Make one circuit current track with another circuit current

[pratto]

I have a self operating chicken coop door out in the yard. It opens and closes based on light, and works fine. I have the 12v wall wart power supply in the house on my workbench, with about 50' of #18 wires going out there (+12v & Grd).

Along with the power supply I have a monitoring display with a fuse, an on/off switch, a digital volt meter, a digital am meter, and a Fuse lamp. I would like the fuse lamp to light up when there is high current draw in the coop so I will notice it. I said the system works fine, and it does, except when I try to incorporate the Fuse Lamp. I tested the FL with my bench DC power supply, and it drops 4v and passes 350mA, and lights up nicely (I guess that makes 11 ohms resistance). With the 4v Fuse Lamp voltage drop, the remaining 8v is in-sufficient to operate the door motor.

I tried numerous series and parallel arrangements, with no luck. I also tried figuring out how to incorporate a zener diode. The lamp was always on.

So I Youtubed some transistor lessons, and tried to make it work that way. Nope. (I modeled it in TINA). So I have decided to ask for help. HELP ! I have attached an image which shows the circuit I came up with. The top one has 47 mA draw in the chicken coop and a 47mA current thru the fuse lamp - it is off, great.

The lower diagram shows the coop drawing 329mA, and the fuse lamp still around 47mA - not lit up.

I just can't figure out how to make the base current more sensitive to the main line current. Image attached

 

[Alec_t]

Something like this perhaps?

 

[Harald Kapp]

Alec has a good and simple circuit there.

Another idea: wind some of the wire going to the coop around an iron core to create an electromagnet (use only one wire out of the wire pair leading to and from the coop, other wise the magnetic fields will cancel).
Use this electromagnet to activate a reed contact (as is e.g. often used in alarm systems to detect open windos or doors).
You can adjust the sensitivity (.e. the threshold above which the circuit will be activated) by the distance between iron core and reed contact and/or the number of windings used on the iron core.
Use the reed contact to control the lamp.

No fancy electronics involved at all. If you want to get started with electronics, go for Alec's circuit. It is simple and easy to build.

 

[pratto]

Alec & Harald -
ok. Excellent. I will do both and get back with you. Thanks.

 

[pratto]

Alec -
I set this up in TINA, and the results don't seem to be anywhere close to sending 350mA thru the Lamp (which I guess you used the LED to simulate). I must admit the LED has me confused, how can it possibly handle 350mA ?
Also, to make the Fuse Lamp light, it must see 4v.
I have attached an image of the circuit, or at least my understanding of your circuit.

What have I done wrong ?

 

[Bluejets]

I've used both methods described here on different occasions, with the only difference in the transistor switch being replacement of R1 in Alec's diagram with a diode.

 

[pratto]

Bluejets-
I replaced R1 with a diode, and it is still no where near working right. I have attached the circuit.
Maybe I should mention what a Fuse Lamp is. Back in the 60's, 70's & 80's, many radios had a lit up front frequency panel. They were lit with an incandescent bulb that glowed brighter as more current flowed thru it, just like a normal incandescent lightbulb in your reading lamp.

In my case, to achieve the desired brightness, i need 4v across it, which sends 350mA thru it.

I am beginning to think that you and Alec are trying to drive an LED light, not a Fuse Lamp.

 

[pratto]

Here is an image of two fuse lamps lit up.

 

[Alec_t]

I am beginning to think that you and Alec are trying to drive an LED light, not a Fuse Lamp.

Exactly so.
I'm puzzled as to why you want to waste 350mA just to indicate that the coop door motor is taking excessive current?
If you're really determined to do that, I can modify the circuit I posted.

 

[pratto]

Alec -
Exactly so. That is precisely what it is. As to wasting the energy, well it only amounts to less than a penny a month (it is only at 350mA for about 30 seconds twice a day).
As to why I want to do it this way, there you pose me. I seems silly I guess, but I like the way they look. And as you can see in one of the pictures I sent, I set up the control panel to use them. And they are just neat.

If you would go to the trouble of, well, figuring out what I haven't been able to, that would be very considerate of you. I still intend to mock up Harald's reed switch because he went to the trouble of offering to help me. But it really is an on/off switch, and doesn't actually track with the coop current. That was why I liked your approach. It seemed that you were using a transistor to, as I said in my original request, track with the current. This will mean different brigtnesses as it ramps up and down.

Thanks.

 

[Alec_t]

Ok. What are the lamp's voltage and current spec for full brightness? (4V, 350mA I presume are less than the maximum rated values ?)
I don't think you'll notice much ramping up and down in reality. Either the door motor will be off or drawing normal current (lamp off) or it will be stalled and drawing excess current (lamp fully on).

 

[pratto]

Each is rated for 12v & 300mA. But I am using them in parallel, so the 350mA is divided in two (approximately). I have cranked up the voltage and current to about 600mA (in the parallel configuration) and it works fine and is much brighter (too bright to look at).

As for the ramping up, that was just a little shorthand on my part to try and keep my messages from being books. Although there is a slight ramping up because of the control circuitry, you are right that it is minor. What I was really referring to was the fact that, when the door goes up, it is fighting against gravity and the load is 350mA, but when the door is going down, it only uses about 250mA. And these different currents will make the fuse lamps light up at different intensities.

 

[pratto]

Alec -
I did want to say that, if making the thing track current is difficult, then just a transistor controlled on/off would be just fine.

 

[Alec_t]

Well, something simple like this could track the current over a limited range (0.3A to 0.6A in the sim)

..... but there would be a lot of wasted power in the transistor (up to ~3W)(blue curve) and R2 (up to ~7W)(red curve). Personally, I'd use on/off switching of the lamps instead.

 

[pratto]

yeah, I think so too. You mean like an emitter follower switch ?

 

[pratto]

I don't seem to have the hang of this. How do I make a transistor switch work for this use ?

 

[Alec_t]

Here's what I'd try, with the lamps connected in series to reduce the total current draw. When the lamps are on, there is about 6V across each (providing they are reasonably matched).

The curves show the sharp turn-on motor current thresholds of the lamps for R1 values of 1Ω to 2.2Ω respectively. (The higher the resistor value the lower the threshold). My lamp simulation model allows for thermal inertia, which is why the lamp current drops back from an initial peak at switch-on.

 

[pratto]

Alec -
I couldn't get it to work until I made R2 3ohms, then hey presto, it worked. I was unable to use the same mosfet as you did, but I guess it was close enough.

I am attaching one image with the coop drawing 320mA and the Lamp drawing 161mA (bright enough). After hours of screwing around with two fuse lamps in series & parallel, I have decided to go with one. It works, and that was the goal.
The second image is with a coop draw of 47mA and 7uA thru the lamp. It is OFF. good.

Thanks very much.

 

[Alec_t]

Good to know it works for you, even with a 3Ω resistor dropping significant voltage. I was under the impression that the lamp was supposed to indicate an over-current fault, rather than normal running current. That's why the max value of R1 in the sim was 2.2Ω.

 

[pratto]

yeah, I'm not the best 'splainer. Once again, Gracias Senor.
Now comes the actual build. I'm kind of dreading it, cause with very finicky values, the tolerances can be a real problem.