Introduction: Intro & Tutorial on Programmable Power Supply!
If you have ever wondered about programmable power supplies,then you must go through this instructable to get a complete knowledge & practical example of a programmable power supply.
Also anyone who is interested in electronics,please go through this instructable to explore some new interesting things....
Step 1: What Is a Programmable Power Supply and What Makes It Different?
It's been a while since i uploaded any new instructable.So I thought to quickly upload a new instructable on a very necessary tool (for any hobbyists/electronic enthusiast/professional) which is a programmable power supply.
So,the first question arises here that what is a programmable supply?
A programmable power supply is a type of linear power supply which allows the full control of the output voltage & current of the unit through digital interface/analog/RS232.
So what makes it different from a traditional LM317/LM350/any other IC based linear power supply?Let's take a look at the key differences.
1)The main big difference is the control:
Generally our traditional LM317/LM350/any other IC based supply operates on a CV(constant voltage) mode where we have no control over the Current.The load draws the current according to it's need where we can't control it.But in a programmable supply,we can control both the Voltage and current fields individually.
2)The control interface:
In our LM317/LM350 based supply,we turn a pot and the output voltage varies accordingly.
In comparison,in a programmable power supply,we can either set the parameters using numeric keypad or we can change it using a rotary encoder or even we can control the parameters via a PC remotely.
3)The output protection:
If we short the output of our traditional supply,it will lower the voltage and supply the full current.So within a short span,the control chip(LM317/LM350/any other) get's damaged due to overheat.
But in comparison,In a programmable supply,we can shut the output totally(if we want) when a short circuit occurs.
4)The User interface:
Generally in a traditional supply,we have to attach a multimeter to check the output voltage every time.Also in addition a current sensor/precise clamp meter is needed to check the output current.
(NB: Please check my 3A variable bench power supply instructable here which consists of inbuilt Voltage & Current readout on a colour display)
Apart from that,in a programmable supply,it has a inbuilt display which shows all the necessary informations like current voltage/current amp/set voltage/set amp/mode of operation and many more parameters.
5)No of outputs:
Suppose you want to run a OP-AMP based circuit/audio circuit where you'll need all Vcc,0v & GND.Our linear supply will give only Vcc & GND(single channel output) so you can't run these type of circuit using a linear supply(You will need two of them connected in series).
In comparison,a typical programmable supply has minimum two outputs(some has three) which are electronically isolated(not true for every programmable supply) and you can easily join them in series to get your required Vcc,0,GND.
There are also many differences,but these are the main key differences which I described.Hopefully you will get an Idea of what a programmable power supply is.
Also,In comparison with a SMPS,the programmable power supply has a very little noise(unwanted AC components/electrical spikes/EMF etc) at the output(As it is linear).
Now let's move on to the next step!
NB: You can check my video regarding my Rigol DP832 programmable power supply here.
Step 2: What Is CV & CC Mode of Any Power Supply?
It is very much confusing for many of us when it comes to the matter of CV & CC.We know the full form but in many cases,we don't have the proper Idea how they works.Let's take a look on both modes and make a comparison on how they are different from their working perspective.
CV(constant voltage) mode:
In CV mode(whether in case of any power supply/Battery charger/almost anything which has it),the equipment generally maintains a Constant output voltage at the output independent of the current drawn from it.
Now let's take an example.
For say,I have a 50w white LED which runs on 32v & consumes 1.75A.Now if we attach the LED to the power supply in constant voltage mode & set the supply to 32v,the power supply will regulate the output voltage and will maintain it at 32v anyway.It will not monitor the current consumed by the LED.
These type of LEDs draw more current when they get hotter(i.e it will draw more current than the specified current at the datasheet i.e 1.75A & can go as high as 3.5A.If we put the power supply at CV mode for this LED,it will not look at the current drawn & only regulate the output voltage and thus,the LED will be damaged eventually on the long run due to excessive current consumption.
Here the CC mode comes into play!!
CC(constant current/current control) mode:
In CC mode,we can set the MAX current drawn by any load & we can regulate it.
For say,we set the voltage at 32v & set the max current to 1.75A and attach the same LED to the supply.Now what will happen?Eventually the LED will get hotter and try to draw more current from the supply.Now this time,our power supply will maintain the same amp i.e 1.75 at the output by LOWERING THE VOLTAGE(simple Ohm's law) and thus,our LED will be saved on the long run.
Same goes for the battery charging when you charge any SLA/Li-ion/LI-po battery.In the first part of the charging,we have to regulate to current using CC mode.
Let's take another example where we want to charge a 4.2v/1000mah battery which is rated at 1C(i.e we can charge the battery with a max current of 1A).But for safety's sake,we will regulate the current to a max of 0.5C i.e 500mA.
Now we will set the power supply to 4.2v and set the max current to 500mA and will attach the battery to it.Now the battery will try to fetch more current out of the supply for first charging but our power supply will regulate the current by lowering the voltage a little bit.As the battery voltage will rise eventually,the potential difference will be less between the Supply and the battery and the current drawn by the battery will be lowered.Now whenever the charging current(current drawn by the battery) drops below 500mA,the supply will switch to CV mode & maintain a steady 4.2v at the output to charge the battery for the rest of the time!
Step 3: There Are So Many Out There!!!!
Many programmable power supplies are available from different suppliers.So if you are reading still now and determined to get one,then first you have to decide some parameters!!
Each & every power supplies are different from each other in aspect of accuracy,no of output channels,total power output,Max voltage-current/output etc etc.
Now if you want to own one,then first you decide what is the max output voltage & current you generally work with for your day to day use!Then select the no of output channels you need in order to work with different circuits at a time.Then comes the total power output i.e how much max power you need (P = VxI formula).Then go for the interface like either you need numeric keypad/rotary encoder style or you need analog type interface etc.
Now if you have decided,then finally comes the main important factor i.e pricing.Choose one according to your budget (and obviously check that if the technical parameters mentioned above are available within it).
And last but not the least,obviously look at the supplier.I would recommended you to buy from a reputated supplier and don't forget to check the feedback(given by other customers).
Now let's take an example:
I generally work with digital logic circuits/Microcontroller related circuits which needs generally 5v/max 2A(if I use some motors & stuffs like that).
Also sometimes,i work on Audio circuits which needs as high as 30v/3A & also dual supply.So i will choose a supply that can give a max of 30v/3A and have a dual electronically isolated channels.(i.e each channel can supply 30v/3A and they will not have any common GND rail or VCC rail).I don't generally need any fancy numeric keypad like thing!(But of course they helps a lot).Now my max budget is 500$.So i will choose a power supply according to my above mentioned criterias...
Step 4: My Power Supply....Rigol DP832
So according to my needs,Rigol DP832 is a perfect equipment for my use(AGAIN,STRONGLY IN MY OPINION).
Now let's take a quick look at it.It has Three different channels.Ch1 & Ch2/3 are electronically isolated.Ch1 & Ch2 can both give a max of 30v/3A.You can connect them in series to get as much as 60v(max current will be 3A).Also you can parallelly connect them to get a max of 6A(max voltage will be 30v).Ch2 & Ch3 has a common ground.Ch3 can give a max of 5v/3A which is suitable for digital circuits.The total output power of all the three channels combined is 195w.It costed me around 639$ in India(Here in India,it is a little bit pricey as compared to the Rigol's site where it is mentioned at 473$ because of import charges and taxes..)
You can select different channels by pressing the 1/2/3 button to select corresponding channel.Each individual channel can be On/Off using it's corresponding switches.Also you can turn them On/Off all at once via a other dedicated switch called All on/off.The control interface is totally digital.It provides a numeric keypad for direct entry of any given voltage/current.Also there is a rotary encoder via which you can gradually increase/decrease any given parameter.
Volt/Milivolt/Amp/Miliamp - four dedicated keys are there to input the desired entity.Also these keys can be used to move the cursor Top/Bottom/Right/Left.
There are five keys under the display which acts according to the text which is showed in the display above the switches.For say,If i want to turn on OVP(over voltage protection),then i have to press the third switch from the left to turn on OVP.
The power supply has a OVP(over voltage protection) & OCP(over current protection) for each channel.
Suppose,i want to run a circuit(which can tolerate a max of 5v) where I will gradually increase the voltage from 3.3v to 5v.Now If i accidentally put voltage more than 5v by turning the knob & not looking at the display,the circuit will be fried.Now in this case the OVP comes into action.I will set the OVP to 5v.Now i will gradually increase the voltage from 3.3v and whenever the 5v limit is reached,the channel will be shut off to protect the load.
Same goes for the OCP.If I set a certain OCP value(For say 1A),whenever the current drawn by the load reaches that limit,the output will be turned off.
This is a very useful feature to protect your valuable design.
Also there are many more features which I will not explain now.For say,there is timer by which you can create a certain waveform like square/sawtooth etc.Also you can turn on/off any output after a certain period of time.
I have the lower resolution model which supports readback of any voltage/current upto two decimal places.
For Ex: If you set it to 5v and turn the output on,the display will show you 5.00 and same goes for the Current.
Step 5: Enough Talking,let's Power Some Thing Up(also,CV/CC Mode Revisited!)
Now it's time to connect a load and power it up.
Look at the first picture where I have connected my homemade dummy load to the channel 2 of the power supply.
What is a Dummy load:
Dummy load is basically a electrical load which draws current from any power source.But in a real load (like a Bulb/motor),the current consumption is fixed for the particular Bulb/Motor.But in case of a Dummy load,we can adjust the current drawn by the load by a pot i.e we can increase/decrease the power consumption as per our needs.
Now you can clearly see that the load(wooden box at the right) is drawing 0.50A from the supply.Now let's take a look at the power supply's display.You can see that channel 2 is on and rest of the channels are off(The green square is around channel2 & all the output parameters like voltage,current,power dissipated by the load are shown).It is showing voltage as 5v,current as 0.53A(which is correct & my dummy load is reading is little less i.e 0.50A) & the total power dissipated by the load i.e 2.650W.
Now let's take a look at the power supply's display at the second picture((zoomed pic of the display).I have set voltage of 5v & the max current is set at 1A.The supply is giving a steady 5v at the output.At this point,the load is drawing 0.53A which is less than the set current 1A so the power supply is not limiting the current and mode is CV mode.
Now,if the current drawn by the load reaches 1A,the supply will go into CC mode and lower the voltage to maintain a Constant 1A current at the output.
Now,check the third pic.Here you can see that the dummy load is drawing 0.99A.So in this situation,the power supply should lower the voltage & make a stedy 1A current at the output.
Let's take a look at the 4th picture(zoomed pic of the display) where you can see that the mode is changed to CC.The power supply has decreased the voltage to 0.28v to maintain the load current at 1A.Again,ohm's law wins!!!!
Step 6: Let's Have Some Fun....Time to Test the Accuracy!!
Now,here comes the most important part of any power supply i.e the Accuracy.So in this part,we will check,how precise these type of programmable power supplies really are!!
Voltage accuracy test:
In the first pic,I have set the power supply to 5v & you can see that my recently calibrated Fluke 87v Multimeter is reading 5.002v.
Now let's take a look at the datasheet at the second pic.
The voltage accuracy for Ch1/Ch2 will be within the range as described below:
Set voltage +/- (.02% of Set voltage + 2mv).In our case,I have attached the Multimeter to Ch1 & the set voltage is 5v.
So the upper limit of the output voltage will be:
5v + (.02% of 5v + .002v) i.e 5.003v.
& the lower limit for the output voltage will be:
5v - (.02% of 5v + .002v) i.e 4.997.
My recently calibrated Fluke 87v Industrial standard Multimeter is showing 5.002v which is within the specified range as we calculated above.A very good result I must say!!
Current accuracy test:
Again take a look at the datasheet for the current accuracy.As described,the current accuracy for all the three channels will be:
Set current +/- (.05% of set current + 2mA).
Now let's take a look at the Third pic where I have set the max current to 20mA(The power supply will go into CC mode and try to maintain 20mA when I will attach the Multimeter) & my Multimeter is reading 20.48mA.
Now let's calculate the range first.
The upper limit of the output current will be:
20mA + (.05% of 20mA + 2mA) i.e 22.01mA.
The lower limit of the output current will be:
20mA - (.05% of 20mA + 2mA) i.e 17.99mA.
My trusted Fluke is reading 20.48mA & again the value is within the above calculated range.Again we got a good result for our current accuracy test.The power supply didn't fail us....
Step 7: The Final Verdict.....
Now we have come to the last part...
Hopefully I could give you some little Idea about what are programmable power supplies and how they works.
If you are serious about electronics and do some serious designs,I think that any type of programmable power supply should be present in your arsenal because we literally don't like to fry our precious designs due to some accidental overvoltage/overcurrent/short circuit.
Not only that,but also with this type of supply,we can precisely charge any type of Li-po/Li-ion/SLA battery without the fear of catching fire/any special charger(Because Li-po/Li-ion batteries are prone to catch fire if proper charging parameters doesn't meet!).
Now it's time to say goodbye!
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