Current sensor

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?

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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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.

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

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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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!

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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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.

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.

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.

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.

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

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.

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?

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.

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...

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.