In this blog posting I will publish the schematics for La Carga DC Load unit. I will also add some commentary to aid with the interpretation. My style of drawing schematics has evolved some but the roots are back in the 1960s. If you find anything of brilliance in these schematics it probably came from the tutorials of Robert Peter Oakes, the engineering in depth wisdom of michaelkellett, the innovation of shabaz, or any other of my mentors who have helped me so much these last two years. In many ways I see myself as "The Borg" never innovating but always assimilating. On the other side if you see something that is surely going to produce magic smoke then I was likely the source.
The first schematic is what I am calling the main board. Some of this schematic will actually be on a circuit board and the rest will be components mounted on the chassis of the unit or on the control panel. The boards I am using are solderable breadboards that will work adequately for the level of complexity and precision needed for this project. I am also going to publish my hand drawings of component layout on the boards. While these will be somewhat difficult to read and interpret I want to emphasize the importance of planning before actual construction. These are my attempt at planning. As much as possible these schematics have been prototyped on regular bread board and tested for proof of concept.
Main Board
Clicking on the Schematic Should enlarge it.
The design has (4) series 0.5 ohm resistors that are each normally shorted out by relays. Also in series with the 4 resistors is a MOSFET and a 0.5 ohm current sense resistor. As the input voltage is applied and increased to the unit relays will open one after another adding 0.5 ohms of series resistance to the circuit. An LM339 comparator is being used to drive gate level MOSFETs which in turn drive the relays. The reference inputs of the LM339 are set by a resistive divider so that relay 4 opens at 6 volts, relay 3 opens at 12 volts, relay 2 opens at 18 volts, and relay 1 opens at 24 volts. While it has been pointed out to me that properly heat sunk MOSFETs will eliminate the need for these resistors I wanted the exercise of seeing if I could design this type of approach.
U1 is a ISL28218 Rail to Rail OpAmp that controls the main Power MOSFET using the voltage level across the sense resistor and a reference voltage adjusted by 10 turn 5K R27. I am using the 5 volt supply for my reference voltage even though Peter has taught me better (should be an REF02). The precision of a unit is however only as good as the weakest link and the last time that I checked the "operator" of this unit has a tolerance of 10%.
The relays RL5 and RL6 are mounted to the chassis of the unit. RL5 ensures that there is support power to the circuitry of the unit before "Test Voltage" is connected. RL6 is tied back to the Limits Board which checks for Polarity of input "Test Voltage" as well as monitoring over voltage and over current faults. RL6 is a latching relay and when triggered disengages "Test Voltage" from all of the circuitry except the Volt Meter. SW1 is a manual reset of the latch.
Here are the hand drawn circuit board layouts for this schematic:
The next Schematic that will be covered is the Power Supply board for the unit. The schematic was drawn with the anticipation of finding a multiple secondary transformer which never materialized so the unit will be constructed with two transformers.
One of the things that I wanted to explore with the power supply board was the use of Recom's R78 series DC/DC switching convertors. These are great substitutes for the traditional linear 78XX series of voltage regulators. You will see when I cover actual board construction that while a tad bigger than the 78XX linear series the fact that no heat sink is needed more than makes up for the slightly larger size. I encourage you to check out these components on the Newark sales site. The 9 volt supply on the board is for the power to the meters. The meters that I chose to use require 9 volt isolated supplies. Since each meter only draws 0.5 mA I used a conventional linear regulator in a TO92 case for this application.
Here is the board layout for the Power Supply:
The next schematic that will be covered is for the Limits Board. This is where such fault conditions as Over Voltage, Over Current, and Polarity Reversal are detected and acted upon.
In this circuit we are using an LM339 comparator to measure the Fault Limit condition and feeding the gate outputs to a couple of OR gates which in turn will trigger RL6. The status of the failure is communicated to the front panel where LEDs indicate whether it is Polarity, OV, or OC that is causing the fault.
Here is the board parts layout for the Limits Board:
The last board in the unit is the meters interface and calibration board.
The purpose of this circuit is to take the 0V to 35V input of the unit and the voltage across the sense resistor and convert it to the proper voltage to be displayed by the 200mV full scale meters. The voltmeter will display 0.0 to 199.9 volts and the ammeter will display 0.00 amps to 19.99 amps though the upper range of this capability will hopefully never be displayed. The calibration is accomplished with a resistive divider that allows an adjustment of + or - a little over 10% of calculated top dead center.
Here is the parts layout for the board:
I will be covering construction of the boards in future Episodes but since the Meter Board has already been produced here is a preview.:
Sorry this got a little long but thanks for looking.
John
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