Current sensor
Submitted: Wednesday, Mar 08, 2017 at 12:40
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qldcamper
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 ?
Reply By: Allan B (Sunshine Coast) - Wednesday, Mar 08, 2017 at 15:42
Wednesday, Mar 08, 2017 at 15:42
-
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.
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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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.
FollowupID:
879112
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
FollowupID:
879138
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.
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...
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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