school project help - Page 1
Forum section: electronics / beginners

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 03:18:01 pm
content: Hi everyone, Me and a friend are doing a school project about subzero refrigeration and are building everything from scratch and we wondered if anyone here could help us with a circuit that we need?we have 0-12V DC signal in, and a load that takes 0-12V DC 8A, and a PSU that provides either 12v or 5v DC 100W

author: lapm
author_rank: Frequent Contributor
timestamp: December 08, 2015, 03:40:17 pm
content: And your problem you needed help was?

author: CrashO
author_rank: Unknown
timestamp: December 08, 2015, 03:45:53 pm
content: Quote from: lapm on December 08, 2015, 03:40:17 pmI think he wants us to do his homework and design everything needed to go from a 0-12V signal to refrigeration

author: dan3460
author_rank: Frequent Contributor
timestamp: December 08, 2015, 04:09:57 pm
content: I think you need to give us more information. Is the signal a square wave from 0v to 12V? how is this signal been controlled? I'm guessing the load is the refrigeration unit.

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 04:54:09 pm
content: Funy rightCrashO?But we said sadly nothing about homework just a school project for inpres our teacher, All we wondered was how it works and which components we should choose

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 04:58:22 pm
content: ohh sorry dan3460, it varies between 1v and 12v equally on both in and out putt's

author: Simon
author_rank: Unknown
timestamp: December 08, 2015, 05:46:10 pm
content: Quote from: teimes on December 08, 2015, 04:54:09 pmthe mention of school usually implies a task set by a teacher. If you give a bit more background it helps.Hint: Don't include any form of number display that will be shown without warning, and no loose wires hanging around especially red ones both can make people nervous......

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 06:25:01 pm
content: it is not a task we have been given by our teacher, we are two who like electronics and building and challenge ourselves.we have no circuit yet, because we have not learned anything about this type of circuit yet and therefore we are here for geting helpthere is no display on it and the only cables coming out of it is that seen on the "drawing", we have a signal that varies between 1V and 12V as its input, and the output is a load on 8A and have the samma output voltage ass the input signal, so we just wonder how we increase the current in the signal coming in so it be able to drive the load.

author: Simon
author_rank: Unknown
timestamp: December 08, 2015, 06:27:15 pm
content: What do these "signals" represent. You need to explain what parts you have so far and how they work so we can understand what you are doing.

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 06:58:21 pm
content: I have a signal that varies between 1v to 12v, ie 1v, 2v, 3v, 4v, 5v, 6v, 7v, 8v, 9v, 10v, 11V, 12V, but the amp is so small that it can not drive the load in the other end so I have to increase the amp, so when the signal in is at 3 Voltage so is the utput on 3 Voltage 8A for example

author: Simon
author_rank: Unknown
timestamp: December 08, 2015, 07:05:20 pm
content: Where does the signal come from, what does it represent ? You could use a buffer amplifier like a power opamp with a gain of 1 but 8A is a lot and this sounds like the wrong way to do it, you can also use a power supply of some sort and vary it's output in rsponse to an input.

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 07:22:20 pm
content: "you can also use a power supply of some sort and vary it's output in rsponse to an input." how?

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2015, 07:56:32 pm
content: If all you need to do is vary the speed of the fan, depending on the input voltage, then you could use PWM (Pulse Width Modulation). There are plenty of ways to implement this.http://electronicdesign.com/power/pulse-width-modulator-features-versatile-operating-parametershttp://www.almico.com/sfarticle.php?id=1

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 08:09:35 pm
content: the fan is controlled by the temperature gauge, and we thought it could also be Controlled load that needs 8 amp's but the fan controler has a max utput of 2 amp's

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2015, 09:15:59 pm
content: I think from your comments so far you are trying to take on something beyond your current level of skills and knowledge.You have a concept, which is good, but you don't yet have the skills to make it real.You should consider taking on something less ambitious that you can build with knowledge you already possess.

author: Simon
author_rank: Unknown
timestamp: December 08, 2015, 09:23:03 pm
content: To control "refrigeration power" using a peltier cell (I assume you are using a peltier) you can PWM the peltier itself, or you can use a thermostat circuit. Look up "comparator" and "Thermistor"

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 11:13:53 pm
content: Quote from: IanB on December 08, 2015, 09:15:59 pmthat's why wi want to learn it

author: teimes
author_rank: Contributor
timestamp: December 08, 2015, 11:24:01 pm
content: Quote from: Simon on December 08, 2015, 09:23:03 pmyes we are, we have a thermostat that is connected to the fan but it gives only 2ampWe thought that we should let the thermostat Control the temperature in it, so it does not risk geting to warm but also that it cools down the space quickly

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2015, 11:29:19 pm
content: Quote from: teimes on December 08, 2015, 11:13:53 pmI understand. But there is a difference between trying to learn "everything" all at once, and learning particular details as you go.This is why you cannot go to college and learn how to be an engineer in just one year. There is just too much to learn.In this thread people have been giving you good answers, like "use a power supply with a control input", but you do not understand how to apply that answer. This puts you in the position of someone with a plate of nutritious food, but you cannot digest the food.So, I do not want to discourage you, but I do think you are being too ambitious for your level of knowledge, and you need to begin with something simpler.

author: tooki
author_rank: Super Contributor
timestamp: December 08, 2015, 11:51:56 pm
content: The problem is that we have no context. To provide any kind of useful answers here, we need to know a ton more about the concept, how it needs to work, etc.

author: bitslice
author_rank: Frequent Contributor
timestamp: December 08, 2015, 11:59:56 pm
content: The power levels are the sticking point here, if you wanted to vary a 12v@1a load, then that is easy.To vary a 100W load, not so easy.And that fan is going to stop working at <6v

author: Ian.M
author_rank: Super Contributor
timestamp: December 09, 2015, 12:35:50 am
content: Quote from: Simon on December 08, 2015, 09:23:03 pmPelter heatpumps don't do well with direct PWM control because the unwanted Joule heating is equal to (I^2)*R and the cooling is proportional to I. That means, assuming you want to run the module at half its heat transfer capability, if you run it at full voltage and PWM it at 50% duty cycle, you get twice the self-heating that you would if you reduced the voltage to reduce the current to 50%. Its therefore essential to filter the PWM to get a reasonably smooth current through the module.Also, they dont do well with 'bang-bang' control (simple on/off control with a thermostat) because the repeated thermal cycling causes differential radial expansion and contraction of the hot and cold faces, flexing the individual thermoelectric elements leading to fatigue and cracking resulting in partial or total module failure.You therefore need to drive them with a variable voltage controlled power supply, and use a PID control algorithm tuned to be somewhat overdamped to avoid rapid cycling.

author: teimes
author_rank: Contributor
timestamp: December 09, 2015, 01:57:18 am
content: Thank you everyone, we have read a bit on what you have proposed have come up with a circuit that works in proteus, so thank you everyone

author: teimes
author_rank: Contributor
timestamp: December 09, 2015, 01:58:21 am
content: Quote from: IanB on December 08, 2015, 11:29:19 pmwe do not learn as long as we do not try

author: Simon
author_rank: Unknown
timestamp: December 09, 2015, 06:57:54 am
content: Quote from: teimes on December 09, 2015, 01:58:21 amYes but there is aiming for something you can achieve and aiming for what is impossible for you now. You were unable to even explai what you had to work with. This project is not an absolute begier one.If i was doing it I would read a temperature and then generate a PWM and low pass filter is with a SMPS output circuit to get a smoothish voltage to run the pltier on. There would ned to be a micro controller i there probably to adjust the PWM to correct the temperature. There may be a more analogue way of doing it.

author: teimes
author_rank: Contributor
timestamp: December 09, 2015, 10:45:54 am
content: Quote from: Simon on December 09, 2015, 06:57:54 amsorry I regret that I could not express myself properly, I have dyslexia so it's not as easy alwaysso what you're saying is that I need something similar as to a rc speed controller

author: Joule Thief
author_rank: Regular Contributor
timestamp: December 09, 2015, 11:30:48 am
content: Quote from: teimes on December 08, 2015, 11:24:01 pmmaybe a few questions to seek clarity1/ how big is "the space" you want to cool?2/ what is the lowest temperature you wish to reach?3/what type of refrigeration unit is used? freon based? peltier electronic?4/ how quickly do you need your space to be cooled to the required temperature?

author: teimes
author_rank: Contributor
timestamp: December 09, 2015, 11:44:42 am
content: Quote from: Joule Thief on December 09, 2015, 11:30:48 am1/4x1.5L bottles PORTABLE refrigerator2/around about -15~-20°C3/peltier4/as fast as possible if possible

author: ABHIRAMSHIBU
author_rank: Contributor
timestamp: December 09, 2015, 12:15:57 pm
content: I think 8 amp is very less about 96 Watts. So for making a simple refrigerator u need a compressor, a brass tube, a thermostat. If u have a thermostat with a low amp rating u can use a relay in between it. Simple!.I am not that much expert and what I tell u have to be clarified with other members. U need a adurino, a photo detector, Lm35 sensor, a infra red led. There are so may codes out there u can use to make these things to perform PWM fan controls. Search google about it.So u need to make a thermally insulated container. And inside u have to keep the thermostat.Hot sideTake the brass pipe and connect it to the compressed gas region( Varies according to type of compressor used ) Coil it and stick lm35 with thermal paste on the pipe. Connect the fan on the other end, other connections should be made to audino ( Follow the guide u used to programme).Cold SideTake an another brass pipe and coil it and connect to extracted gas region of compressor. Put a fan also in this coil. Make a box for this setup and make a hole for air intake. Make another hole parallel to the setup with the fan and coil on cool side and take a tube and collect the air to the refrigerator. Leave some small holes ( Very small ) on refrigerator for the air to escape.The compressor and the fan (Cold side) should be connected to the thermostat or the relay. This setup should work.Fill the compressor setup with any refrigerator gas.If u have any doubts please ask. Any way I am not an expert but I will help with what I know...

author: Ian.M
author_rank: Super Contributor
timestamp: December 09, 2015, 01:21:49 pm
content: That certainly isn't going to be easy with a Peltier module.Summer temperatures in Norway average about 18°C, which is 38°C differential from your desired -20°C, and as you want to be able to regulate the temperature, you will need reserve cooling capacity. Also the heatsink has to run above ambient to get rid of the waste heat. and the closer to ambient it is, the bigger it and its cooling fan will need to be, which significantly increases the temperature differential seen by the module.I did a quick search for the most powerful Peltier module I could find that would work with your supply voltage and current limits:http://uk.rs-online.com/web/p/peltier-modules/4901525/, and it looks like with 50°C differential, you'd only be getting 20W of cooling at the cold side and would need the heatsink to be able to dump 120W with only 10°C differential to ambient.A 0.08°C/W heatsink that can handle 120W isn't small, light or cheap, and you would need to make the box with something exotic like layered vacuum insulation panels to get the total heat flux low enough.Next, how fast does it cool down?Water has a heat capacity of 4.187 kJ/kg-KWater's specific heat of fusion is 334 kJ/kgIce has a heat capacity of 2.108 kJ/kg-K.4*1.5L bottles is about 6kg, so to cool from 18°C to -20°C, requires the removal of approx 2710kJ of heat, most of that at 0°C. At 0°C there is about a 30°C differential across the module, at which point it can cool at about 40W. Given absolutely perfect insulation, it could cool down in about 19 hours. In practice the insulation will not be perfect, and if it is designed for a manageable heat flux at -20°C, the cool down time will be closer to 25 hours.(Edit: corrected cooldown calculations)Peltier modules are horribly inefficient, so as a previous poster suggested, you should investigate low voltage compressor driven refrigeration. It is significantly more complex than his brief description suggests.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: December 09, 2015, 01:44:42 pm
content: Quote from: Ian.M on December 09, 2015, 01:21:49 pmEmphasis mine. Seriously though, teimes; I'm not sure it is very feasible to do what you want - how you want to do it.Also, the smart guys on here would probably find it easier to help you if you tell us more about each part the problem. I still don't know exactly what you want (may be because I'm not one of the smart guys though)Good luck!

author: teimes
author_rank: Contributor
timestamp: December 09, 2015, 11:10:47 pm
content: just hypothetical, on a voltage regulator, it is possible to replace the variable resistor with a signal from the fan crontroller to reggulere the voltage?

author: Ian.M
author_rank: Super Contributor
timestamp: December 09, 2015, 11:53:46 pm
content: Not simply. Some linear regulators can work that way, but would waste so much power as heat that it would be totally impractical for your application. Switching regulator chips and controllers generally use a feedback divider to compare a proportion of the output with the internal reference. You'd either need one that accepts an external reference (very rare) or an analog multiplier or multiplying DAC in the feedback path.

author: teimes
author_rank: Contributor
timestamp: December 10, 2015, 12:13:09 am
content: what with a DC Buck Converter instead of a linear regulator?or like this

author: Joule Thief
author_rank: Regular Contributor
timestamp: December 10, 2015, 06:45:37 am
content: Please consider building the cooler and container first to see if you can accomplish the temperature and speed you require. The initial calculations show the controller circuitry would not be used for the first 19 hours of on time. Then proceed to the design and testing of the circuitry to control your cooler.

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 07:06:14 am
content: I've got an even simpler and far cheaper test for you: The heat transfer through the insulation will be pretty much the same for the same temperature differential regardless of whether the interior is hotter or colder than ambient.Get a commercially available cold box (not a powered cooler), or an expanded polystyrene fish box + lid or similar. Take two 10W car bulbs + holders. Wire them in parallel with enough wire to exit the box and attach them to a small plank of wood. Put that in one corner of the box bottom and place a thermocouple probe 5cm from the bottom or sides in the opposite corner. Put the lid on the box and connect a 12V PSU to the bulbs. If it cant reach and maintain an interior temperature 40°C above ambient with only 20W of heat input, it needs more insulation. Try wrapping the box in aluminiumised bubble wrap or whatever other insulation you fancy. I suspect that you will find that the insulation required for an 8L cavity will be far too bulky for convenient portability.N.B. DO NOT leave it running unattended as a heat source in an insulated box is a potential fire risk.

author: Seekonk
author_rank: Super Contributor
timestamp: December 10, 2015, 04:45:27 pm
content: You will be rich and the smartest guys in the room if you ever get this to work. I've seen countless attempts of this on RE sites with very disappointing results.

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 05:11:00 pm
content: It can be done, but Peltier coolers suck efficiency wise, and aren't mechanically robust. There are excellent for cooling very small volumes, or slightly larger volumes when you don't give a s--t about efficiency, but want the lowest possible up-front cost,or need a no moving parts solution.Compressor units have got small enough to be practical for portable applications:http://www.appliancedesign.com/articles/93330-miniature-bldc-rotary-refrigeration-compressors

author: teimes
author_rank: Contributor
timestamp: December 11, 2015, 10:23:05 pm
content: Quote from: Seekonk on December 10, 2015, 04:45:27 pmTHANKS I think?

author: teimes
author_rank: Contributor
timestamp: December 11, 2015, 10:29:14 pm
content: Quote from: blueskull on December 10, 2015, 05:25:27 pmis thet possible?

author: Ian.M
author_rank: Super Contributor
timestamp: December 11, 2015, 10:32:38 pm
content: You've got a better chance than most being so far north. Summer temperatures in central or southern Europe rise so high that single stage Pelter refrigeration to 20°C below zero of even as small as a 2L volume becomes totally impractical or even actually impossible due to the temperature differential required. Its hard enough for a Peltier fridge to even get a six-pack of beer to a drinkable temperature during an eastern med. heatwave.

author: teimes
author_rank: Contributor
timestamp: December 12, 2015, 01:42:32 am
content: Quote from: Ian.M on December 11, 2015, 10:32:38 pmif i use water cooling as in gaming computers, then it becomes the pretty effectively, and a slightly large and thin steel or copper plate on the cold side, so it gets colder faster.?

author: Ian.M
author_rank: Super Contributor
timestamp: December 12, 2015, 02:10:44 am
content: You need *GOOD* thermal coupling on the cold side so that needs a fairly substantial large area heatsink inside the coldbox. You also have to contend with it iceing up as water vapour condenses and freezes As solid ice has a thermal conductivity about a hundred times worse (smaller) than that of aluminium, and powdery frost is another order of magnitude worse, you can easily end up with the cold plate of the Peltier element at -25°C and the rest of the box at 0°C or above because the ice on the cold plate heatsink has grown, till its thick enough that the heat transfer through it isn't enough to freeze any more water at its surface. The only cure is enough cold plate area so that the maximum possible water vapour concentration before cooldown cant form a thick enough layer of ice to support more than a few degrees differential at the maximum heat flux the Peltier module can pump at just below zero. It also helps to orient the cold plate and heatsink vertically, so condensate that forms above zero can run off before it freezes.Water cooling could work on the hot side, but you cant afford the temperature differentials that would make passive convection circulation work and if you need to add a pump, the complexity is approaching the point that compressor based refrigeration becomes viable. However if you have an effectively inexhaustible source of cold water, e.g. in a boat on a lake or the sea with an intake well below the surface, water cooling becomes far far more attractive, because you loose the weight and power penalties of the reservoir, radiator and associated cooling fan, and are able to dump heat at anything up to 10°C below the air temperature.