A couple months ago I ran across a circuitry problem that may interest those with a little electronics knowledge. We were getting failures on a high percentage of new product, but not on all of them. The product was warning that its internal -15v supply was out of tolerance (and faulting). A manual check with a voltmeter showed the -15v supply was within the required +/-10% tolerance. Other internal supplies were monitored as well by the computer and they never exhibited this issue. Below is a schematic of the components involved, what I found and the interesting solution.
What this schematic is showing is three resistor divider circuits that divide down the +15, +5v and the -15 volt supplies to feed the A/D converter inputs (there is some additional filtering and protection not shown). This A/D converter can resolve a range of 0v to +3v signal thus the need to divide down the voltages before conversion. Additionally the -15v rail voltage is "translated" to a positive voltage by raising it with the +5V. The code in the computer then reads the A/D converter and looks to see if the value read is out of +/-10% tolerance.
There is a problem though with the -15v divider circuit. Do you see it? The -15v circuit is dependent on the +5v voltage as well. So for instance if the +5v rail is 8% too high (within spec), and the -15v signal is 8% too low (also within spec), the resulting divided/translated voltage may be more than 10% out of whack (this is a highly technical term). The unit would then erroneously fault.
I found a simple software change that put this issue to rest. We need to first read the +5 rail voltage. Then the -15v rail voltage is read and we subtract out the component of +5v rail voltage from the -15v reading. To do this requires some math and Kirchoff's voltage law.
VAD3 = V+5v - ( V+5v - V-15v) R5 /( R5 + R6)
Next we have to solve for V-15v :
V-15v = (V+5v - VAD3)(R5 + R6)/R5 + V+5v
I implemented this equation in the firmware of the computer and it worked like a charm! I implemented it, but did not consider this solution to be optimal. In subsequent designs I made sure we did not use the +5v supply to translate the -15v supply voltage, but for this product this software solution had several benefits:
There were some other issues with this solution, scaling issues and floating point issues, but those are for another lab note.
- Easy fix for units already in field. No recall.
- No waste for already manufactured inventory. We can use existing stuffed pcb's.
- We did not need to delay an already delayed product.
- No additional cost for the pcb NRE.
Did you know you can get all of my older lab notes by hitting the archive button on my top banner or to the left?