# Control Circuit (9VDC) For 12VDC Circuit

Darendor
Hello, I've been trying to figure out how to design a control circuit for a project of mine with no success. I could use a little guidance.

The proposed circuit is a board powered by a simple 9V battery. It will have a temperature probe connected to it. The circuit is supposed to close and open a 12VDC connection (i.e. wire from the negative terminal of a car battery) based on the temperature, thus:
- close the circuit if temperature is equal to or less than -19 degrees Celcius
- open the circuit if temperature is greater than -15 degrees Celcius

The board will have a simple SPST on/off switch. It will also (when on) measure the car battery's voltage and display it in a digital readout and also display the temperature probe's reading.

Finally, there will be a green lamp blinking when the circuit is closed, and a red lamp on solid if the car battery voltate is 5VDC or less.

I was wondering if anyone would be willing to help me out in getting this thing started?

Fish4Fun
Darendor,

Welcome to PF!

I assume you live in a cold environment, and the "board" you are referring to will control a resistive engine heater of some sort?

Regardless, from this statement:

I was wondering if anyone would be willing to help me out in getting this thing started?

I assume you haven't started designing the circuit yet, and want someone to undertake designing it for you? If this is, in fact, the case, you probably aren't going to have much luck getting anyone here to volunteer for that project. If, on the other hand, there is some particular aspect of the circuit you are having trouble with, many here will gladly lend a hand; just post a schematic and describe the problem you are having.

Fish

Darendor
I live in Canada, so yes it gets -40 degrees during the wintertime.

Necessity being the mother of invention, I decided I had enough of people stealing my vehicle's block heater extension cord and had the great idea of making an automatic system for heating an engine's coolant during extreme cold where you need your vehicle to start no matter what.

The board controls the connection from the 12VDC car battery to the power inverter.

I can attach schematic if required for clarity.

Darendor,

Welcome to PF!

I assume you live in a cold environment, and the "board" you are referring to will control a resistive engine heater of some sort?

Regardless, from this statement:

I assume you haven't started designing the circuit yet, and want someone to undertake designing it for you? If this is, in fact, the case, you probably aren't going to have much luck getting anyone here to volunteer for that project. If, on the other hand, there is some particular aspect of the circuit you are having trouble with, many here will gladly lend a hand; just post a schematic and describe the problem you are having.

Fish

Darendor
The schematic...

#### Attachments

• autoheater2.png
46.6 KB · Views: 506
Fish4Fun
Darendor,

Oh my.

Battery: $100 Solar Charging System:$150
DC to AC Power Inverter: $70$350

Control Unit: Find electronics student at RDC and offer $100 to design it...? ...where to start? You had best save your$100 for the "student" and the $150 for the solar charging unit (For the record,$150 + $70 +$100 = $320, and if you add the$100 for the "student", you get $420). Let's assume a 200W heating element, as that seems on the low side of "standard". 200W/12V = 16.7A. Let's assume the battery you are referring to is a standard 12V deep-cycle lead acid battery @ 105Ah capacity. 105Ah/2 = 52.5Ah/16.7A = 3.14 hours of operation. Let's assume you can find a 36W solar charger for your$150; 36W/12V = 3A. 60Ah/3A = 20 Hours to recharge 52.5Ah battery @ 87.5% efficiency. So, it is safe to assume that a $150 solar charger is NOT going to keep your battery charged. Why, if I stated your battery had a capacity of 105Ah, did I divide that figure by 2? If you drain a lead acid battery below ~50% capacity it shortens the life of the battery considerably. Granted, "deep cycle batteries" do better than "cranking batteries" wrt "deep discharge cycles", but in the climate you are discussing, if you expect your battery to last a year, draining it to more than 50% capacity should be avoided. So, we can eliminate your$150 solar charger. If you need more than ~3 hours of heating @ a time, you should spend some of this "savings" on more batteries. You should charge your batteries via you vehicle's alternator. Assuming a 75A alternator, you should be able to charge one battery (3.5 hours of operation) per hour of vehicle run time.

Assuming you are not going to need the heater while the car is running, and assuming you have some notion of what the temperature is outside, instead of worrying about a temperature sensor with digital display, a power switch and some blinking lights, you might consider buying an inverter with a power switch already installed on it, and simply turning the switch on when you exit the vehicle and off when you return.

So far, you have only batteries and an inverter on your shopping list. If you add a cigarette lighter plug adapter with battery clips to your list (~$10.00), your system is almost complete! Your vehicle's charging system will keep your battery charged to the proper level w/o any auxiliary charging system as long as the usage time to drive time is maintained. If your usage involves a 30 minute drive to work and 9 hours of parking with the heater on, you would need to add a battery charger to your shopping list, and attach it to your batteries @ night. To figure the charger size: 9 hours * 200W / .8 = 1.44kWh ==> 1.44kWh / 12V = 120Ah. 120Ah/52.5 = 2.28 batteries, round up to 3 batteries to ensure acceptable battery life. 1 hour drive time * 50A = 50Ah supplied by vehicle, 70Ah required by charger over night @ home. Assuming minimum charge time of 7 hours, you would need a 10A battery charger to ensure your batteries were fully re-charged over night. A 10A battery charger ranges from$50 to $400. If you drive more and work less you might save on batteries and the charger. Finally, the$100 for the "design". The national average for EE's is slightly over $100k/year. Assuming 50 weeks/year @ 40hrs/week this implies$50/hr, generally with benefits. An "off the cuff design" to replace you having to switch the heater on and off might take a couple of hours, a formal design could easily take 20 hours, a full-blown, safety approved, production design could take well over 100 hours. Having a single unit built from any of the above designs could easily run $1000. My point is, keep it simple. Just because 10,000 "boxes" could be designed and built to retail for$50 each, does NOT imply that a single unit could could be designed built for $500. That is the nature of mass production.$10,000 in engineering is only $1/10,000 units or$0.10/100,000 units, but if you only need 1 unit, it is $10,000/unit. Anyway, it looks to me like you can "save" on extension chords by purchasing$100-$300 worth of batteries,$70 on an inverter and $50-$400 on a charger. With a 25ft extension chord running ~$10, what you have to ask yourself is: "Am I losing 22 to 77 extension chords a year?" Fish Darendor ... Okay, so I was of the notion that using an alternator to charge more than one battery at a time would damage my vehicle's electrical system...which is where the solar charger would come into play: It would charge the battery during the daytime. The idea of the block heater is to allow the engine to turn over in extremely cold temperature, so there would be no point in running the heater with the engine running at the same time. I'm not certain if the rest of your post was derisive sarcasm or not, but all I really wanted was some pointers on getting my control unit going since I've never delved into this before. Thanks for the input though. Darendor, Oh my. ...where to start? You had best save your$100 for the "student" and the $150 for the solar charging unit (For the record,$150 + $70 +$100 = $320, and if you add the$100 for the "student", you get $420). Let's assume a 200W heating element, as that seems on the low side of "standard". 200W/12V = 16.7A. Let's assume the battery you are referring to is a standard 12V deep-cycle lead acid battery @ 105Ah capacity. 105Ah/2 = 52.5Ah/16.7A = 3.14 hours of operation. Let's assume you can find a 36W solar charger for your$150; 36W/12V = 3A. 60Ah/3A = 20 Hours to recharge 52.5Ah battery @ 87.5% efficiency. So, it is safe to assume that a $150 solar charger is NOT going to keep your battery charged. Why, if I stated your battery had a capacity of 105Ah, did I divide that figure by 2? If you drain a lead acid battery below ~50% capacity it shortens the life of the battery considerably. Granted, "deep cycle batteries" do better than "cranking batteries" wrt "deep discharge cycles", but in the climate you are discussing, if you expect your battery to last a year, draining it to more than 50% capacity should be avoided. So, we can eliminate your$150 solar charger. If you need more than ~3 hours of heating @ a time, you should spend some of this "savings" on more batteries. You should charge your batteries via you vehicle's alternator. Assuming a 75A alternator, you should be able to charge one battery (3.5 hours of operation) per hour of vehicle run time.

Assuming you are not going to need the heater while the car is running, and assuming you have some notion of what the temperature is outside, instead of worrying about a temperature sensor with digital display, a power switch and some blinking lights, you might consider buying an inverter with a power switch already installed on it, and simply turning the switch on when you exit the vehicle and off when you return.

So far, you have only batteries and an inverter on your shopping list. If you add a cigarette lighter plug adapter with battery clips to your list (~$10.00), your system is almost complete! Your vehicle's charging system will keep your battery charged to the proper level w/o any auxiliary charging system as long as the usage time to drive time is maintained. If your usage involves a 30 minute drive to work and 9 hours of parking with the heater on, you would need to add a battery charger to your shopping list, and attach it to your batteries @ night. To figure the charger size: 9 hours * 200W / .8 = 1.44kWh ==> 1.44kWh / 12V = 120Ah. 120Ah/52.5 = 2.28 batteries, round up to 3 batteries to ensure acceptable battery life. 1 hour drive time * 50A = 50Ah supplied by vehicle, 70Ah required by charger over night @ home. Assuming minimum charge time of 7 hours, you would need a 10A battery charger to ensure your batteries were fully re-charged over night. A 10A battery charger ranges from$50 to $400. If you drive more and work less you might save on batteries and the charger. Finally, the$100 for the "design". The national average for EE's is slightly over $100k/year. Assuming 50 weeks/year @ 40hrs/week this implies$50/hr, generally with benefits. An "off the cuff design" to replace you having to switch the heater on and off might take a couple of hours, a formal design could easily take 20 hours, a full-blown, safety approved, production design could take well over 100 hours. Having a single unit built from any of the above designs could easily run $1000. My point is, keep it simple. Just because 10,000 "boxes" could be designed and built to retail for$50 each, does NOT imply that a single unit could could be designed built for $500. That is the nature of mass production.$10,000 in engineering is only $1/10,000 units or$0.10/100,000 units, but if you only need 1 unit, it is $10,000/unit. Anyway, it looks to me like you can "save" on extension chords by purchasing$100-$300 worth of batteries,$70 on an inverter and $50-$400 on a charger. With a 25ft extension chord running ~$10, what you have to ask yourself is: "Am I losing 22 to 77 extension chords a year?" Fish Fish4Fun Darendor, I'm not certain if the rest of your post was derisive sarcasm or not, but all I really wanted was some pointers on getting my control unit going since I've never delved into this before. My post was an assessment of the cost to benefit ratio of your project with specific examples to validate it. If you want to proceed with your project as outlined in the OP, then I would suggest you begin by choosing a uController; it is by far the easiest solution to the various functions you want your box to perform. I would suggest the AVR family, I like the ATMega16 and 32 as general platforms, though you could certainly use a less robust model. I would simultaneously work on the firmware and the analog segments of the project. You can use the on board ADCs for temperature sensing and voltage monitoring. Next I would select the display unit as the interface with the uController will dictate some portion of the firmware and uController's I/O resources. I would consider something like: http://www.matrixorbital.ca/manuals/LCDVFD_series/LCD0821/LCD0821_140.pdf [Broken] For control of the heater, I would consider simply switching the DC input to the inverter. You can accomplish this with a relay or a semiconductor. You would want a relay with contact ratings > 20A, something like this: http://www.components.omron.com/components/web/pdflib.nsf/0/446205B0DF5DAF5385257201007DD6CF/$file/G8V-RHDataSheet_ocb_0906.pdf

For a semiconductor switch, you would want something like this:

http://www.irf.com/product-info/datasheets/data/irfp4368pbf.pdf

I would NOT use a 9V battery to power the unit as 12V is readily available, and this would simplify your project.

Choice of temperature probes is trivial, as is circuit Voltage Regulation, 12V voltage monitoring, LED selection and power switch selection. Is that the "start" you had in mind? Drawing up the schematic is fairly straightforward, and I did a quick one for you...You would want to put a 10k resistor between the I/O pin and the mosfet gate, decoupling capacitors on Vcc inputs and a few other details, but for a "off the cuff", 2-hour design, it's as good as you are likely to get.

Are you up to writing the firmware? Can you take the schematic and turn it into a PCB? Really, why wouldn't you just do as I suggested in my last post and simply turn the inverter on and off?

Anyway, good luck :-)

Fish

Last edited by a moderator:
Darendor
Well I can try to construct the PCB. I'll take the schematic you drew up for me (thank you) and see what happens I guess.

I'll keep you apprised of my efforts.

Thanks.

Gold Member
2021 Award
Just a thought...

Why would you even want to use an inverter ? why don't you use a 12V based heating system ? it would be much more efficient :)

But I do agree with Fish4Fun's earlier assessment ... its going to take a lot of power and a standard car battery "aint going to last long". 1 or 2 smaller type solar panels are not going to be able to recharge the battery well enough.
Fish's costing assessment is a pretty good place for you to start, to be able to consider your possibilities

Dave

Darendor
A standard block heater plugs into 110VAC here in North America, hence the inverter?

Fish4Fun
Well I can try to construct the PCB. I'll take the schematic you drew up for me (thank you) and see what happens I guess.

I'll keep you apprised of my efforts.

Thanks.

http://www.atmel.com/dyn/products/tools_card.asp?category_id=163&family_id=607&subfamily_id=760&tool_id=2725 [Broken]

If you have never developed firmware in assembly language, you might look here: http://www.avr-asm-tutorial.net/avr_en/ to get started.