snag a small breaker box and stuff it with breakers. Not sure you will find breakers in the low ranges, but worth a lookie.

Option 1 could be done but would carry a cost. You could monitor the AC current placing your load on a contactor. Set the contactor to drop out at a desired current.

Ron

 

[BrownOut]

If you're doing some bench trouble shooting, then your electronics knowledge should be pretty sound. If so, you can build a solution. Actually, I've thought about the same thing. If you are interested, I can try to hammer out a schematic later when I get the time. The circuit I have in mind would be of medium complixity. I'd like to have exactly what you're describing.

 

[KeepItSimpleStupid]

I believe there was a Nuts & Volts article do do just that espacially for refrigeration troubleshooting. I don't remember if it included a start-up delay. I also think there is an Electronic Circuit Breaker IC/

One thing does need to be known though; Range of voltages e.g. 24 VAC to 277 VAC +-10%
Covering from say 100 mA to 20A with one box makes some sense. 2 wire? 4 wire? Lotsof unknowns.

20 to 40 A AC may be harder. For the latter, I'd consider something like a 40 A shunt trip breaker and a current measuring device that trips the breaker. It's hard to guess a use for such a device. Wire leads? Trip the neutral if 120V

DC, the same holds. Are you looking at say 3V to 700 VDC or what? Current range?

Purpose? Portable? Resolution?

Just asking.

On my bench I use a 3A variac with an isolation transformer and analog current and voltage meters.
The ODD part of my setup is I have binding posts available and I have a 15A isolation transformer. The gizmo has a center off switch that allows OFF, 3A variable and 10A continuous. There is a Front Panel Fuse as well.

 

[YaranAyr]

Okay. Contactors, variacs, isolation transformers, parallel breakers... Cool. Sounding good. Thanks everyone.
And BROWNOUT, I'd LOVE to see any schematic ideas. Anythings got to be better than the road my head was going.
To start, this Breaker Box would be for bench testing 110 VAC devices in the home shop - TV's, computers, power tools, etc. So yeah, right, it's going to be on the 20, 30, or whatever amp breaker anyway... so 0.5A to 30A.
As for DC, more of the <=24 VDC range - Battery-op and Automotive-style stuff. <=30A.

 

[YaranAyr]

BROWNOUT, I'd LOVE to see any schematic ideas. Anythings got to be better than the road my head was going. Good to see that someone else has a similar "crazy" notion.

 

[BrownOut]

My time for doing schematics is limited, but I'll try to cobble something together this week, just so you can look and see if it's something you want to tackle.

 

[Reloadron]

The more I think about doing something like this, per the original post, I am going to reverse my thinking as to the "farm" approach.

Looking at this from a US standpoint where typical household mains are 120 volt (derived from 240 volt split phase) and the typical service outlets (wall outlets) are 15 or 20 amp service. You can buy a truckload of 15 or 20 amp circuit breakers for under $4.00 each. Lowes, Home Depot or any home improvement store will sell you all you could want. Just like the pages of them that can be found here. However, when you want circuit breakers that are low current starting with .5 amp all bets are off and it is a new ball game. Now we get into breakers like those found here. They average $20 a pop or more. Therefore if you want 10 EA for 10 current ranges figure on $200 plus. Hey, the bright side is they are DIN Rail mounted.

Constantly switching breakers in and out would also be a pain. Plus forgetting to switch one out when changing current ranges could be a mess in the happening. Something else to consider is when we get above 20 amps then we start looking at the wire gauges as well as plugs and sockets. You want 30 Amps? Then we have AWG 10, you want 40 Amps, now we get to AWG 8. Not to mention what the mains breaker is out there somewhere. There is no escaping the wire sizes needed.

Here in the US I would come in with a 20 Amp GFCI circuit breaker. I would monitor line current and voltage. I would monitor the current using a hall effect sensor and convert my 0 to 20 amps to 4 to 20 mA and run that through a 500 Ohm resistor giving me 0 to 20 amps scaled to 2 to 10 volts. I would build a simple comparator circuit with a measured and variable reference voltage. Use a scalable meter to monitor and set the reference so it displays your trip current. The output of the comparator drives a contactor. When a current exceeds the preset limit the contactor drops out and stays out requiring a manual reset. I would use a contactor rather SST to avoid the leakage current of a SSR.

Anyway, I would go about it something like that, give or take a few meters.

Ron

 

[KeepItSimpleStupid]

yep, limiting to 20 A, 120 V makes sense because that's what your limited to anyway.

You do have to program in a time delay.

But, all of us seasoned people have used an incadesent lamp in series with the load. It's actually a current dependent resistor, but who knows how long they will still be available. The bulb size limits the max wattage the system will draw. Want to limit a motor to 100W, use a 100 W light bulb.

I believe the system I saw in N&V used a PIC microcontroller.

Isolation is essential these days unless all of your instruments are battery powered.

 

[Reloadron]

Good point Kiss, need a delay in there or every time we get inrush exceeding the limit it trips. Damn picky little details.

OK, we shove a PIC in there. May as well let it do some thinking and delay.

Ron

 

[YaranAyr]

Great ideas and info. Thanks for those breakers; I don't think I would have found them.

 

[kg8gp]

Would Something like this work?

Maybe instead of using a voltage divider make selectable taps on current transformer secondary. or maybe both for fine tuning?

 

[BrownOut]

Here is the idea I had. The posted circuit needs some refinement, but I post it here so you know what might be involved. There are lots of different ways to do this, and I make no claim this solution does it with the fewest parts. Also, please note that U4 is a traic, and not an SCR. I used the SCR symbol because I still don't have a good triac model. I'm working on that.

In the diagram, R3 represents a load that will be plugged into the unit. A voltage that is proportional to the current drawn by the load appears across D1 and R1, which is amplifed and buffered by Q1. That signal is compared against a variable reference by U2, and when the current is higher than the preset value, U3 fires, which in turn inhibits U4 from passing current to the load.

The trip point is set by adjusting R7 and/or R9. The circuit is reset by cycling power. A switch can momentarily disconnect V2 to reset the circuit. Also, V2 can be generated directly from the line voltage, since it's not isolated.

This circuit, if built, is of course very dangerous, and will need to be housed in a totally electrically insulated enclosure.


I'm working on getting a triac model and refining the circuit to achieve the required amperage range. This is just a concept at this point. Suggestions are welcome.

EDIT: One weakness of this simple solution is that the negavtive half cycles aren't being monitored, and so the device bring protected might experience the full force of the negative half cycle of power before the circuit kicks in. This can be easily solved with the addition of a few more parts.

 

[YaranAyr]

Wonderful stuff guys. All I could think of was: Comparitor and Relay. But I would have never gotten these ideas done, especially since I don't have any SCR or Triac experience to go with it. My Dad just found this schematic somewhere. You can see what you can take from it, too.

I'll be transitioning into the parts acquisition and development stage. Still I'd love to see any developments from you all. Meanwhile, I guess, do you have any suggestions for SCR, Triac, Transistor, etc. values or numbers that I should have on hand to start with?

 

[Reloadron]

The posted circuit would work fine but is a far cry from your original post:

If S1 is open, the range is
approximately 300-650 mA, and 0.8-2A when it is closed.

Two current ranges available 300 to 650 mA and 800 mA to 2 amp. Pretty long way from your original plan that involved much higher current levels.

I think what you need to do is sit down and give some thought to current ranges. I am assuming your mains voltage is 220 volts. Your initial post had currents of as high as 40 amps listed. I commented on wire gauge required alone.

If your mains voltage is 220/240 VAC I think anything above 10 amps is likely overkill. If your mains were 120 VAC I would suggest 20 amp. Once you have a good idea of what you want then it becomes easy to build on things. Additionally the budget plays into things. Really cool whistles and bells like meters are nice but are they needed? Extras come with a price tag.

Ron

 

[BrownOut]

I think we've decided on 110V and .5A - 20A ( see post #6 & #10 ) The circuit I posted should allow .5V - 10A as it stands. With some tuning, it should easily allow .5A - 20A or so. Any more current would require special outlets, heavy gauge wire, etc. A circuit could be designed for those specs, but would it really be needed??? I'll try to update the design as my limited time allows.

PS: Something I like about the last circuit posted is the "snubless" triac, and I think we should use that in our design, if it is rated for our target current. Designing a snubber for every possible load would be a near impossible task, so using such a device would be a tremendous benefit.

 

[BrownOut]

FWIW, I'm still working on this as time allows. I'm adding a transformerless power supply to replace the DC "source" in my schematic, and improving the range. How you aren't in a hurry.

 

[YaranAyr]

Still on chill. Ditto entry #15.

 

[DJDAudio]

I know this is an old thread but this is the thread that got me to join up here after a google search.

I am working on a very similar project,

The start,
**broken link removed**

And here is a custom built stacked Variac for the project,

**broken link removed**

0120V 40Amp

and here is the 0-40amp AC/DC gauge I setup for it,

**broken link removed**

I plan to use a large Variac as a variable current limiting device. I will wire it with wiper arm closed back on itself. So at full open the induction of the coil will prevent any current from passing down the line. And with it fully closed, it is a dead short bypassing the coil, and full current can pass, this Variac is rated at 20amp so I will install a resistive load across the wiper and mains, that will limit the load on the variac to a safe level.

I will start up my own thread when I get more into the construction of it.