Power supply units are essential tool for every person involved in electronics. It happens for me to own a Korad, that is a linear (heavy) power supply which is at good price and have received good reviews.
What is a power supply and what is the problem this instructable solves.
The main task of a power supply is to supply a load with a configured constant voltage or constant current. However if the cables we are using are long or of bad quality, and the current of our circuit is considerable, then there will be a significant (depending on the application) voltage drop across the wires. That will result in lower voltage than specified across the load. A workaround, is to use short wires with very little resistance (good quality), but that is not always achievable.
Many power supplies, more expensive than korad, to eliminate this issue are capable of remote sensing.
How remote sensing works
I couldn't describe remote sensing better that tinbin here :
I highly recommend this article, so for those of you who click this link or know about the issue you can skip the following two paragraphs.
By default power supplies sense the voltage across the load probing the output of psu. For example if we have set the output voltage to be 5V and for whatever reason psu sense locally 4.8 volts at its output, then will increase voltage till it senses again 5V. However if there is voltage drop of 0.2 volts across wires, so in effect load "sees" only 4.8 volts, psu will never increase voltage at its output since it still senses 5V locally before the running wires.
For that reason higher end models are capable of remote sensing. That is they have an extra output port where we can plug a pair of wires for remote sensing. The other end of the sensing wires connect to the load. So if for whatever reason (including losses across wires) the voltage across the load is less than specified, the psu will sense it (at the load this time) and increase the voltage at its output until the load voltage be again the specified. For example if we see 5V volt output and voltage drop across wires is 0.2V then power supply will remotely sense 4.8 volts at load so will increase the output voltage until voltage at load be again 5V. I repeat myself I know!
The main idea for this instructable.
Korad has a pair of sense wires internally that are connected locally at its output ports. The main idea is to create a new port where we can connect our sense wires, and also with the aid of just a switch to select either local sense (at the output) or remote sense (at the load)
Step 1: What Will You Need
- Wires 24awg: (Choose whatever colour you like, but I think red and black are consistent with other wires of korad)
- 50 cm red wire,
- 50 cm black wire
- 5 cm white wire
- 5 cm blue wire
- 20 cm yellow wire for ground (I used again a blue one)
Consumables that you'll need
Tools you will need
- Soldering iron, or soldering station
- Wire stripper/cutter
- Optionally a rotary or drill tool (Maybe you can melt enclosure with soldering iron, but I don't recommend it)
- Exacto knife
- Hot glue gun
- Maybe you'll need a tool for cutting large FR4 board ( I use a hand metal cutter, like a big scissor)
Cost is ZERO, I don't bother to take into account usb connector and wires or the tiny board. The most important cost here is labor time.
Step 2: Disassemple Korad, Locate and Cut Sense Wire
I will not be too verbose in this step (screws are at obvious positions).
- Unplug all the wire to board connectors of the front panel
- Locate the sense wires, is the thin pair of wires that runs from port to main pcb, like shown in the image
- Cut the pair of sense in middle. Don't through any part of it
- Unscrew and remove the main green pcb
- Unscrew and remove the lcd pcb
- Optionally remove the yellow pcb and optical encoder pcb (I suggest it to avoid damaging it during drilling). Note the rotary knob is easily removable, just pull it don't be afraid you won't damage it.
Step 3: Drill Holes for Switch and Usb Port
If you have removed the protective layer of lcd panel is now a good time
to make a protective mask for your screen putting maybe an adhesive tape
- Looking at the image locate the spots where the usb port and switch will be accommodated
- There is not enough room between top of the front panel and main pcb of the board. So in order to make room for the switch we have to cut a little bit the inner bezel of the panel.
- With a switch at hand make on the panel where you will make cut. Precision is not of importance. All we need is to make enough room for the switch to fit in.
- With a rotary tool cut horizontal lines from inner to outer surface. Rotary tool will maybe push you against the screw pole. Be careful and don't cut it like me. Scrap the pieces out of panel
- Check with the lcd board on if switch fits. Note that you must maintain the gap between outer and inner bezel in order outer metal box fit again in when assembling back the device
- Mark the position where you will make the hole for the switch. This time you have to be precise. The center of hole must be half height of switch bellow the infra surface of inner bezel (that is now missing). I suggest to make these measurements ( I didn't keep them to readily provide you). Height of switch, gap height and thichness of inner and outer bezel.
- Drill a hole. I used my rotary tool for that purpose mounted on a DIY drill bench stand. My rotary tool can't host thicker drill bits so I had to route it a little bit inside the hole to enlarge it. You can use whatever drill tool you have available, or melt a hole with your solder iron. You can make the hole slightly bigger so you have the freedom to move around the switch a little bit to center it perfectly
- As you can see the location for the usb port is ideal because the white plastic is already cutted, so the drilling or melting will be easier, and also there are two poles in each side that we'll use them to secure usb port in place
Create extra masking with two pieces of adhesive tape before drilling or cutting
If you use a drilling tool drill multiple holes inside the black area in order to get a coarse opening.
- With a precision knife smooth the edges
Step 4: Make Wires
The circuit will be the one shown in the first image. It is the super-cluttered collage sketch of what I describe here.
It is easier to cut the appropriate length of wires and solder them firstly on the switch.
When switch is up will select the remote sense pair and when switch is down will select the local sense pair. For that reason the local pair will be connected to the upper switch terminals and remote pair will be connected to the bottom terminals (reverse).
- What will you need is about 17cm of red and black wire for the local pair. So solder this pair on the top of the switch.
- In turn use about 8 cm of red and black wire for the pair that goes to korad pcb and solder that again in the middle terminals of the switch.
- To complete connection use about 5 cm of blue and white wire (or whatever colors you prefer) for the pair that will connect to usb connector and solder it on the bottom terminals of the switch
Twist slightly all the wire pairs. In that way you will have increased noise immunity
When you are done the switch along with the wires should look like in the picture
Unplug the original upper half of sense pair, twist it in the opposite
direction you did before and solder it with the middle pair of the switch. Cable tighteners are used to hold against untwisting during soldering
- Take a washer or a more appropriate ground terminal and solder it on a ground wire (I used the same blue wire as before) of length about 18 cm. The end result of screwed ground wire should look like in the last picture
Step 5: Make the Pcb Board That Hold Usb Connector
Cut an fr4 board to these dimensions (to be written soon). Drill two holes using a 4mm drill in either direction. I can't recall the exact position of screw holes.(My first instructable! Sorry)
Mark the exact position where you will solder usb port on board in the following way:
- Place usb port into the hole
- From the outer side of panel plug a flash drive or a usb cable, in order to handle the port
- Screw the right side of the board, and leave it in a rotated position so you have access to underneath connector
- Put a bean of hot glue on the back side of usb port near its pins (glue will also protect pins during soldering)
- Immediately rotate board to align its left hole with left screw pole and using flash drive as handle push against board and make fast small movements to align usb port as you wish.
- Let the glue get cold, unscrew the board and lift it from behind the panel
- Secure the position of port on board with a rubber band. (During soldering hot glue will melt and if have not fixed position, usb connector will get misaligned)
- Solder the usb connector onto board.
- At this point make sure, using continuity test, there is no short circuits between usb pins and ground (outer surface of usb connector). In such a case use another usb connector
Step 6: Make the Rest of Soldered Connections and Reassemple
- Solder the blue(-) wire (not ground) to the second pin of usb connector (as you see it from behind
- Solder the white(+) wire to the third pin of usb connector
- Solder the other side of ground wire on board
- Solder the other end of the local sensor leads to the bottom pair of cutted original sensor cable
- Put back and screw the lcd board
- Put back and screw the yellow pcb board and rotary board with the knob
- Put switch into its hole and screw it
- Put the usb board into its position and use two self tapping screws to screw it (I borrowed two of them from the main pcb board, shhh don't tell anyone)
- Put back and screw the main green pcb board
- Put every thing back together. Don't forget to screw the new ground wire along with the old one
Step 7: Make a Remote Sense Cable
For remote sense cable you can use a ubiquitous usb2.0 cable. Most of them come with a shield and have a twisted pair.
- Cut one end of the cable approximately at desired end
- The twisted pair does not involve the thicker red and black wires (which are not twisted). In my case the twisted pair is consisted of one green(+) and one white(-) wire.
- Solder on the twisted pair the desired connectors (I prefer crocodiles for that case)
You can make cheaply multiple sense cables by resurrecting unused usb cables or buy new ones
Step 8: Enjoy Remote Sensing Capability
More than enough said, just enjoy the pictures
Note: Since I ran into trouble for this project I changed also the output led to blue one. But that is a different story.