I mentioned back a few posts when discussing using a separate power supply for the meters, that I could use the ICL7660 IC to provide the -5V side of the power supply for the meters, rather than a cascade of negative voltage regulators stepping down from 24V. I’m not exactly sure why the 7107 requires a symmetrical supply, but it makes sense because it will have to perform negative voltage measurements as well as positive. No problem.
I conducted a few quick tests to see where the current is flowing and found some encouraging results. On the -5V line, I was only able to detect about 90µA of current flowing through it when it’s doing nothing. This is encouraging as it means I most likely can use one 7660 IC for two meters rather than shell out for another chip. I say most likely as I am having a hard time finding what the actual max sink current is for the 7660. If someone knows, please let me know. I figure I will just go ahead and build the ammeter (learning from my mistakes with the voltmeter) and attempt to power both from my single 7660 and see if it melts on me. They are replaceable to why not? Even if the sink current doubles (which is what I expect), it will still only be 180µA.
I also measured the +5V and 0V leads and found that the bulk of the current, no doubt to power my humungous 1″ LED displays, travels from +5V to 0V and the -5V rail is just to provide a reference for the meter. Displaying all zeros, it draws 56mA of current.
Soon, it will be time to test and build the control voltage board. I have a junk transformer somewhere on my bench and I will test it’s suitability tonight. If one pair of secondary leads can be rectified to a couple of volts above 24VDC then I’m in business. I know this one to have several secondaries so that even raises the possibility of having separate secondaries for 24V/12V/5V supplies which would be even better. Otherwise, I’ll have to go shopping which I enjoy anyway.
Another thing to do would be to measure the total current draw of 4 relays when they are closed, add the odd 120mA for both meters, and the tiny quiescent currents of the regulators. I expect all of this to fall under an Amp which should do nicely.