A Circuit for Which Voltage Should be Constant and Current Should Vary

What you need is a voltage regulator

http://en.wikipedia.org/wiki/Linear_regulator

Also try an internet search of "voltage regulator".

Not voltage regulator sir...

It's some similar to that but output voltage must be constant by changing the input voltage and by changing the input voltage the output current should vary........

I am still not sure what you are requiring (possibly a current transmitter). Can you explain your application in more detail and also the value (or range) of input and output voltages and currents you are working with.

A current transmitter, voltage divider network or even a single resistor would likely work but the output voltage won't be constant.

Perhaps you mean a "voltage to current converter" that produces a variable output current proportional to a varying input voltage.

http://en.wikipedia.org/wiki/Voltage-to-current_converter

http://en.wikibooks.org/wiki/Circuit_Idea/Passive_Voltage-to-Current_Converter

http://en.wikibooks.org/wiki/Circuit_Idea/Op-amp_Inverting_Voltage-to-Current_Converter

No sir........

Iam designing thermoelectro cooler(TEC) in which iam getting noise at output. And if i connect this TEC output to another circuit then the output of PWM in TEC is going to be induced in the output of the circuit for which i connected.......

So i want a circuit for designing of TEC with very less noise and Good resolution.......

No sir........

Actually iam designing a thermoeleectro cooler(TEC). In that iam getting noise and when iam coonecting the output of PWM in TEC circuit the it is going to be induced in the output of the circuit to which iam connecting........

So i want to design a TEC with less noise and good resolution.......

If I may give some general advice :- Take care to define your problem clearly and fully, you will get a helpful reply sooner. As far as possible, use short sentences which each say just one thing. For example - "I am designing a Thermo-Electric Cooler [TEC]. It must work at normal room temperature. This TEC must keep a test box at -10 Celsius +/- 1 Celsius. I am powering the TEC from a fixed DC voltage through a Pulse Width Modulated [PWM] Switch. My problem is that PWM electrical switching noise is causing wrong/unstable temperature readings from the thermocouple and amplifier used". Addressing your problem :- If you use a LINEAR circuit to drive the TEC, it will not make noise like a PWM switch. If there are reasons why PWM is essential, then you must minimise pick-up of PWM noise by the temperature measuring circuit and/or reduce the noise of the PWM. My first questions are then :- 1) Are you powering PWM and temperature amplifier with one supply??..... 2) Are the TEC power and temperature wires long?? Mixed together?? 3) Are the two power wires to TEC touching insulation [good - even better twisted, with metal braid screen]- or forming a big circle (which maximises electric & magnetic fields to couple to the temperature measurement)?? 4) Same question for temperature wires... It is possible a resistor-capacitor filter at the temperature amplifier input will fix all your problem, but usually one has to do several things better. Awaiting your reply....

K sorry sir...

Sir can u give a circuit for designing a good TEC?

No. You have not given the requirements in a specification, as I suggest. What basic cooling hardware and circuit you already have is not known to me. God knows, but I cannot -unless you tell me. For example - What cooling power, watts, is required at what temperature? - or what cooler do you have? Is low power consumption most important? - or lowest possible temperature? - or precise control of low temperature? Your problem of switching noise suggests precise control. A home refrigerator of the type which uses an electric heater to evaporate a fluid is a "Thermo-Electric Cooler". They come with an effective temperature control and can keep beer or fruit juice cool in a nice big box. If you have a clever semiconductor device in mind instead, what make and type is it?

Try PELTIER cooler in a web search. A quick look found www.marlow.com, manufacturers of Peltier devices. In their "downloads" page you will find a design guide for selection, heat-sinking, power supplies etc. Looking at their type RC3-4 data sheet, the voltage/current characteristic is near linear resistance. Thus your original request for a circuit which had a fixed output voltage, but controlled output current to vary power, would not work (if it existed!). If you apply a fixed voltage V to a resistor R, Ohm's Law states you get a current I = V/R Current I cannot be changed except by changing V. Hence your first requirement of your original forum question (that V must be constant) makes change of I for a Peltier cooler RC3-4 impossible

I want a circuit which can control the cooling in TEC which reaches to -30degrees/centegrate by passing 2amp current............

So what is the voltage across this TEC when it draws 2 amps current?? 115 volts root mean square alternating voltage would be about right for an "Electrolux" refridgerator.

This circuit is a simple but effective current source for about 2 amps. BC108 base voltage is about 600 millivolts. BC108 temperature needs to be steady because regulated current will change about 1% for 3 Celsius change. A good heatsink, 2.5 Celsius per watt is needed for the BUZ10. Do not forget the Peltier cooler will just melt if you do not have a good heatsink on its hot side. To get -30 Celsius, since 60 Celsius hot to cold differential is as high as you can get [with 1 stage], the hot side must be at less than 25 Celsius. This is a water cooling job - or an almighty air blast!

Try the National Semiconductor website.

Have a look at the switchmode regulators with the buck boost feature like LM2596

http://www.national.com/analog/power

http://www.ti.com

CR4 Admin: Texas Instruments bought National Semi. Search the TI site for National Semi products.