Current sensor

Submitted: Wednesday, Mar 08, 2017 at 12:40
ThreadID: 134435 Views:2735 Replies:7 FollowUps:17
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Hi again guys,

I need to install a fan to help with ventilation to my fridge in my ute tub.

Looking at using an Ua 741 op amp with a couple of thermisters to sence when the fridge compartment is above the temp of its surrounds, easy enough.

But then i thought to maybe use the voltage drop across the fuse as the input of the op amp so the fans only turn on when the compressor in the fridge is running.

Question is this, how little a difference across the + and - inputs is required to force the output to change states ?
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Reply By: Malcom M - Wednesday, Mar 08, 2017 at 14:32

Wednesday, Mar 08, 2017 at 14:32
Measuring the drop across the fuse is a real bad idea as the fuse could blow tearing the chip apart.
The 741 is also a very old design so unless you have a few kicking around your shed from 35 years ago, I'd be looking at a modern dedicated current sense chip.
Something like a Maxim MAX4172.
Forget the fuse idea and put in a current shunt such as this one.
AnswerID: 609263

Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 14:51

Wednesday, Mar 08, 2017 at 14:51
your correct about having one laying about, hence the reason i was originally going to use one with the thermisters.

If the fuse blows..... specs say it can handle 15 volts.

I did look at the chips similar to what you suggest, but their output isnt sufficient to run a fan so decided to go with what i got with an output transistor.

Thanks for the suggestion, but any idea of the answer to my original question?

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FollowupID: 879095

Reply By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 15:42

Wednesday, Mar 08, 2017 at 15:42
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It depends on just which 741 you select. Maker, model etc.
Then it depends on each one you pick up..... the manufacturing spread.

Generally, 741's may require only a few microvolts to obtain a null output, or it can be up to 10 millivolts. Then from null, the differential for max output would again depend on brand, model, individual performance etc. Temperature will also have an effect.

On top of all that, the 741 requires a balanced supply voltage, e.g. plus 12v and minus 12v, not plus 12v and zero which is what your vehicle will offer.

Sensing volt-drop across a fuse would also be unreliable for a number of reasons.

An op amp could be used in a current-sensing (mV from shunt) circuit together with a number of other components but a 741 would not be a practical way to do it. If you need to be told that then I would question your ability to design and construct a reliable circuit.
Cheers
Allan

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AnswerID: 609265

Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 16:17

Wednesday, Mar 08, 2017 at 16:17
Your right, i have no formal training in this type of electronics, it is just a hobby hence the reason i am asking, what you might see as a silly question.

Started when i was fresh out of school and couldnt get anyone to repair my CB radios at a price i could afford back then so had to learn to repair them myself. I found it a challenge as the years went on because there were no replacement parts available so i had to study winding books and such to make replacement stages.

I have always learned from the old retired guys as there was no internet back then, but i am fast approaching that status myself.

I was just starting to play with op amps a few (10) years back when my life took a big change and i lost the time i had to play.

So your saying that the Vss and Vdd isnt just 12 volts as i interpreted the data sheet to mean. And please take this as a question not a challange to your advice, im still learning.
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FollowupID: 879100

Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 17:13

Wednesday, Mar 08, 2017 at 17:13
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The internet abounds with info and applications for op amps, LM741 included.
Take a look at TI's datasheet as an example. Although it calls for a 'balanced' supply, operation of some applications may be achieved with a single-sided supply although you may have trouble cancelling out the 'offset' when dealing with small value dc inputs as you intend.
Another site of interest is here.

Try Googling "current sensing relay circuits" although I still do not think this is the best way forward.
Cheers
Allan

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Follow Up By: vk1dx - Thursday, Mar 09, 2017 at 02:40

Thursday, Mar 09, 2017 at 02:40
Now your stirring up memories. I used heaps of the 741's to detect when a train was on a section of track. Ahhhh Allan.https://www.youtube.com/watch?v=SRvVLC9CfGM

I will get shot for this as it's OT. It's 2:38 AM and my brian has finally settled down. Back to bed. Chemo brain is not a rumour.

Phil
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Reply By: Member - Jack - Wednesday, Mar 08, 2017 at 16:22

Wednesday, Mar 08, 2017 at 16:22
Hey qldcamper ...

You may be the very first person to call Al an 'old retired guy', but from now on I don't think you will be the last :)

The hurrieder I go, the behinder I get. (Lewis Carroll-Alice In Wonderland)

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Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 16:31

Wednesday, Mar 08, 2017 at 16:31
Thanks for pointing that out lol, but i didnt actually call him one....directly, just informing him where i found a wealth of knowlege.

Hope he has a sense of humour...... he does doesnt he?
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 16:39

Wednesday, Mar 08, 2017 at 16:39
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Thanks Jack....... I think!

But I AM an "old retired guy". From some comments I have made in past posts you would have a fair handle on it.
Just so there is no doubt, in May of this year I will celebrate my 85th birthday. You might say that I am one of the 'Elders" on this forum.

And I have been foolin' with electrics since the batteries and torch globes when I was aged 4 years. So I might say that "I have seen it all", but it is questionable how much of it I can remember! lol

Sense of humour? Mate, with my face I have to have one!


Cheers
Allan

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Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 17:11

Wednesday, Mar 08, 2017 at 17:11
Ha, the torch batteries and bulbs and a length of bell wire, some of my earliest memories..... having molten yellow plastic from the bell wire stuck to my fingers.
Dad was always looking for the torch parts.

I bet you had one of those 100 in one electronics boards from tricky dickys too. Or maybe not, you got a few years on me so you were probably in formal training before those things hit the market. Come to think about it, you would have completed your apprenticeship by then.

I would have loved to get a start as an electronics engineer but life had a different rout planned for me so it has been just a hobby.
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Reply By: Member - Milton477 - Wednesday, Mar 08, 2017 at 16:37

Wednesday, Mar 08, 2017 at 16:37
I have wired a 12V relay in parallel with the fridge's own fan. The relay then drives the external ventilation fan.
AnswerID: 609267

Follow Up By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 16:59

Wednesday, Mar 08, 2017 at 16:59
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I must say that this is the way I would go if I had need. Using a plug & socket on the fridge would allow easy disconnection of the external fan if removing the fridge from the vehicle.
Cheers
Allan

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Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 17:16

Wednesday, Mar 08, 2017 at 17:16
That would be too easy, honestly where is the challenge in that?

Ok honestly honestly this time, the fridge is in there to stay, major job to get it out to remove the covers to do it the easy, logical way.
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Follow Up By: Zippo - Wednesday, Mar 08, 2017 at 18:40

Wednesday, Mar 08, 2017 at 18:40
If you're looking at an in-line current sensing method, I made up a simple current-operated relay for a monitoring task. About five turns of copper wire wound around a reed switch, it operates at ~1A. Obviously the average reed switch isn't going to drive your fan, but the in-line volt drop of the coil is insignificant and it could drive the relay/transistor-switch of your choice. Have used it in-line with my Engel to allow logging of duty cycle.
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FollowupID: 879120

Follow Up By: qldcamper - Wednesday, Mar 08, 2017 at 20:06

Wednesday, Mar 08, 2017 at 20:06
I have tried a reed switch but not like that, i put 6 turns of copper cable round a core which developed some flux but not enough to operate the reed switch. The reed switch is rated at 500 Ma so will operate the fan at 0.22A so may be a larger switch but looks like similar. I will give it a try in that format tomorrow.
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Follow Up By: Al-one - Wednesday, Mar 08, 2017 at 20:43

Wednesday, Mar 08, 2017 at 20:43
Forget the electronics and the reed switches. Milton has the right idea using the condenser fan power to drive a relay which in turn can be used to control another fan with 12 volts sourced from the van battery.
Cheers,
Al-one
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FollowupID: 879132

Follow Up By: Zippo - Wednesday, Mar 08, 2017 at 22:28

Wednesday, Mar 08, 2017 at 22:28
Al, the O/P indicated it would be a major job to access the internals. That's why I suggested the non-invasive in-line sensing approach. In a different situation, I'd probably go with a different approach too.
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FollowupID: 879135

Reply By: Member - Boobook - Thursday, Mar 09, 2017 at 05:53

Thursday, Mar 09, 2017 at 05:53
Try these

ebay differential voltage sensor

They have the circuit diagram on a link. There are lots of similar and cheaper ones on ebay. Easier, cheaper, quicker and more reliable than making your own breadboard.

Use a couple of thermistors in series with resistors for the input voltages.

AnswerID: 609278

Follow Up By: qldcamper - Thursday, Mar 09, 2017 at 07:56

Thursday, Mar 09, 2017 at 07:56
Thanks for that Tony,

Pretty much along the same lines as what i am trying to achieve with the 741, just without all the protection and filters to cover many different applications.

It is a learning curve for me as well, I see you are an amateur so you would understand the challenge of doing something yourself rather than just buying what you need. How many days....weeks, maybe months have you spent playing (fighting) with antennas that you could have just bought on ebay?
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FollowupID: 879140

Reply By: Member - Jim B8 - Thursday, Mar 09, 2017 at 06:26

Thursday, Mar 09, 2017 at 06:26
In answer to your question, the sensing voltage is dependant in the amplification ratio that you set up
In answer to your problem, wire the fan to the wire feeding the compressor and have the fan run every time the compressor runs, if chosen correctly, the fan will draw very little current and will help the efficiency regardless of temp.
I use 2 fans on my fridges, condensor and evaporator, and have no problems with excess current draw
I used to use an AD590 to sense temperature, probably not made any more, And for a different application. Very accurate. Very linear, but needs sdditional circuit to work
I suggest that your cure may be over done a bit, and it needs simplifying. But sounds like fun
.
AnswerID: 609279

Follow Up By: qldcamper - Thursday, Mar 09, 2017 at 08:14

Thursday, Mar 09, 2017 at 08:14
Thanks for the suggestion but if you read the thread that has been covered a couple of times, had i realized the need for such a fan before i mounted the fridge permanently to the slider and bolted the slider to the floor of the tub with bolts in very difficult to get at places i would have made provisions for the painfully obvious practical way of doing it.

The fridge works very well as it is but the outer casing gets very hot so the poor thing must be struggling and using up a lot more power than it needs to be, i figure if a fan or maybe two shortens the compressor run time by 1 hour a day and reduces the hi side pressures, then i will be miles in front.




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FollowupID: 879141

Follow Up By: Malcom M - Thursday, Mar 09, 2017 at 08:50

Thursday, Mar 09, 2017 at 08:50
Apart from the obvious problem that you want to build your circuit regardless of suitability, you could look at this solution.
You say you can detect heat by hand so place a thermistor against the external case...
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FollowupID: 879142

Follow Up By: qldcamper - Thursday, Mar 09, 2017 at 09:49

Thursday, Mar 09, 2017 at 09:49
Thanks again Malcom,
It gets rather warm in the tub when the cover is on regardless if the fridge is running, that is why, after no success with a reed switch, i was originally going to use 2 thermisters to set the inverting and non inverting inputs of an op amp (as mentioned in my original post)making it so the fridge compartment would have to be hotter than its surrounds, if only one temp sensor was used the fans would come on every time i parked the ute in the sun weather the fridge is turned on or not. There are many commercially available devices that would serve that purpose.
I have put a bit of thought into this and ruled out some of the more obvious solutions.
Looks like the simple reed switch may be a goer with the suggested configuration by another member, i just have to pick up a bit of winding wire to give it a try.
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FollowupID: 879146

Reply By: noggins - Thursday, Mar 09, 2017 at 10:35

Thursday, Mar 09, 2017 at 10:35
Why not do it an even simpler way and mount a very small solar panel to the ute and use it to drive a magnetic hub fan .
I did this in the tub of my Ranger and I recon I cut the power usage by 50% .
At night it wont work obviously , but there's no sun to heat it up either.

Ron
AnswerID: 609283

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