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Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
I can't figure out what is going on with this project!!!
I have the following circuit mocked up, IC1 and IC2 are ULN2803A's:
Basically I'm trying to switch my twelve loads (R_load) using 8x signals from my Arduino. To do so, I switch on either B1 or B2 to "select" the proper ULN2803, and then switch on the proper C1 through C6 line to select the load. I simulated this is Pspice and LTSpice and it works beautifully.
However nothing works right on the breadboard.
Initially I didn't have the 12x diodes present, because I didn't think I needed them. But in practice turning on any "C" line made every load on that ULN2803 turn on. So I added the diodes and that appeared to help...
... except now when I look at the voltage at point A, I see some influence from other channels:
None of this was expected in Pspice. Pulse width as shown is about 5us. I tried adding a pull down resistor to each base of the darlington, it lowers the height of the extraneous peaks, but doesn't eliminate them.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I think what you're doing wrong is switching the ground pin of the ULN. :P
When the MOSFET is off and any of the control lines is high it will act like an emitter follower and pull its ground pin up to 5V less a few diode drops. This voltage will then feed back to all the other input pins through the ULN's internal pulldown resistors. It might look ugly, but it should work. I can't see any reason why it wouldn't. Inductive loads might be sketchy, I recommend connecting the ULN's diode pin to the positive rail to catch any spikes.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
What is the problem then? You never specified. Does it fail to switch the loads somehow?
I would gang multiple ULN2803s by driving each one off an octal latch like the 74HC573, or an I2C to GPIO expander chip like the PCF8574. Or simply a bigger micro with more GPIO pins.
We use the PCF8574+ULN2803 combo at work for driving relays off I2C. Be aware that the PCF8574 can't source much current and needs pullup resistors.
If you look at the schematic on page 2 of the datasheet you can see why the inputs would crosstalk when the MOSFET was off, but I can't see why they would crosstalk when the MOSFET was on. What MOSFET are you using? Is the Rds(on) low enough?
Registered Member #43278
Joined: Sat Feb 22 2014, 09:18AM
Location: Sydney
Posts: 9
Turning back to the OP, why did you decide to 'switch' the darlington arrays by controlling their ground pins? Wouldn't it have been more straightforward to use one array rather than two, in a multiplexed configuration, with two mosfets (or BJTs) switching the +24V side of the loads? (You would eliminate one array and the diodes on the inputs but would probably need diodes on the outputs to prevent back circuits.)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The problem with that is that if you put the switches in the 24V rail, you need a high-side driver for them. Not really that complicated (PNP BJT switches would make it particularly simple) but not trivial either.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
More importantly, my loads have a common +24V rail. Thus my "solution."
I'm replacing all the blow stuff tonight and will give it another try to see if it blows again. As far as I can tell everything is working properly..........
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