Bench Power Supply
Page Two
Shunts , Current limiting , Digital panel meters
The drawing below shows all the elements required to be added to the main power supply circuit previously described to enable us to control the current and display voltage and current .
The two shunts RS1 and RS2 are two high precision ( 0.5% ) resistances used to by-pass most of the current flow from the digital voltage display to obtain two ranges , 0 to 199.9 mA and 0 to 1.999 A by measuring the voltage drop across the selected shunts . Switch S1 , a miniature DPDT switch , is wired on one side to select the range to the inputs normally labelled " HI and LO " of the digital module and the other side to select the decimal point setting of the display .
On my power supply , I invested a little more money by using two miniature 3 poles 2 positions switches and connected small LEDs indicating low (red) and high range
(green) that can easily seen at a glance .
S2 is used for voltage ranging in the same manner with the exception that the voltage is measured across from positive to the negative regulated voltge output and after the current limiting variable resistor ( RSV1) if use . Two voltage ranges are used ,0 to 19.99 volts and 0 to 199.0 volts .
Some module do not include voltage dividing resistors for range selection and additional resistances need to be connected externally to achieve the proper range . If this is required read the instructions in the section titled Metering a power supply for voltage dividing methods and resistance values .
3-1/2 Digit LCD voltmeter modules are readily available at very reasonable cost , the larger the module the higher the cost and a size 2-1/2" by 1" module will be easy to read a fair distance . If possible obtain all the information about available functions on board before you select one .
Most LCD modules are powered by a " floating " supply , that means that an independant supply source must be used and NO connection is permitted to any other part of the circuit from which you take input measurements . Usually a single 9 volts battery will do the job
for the two modules and will last a long time . Just make sure that the ON/OFF function is either made with a single switch or to one side of a DPDT switch substituted
for S1 used for the main board as listed in the parts list .
A few LCD modules use a 5 volts common supply at a slightlty higher cost . it means that the module negative supply should be connected to the common supply ground . It is convenient that a single small 5 volts regulator can be used to power the modules and the power source can be taken from the rectifier output .
LED digital modules can also be used and ususally have also a common ground connection but they do require a much higher current source and a regulator to match . Bear in mind that with all LED segments on the current may exceed a 100mA rating regulator
and a TO-220 -1.5A regulator must be used for both modules current source .
Where to get digital modules
In Canada on the west coast , you can order different sizes modules from RPE Electronics at a very reasonable cost starting at $Can 16.95 ( $ US 11.61 ) for the smallest one , they have a good selection of LCD and LED modules with or without common supply application .
In the USA Digi-key is the place to order from if you do not have a local source .
Adjusting current limit
The LM317 regulator has good over current shut down protection to protect the power supply , BUT, your projects under test have no protection and the possibility of a
failed smoke test when turning on power could be catastrophic resulting in a heart breaking if not expensive experience . Knowing what the estimated amount of current required to power your project under test , it would be very reassuring to know that your current source is limited to approximatly and only a few addtional miliamps. You could safely determine by the amount of current flow indicated on your current meter that if it is excessive then something is not right and indicate a faulty or shorted circuit but without total destruction of the circuit under test .
RCV1 is used to set the current limit at any voltage to a load from your power supply . I could not be without one . I have installed a 5 watts wire wound variable
resistor and since most of my projects use less than 100 miliamps up to about 12 volts the current limit is set to that amount of current . For other applications requring more current I simply increase the current to the maximum required amount for that application up to the maximum capability of the power supply . At that point I can determine if more current is required in excess of the power supply .
To set the current limit it is only required to select a voltage and short the output of power supply then adjust the current limit with RCV1. But this is not a safe and good way to do this , and I always use a load across the output for that setting .
Not shown on the circuit , you might want to install as I have on my power supply and additional normally open push button rated at 3 amps which is connected in series
with a 30 ohms 10 watts ( square ohms type ) between the positive and negative output of the power supply . When the voltage selected is 30 volts the current flow will be one amp , so by pressing the button momentarely I simply adjust for the required current limit then release the button .
At 30 volts setting the current limit set at one amps will then be 700 mA @ 20 v , 500mA @ 15 v , 300mA @ 9v and so on , so I have no fear of facing any disaster.
This is only one setting exemple and in most cases should be set at a lower level .
For lack of using a wire wound variable rehostat you migh want to use instead one or more high watt resistors in the same manner connected to a rotary switch
to select pre-fixed current limit settings . Select at least 10 watts resistances by dividing voltage by the current required .
Other uses for a current limting function are batteries charging , small DC motor testing , voltage and current reference source and endless unmentioned tasks .
To charge batteries , simply select the required charging voltage , press the button and adjust maximum charging current indicated on the battery under charge and power on . The current meter will indicate the charging current and gradually diminish to zero indicating that the battery or batteries in parallel are fully charged .
Power supply enclosure
Select a sturdy plastic enclosure large enough to accomodate your transformer and PCB or wired circuits with separate removable cover and face plate .
The face plate should be large enough to accomodate the displays modules and all the controls . In addition to the parts listed , you may want to install LEDs .
You will also require two output plugs one red (+) one balck or green (-) , a power cable , colour ribbon wires , some dry transfer lettering for functions identification .
For a good job protecting the lettering a few ligh spray over the front panel with a flat clear finish and the total cost should be les than $75.00 in any currencies this is well below the purchase price of a comparable commercial power supply , better quality , and the pride of your efforts .
Your power supply could look like this
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