Need advice on how to read High DC voltages (40+ VDC)

Tired2

Hi everyone.

I have been out of the propeller game for a bit, but I'm finally picking back up where I left off.

I have been designing a motor controller with a friend for a while now, and I have made good progress. Its like the multi cog system was designed for my application... it is very nice.

So, here is the problem I have currently... I have an 8ch ADC chip attached and working properly. I need to feed it 0-5vdc to provide the voltage of my battery pack(s) to the prop.

The prop is running on its own 12vdc supply battery that must be kept separate (isolated) from the rest of the high power section of the controller. So far, it is fully isolated from the driver section and the rest of the main battery packs.

So, my plan is to use a voltage divider to get my voltage down to something usable, but my problem is that I cannot connect the grounds together to get any ground reference... or I'll lose isolation. I have found some linear opto isolators (HCPL-7800) that might do the trick, but I could use some guidance.

I have also heard you can convert the analog voltage to a PWM duty and use a normal digital opto, but how could I do that without having a full DC-DC power supply on the battery pack side? The pack is 40vdc or more, so I'd like to avoid that.

Any suggestions would be very much appreciated.

Also, what is the best way to keep the voltage divider from draining the power pack down all the way over time? Can I get away with a transistor switch? A relay would work, but those get expensive

Thanks!

Rayman

One way to go is to use digital optoisolators to talk to your ADC chip.

As for bleeding the battery with the voltage divider, one way to go would be to use a high resistance divider and then use a buffer amp between the divider and the ADC chip.

Tired2

Thanks for the quick response.

Can you elaborate a bit on how to use digital opto's to convey an analog signal to the ADC?
Its not coming together for me...

For the second part, I planned on using an op amp to buffer the signal on the low voltage side of the isolation, but even if I have a very high resistance divider, it would still drain the pack because its basically shorting the pack at all times.

I = 40/100,000ohm is only like .0004 A, but that still seems too high to be acceptable for a pack to sit at. I guess the pack's own internal resistance will do the same thing... right?

Rayman

If you're using something like a MAX1270, you need 4 digital signals to communicate. What I meant is that you can use optoisolators on those 4 digital signals and leave the MAX1270 on the battery side....

If you're really worried about draining the battery then you probably do need some kind of semiconductor switch or relay to seperate it.

Tired2

Ah, I see. That could work, but I can't have the ADC on that side of the circuit or transients will jack with my readings... even if I get it on a stable supply (DC-DC), I'd end up fowling it all up when I tried to connect my voltage lines to it, and I'd be back in the same boat.

Rayman

You can normally use some simple RC filter elements to get rid of any transients, especially if measuring a DC voltage...