Last night I was doing the preparatory work prior to the assembly of the final board for the DC Load. This is the board that handles the limits I want to protect like: Over Voltage, Over Current, and Reversed Polarity. I bread boarded the circuit that I presented to you in Episode II Schematics. To my dismay I found that it was not working as I had planned. Since my initial design and bread boarding I had made minor changes. Also my expectations for this circuit had also expanded. Finally it seems that my original bread board tests had been less than comprehensive. The results were that a trip back to the drawing board would be needed.
By 4AM I finally had a working replacement circuit. The most difficult part for me was devising a way to test for polarity reversal. My original circuit had used a comparator circuit but I could only make this work if both Pos and Neg inputs were isolated from the ground of the circuit. This was not the case for this unit so the comparator was out. After a lot of trial and error I built a working circuit using a P Channel Enhanced MOSFET, a diode, an LED, and a couple resistors. My original circuit, that I posted last week, was only designed to trip a latching relay cutting the test input from the Load Unit. In this revised circuit I wanted the fault condition, whether Over Voltage, Over Current, or Polarity to be displayed on a set of indicator LEDs and remain lit until I pushed the reset button. Fortunately some of my original design was useful with the appropriate modifications.
Here is a revised schematic of the Limit Board. I have been over it with a fine tooth comb but with the low level of sleep I probably missed something.
To help understand how this piece of circuit fits into the rest I will attempt to cover the obscure parts. There is a +5 volt denoted as "B" and a +12 volt rail denoted as "C". "M" is the positive input jack of the unit. "M" is in the circuit before the cut off relay. This means that even when the unit has detected a fault and cut the connection to the test equipment the Limit board can continue to monitor the voltage and polarity of the input. If the fault was caused by Over Current the Unit retains memory of this on the lit LED on the front panel but since the latching relay has opened the connection to the Load there is no more Current in the unit. The Reset switch of the unit is a normally closed push button with one pin tied to ground. OC Sense IN comes from the high side of the current sensing resistor. OC Sense Ref comes from a resistive divider on the Main circuit board that sits at a slightly higher voltage than the max voltage available to the main MOSFET's Control circuit. If the MOSFET were to short out this will be detected and the latch relay should immediately open the connection to the unit under test.
I have also made a preliminary board layout for the components. This is hand drawn and fairly crude. I have also packed the components tightly so that I could get them all one a standard 30 column solderable breadboard. The relay (RL7) is actually mounted to the chassis and is displayed in the drawing simply for wiring clarification.
The main purpose of showing the board layout is to illustrate the process that I go through when I build. Since it is heavy with my own conventions and habits it may be difficult to interpret. At this point I am back on track and in a few days I will put together a blog with more details on the building of the boards for this unit.
Thanks for checking it out.
John
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