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DOUT + ULN2803


jwillans
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I have a question regarding the configuration described in:

http://www.ucapps.de/midio128/relay_example.pdf

There is only a single ground on the ULN2803, should this be used to ground both the lower voltage switches (from the DOUT) and also the higher voltage switches (shown as relays in the example, but lamps in my case)?  Can different voltages share the same ground in this way (apologies for such as basic question).

James

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Yes, not only can the two device share the same ground but it could create problems if they do not share the same ground.

The idea of ground is very much like providing the foundation for everything. The voltages are all measured with reference to ground.  So it is necessary to use the same ground for everything so the relative voltages remain the same throughout.

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Thanks JimHenry, that makes sense.  As I have indicated there will be two voltages at play - that of the midibox control system 5V and also that of the lamps 20V.  Does it make sense to have two different power supplies for this purpose, or should I use a single 20V power suply which is reduced to 5V for the purpose of midibox?  What is the easiest way of achieving a 20 to 5V reduction?

Thanks,

James

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It probably will be easiest to use two power supplies. The lamps that need 20 volts (not 24 volts?) probably don't need a very regulated voltage level. The 5 volt logic supply should be more carefully regulated and that regulation is provided by the Core module.  So an appropriate wall wart supply to Core will give your 5 volts and whatever DC supply works for the lamps can be used for that. Just be sure the negative sides of both supplies are tied together to be the ground and you have about +9v to the Core and about +20v to the lamps.

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  • 3 weeks later...

I have received a core module + a dout.  I plan to order the extra components to start experimenting with the lamps in the coming weeks.  I'd just like to outline what I am planning to do, to make sure I have it straight. 

I am planning to use two transformers fed from the mains. The first transformer will provide 5V directly into the core module (my understanding is that the core module will do the rectification/smoothing/regulation).  The second transformer will provide 12V to a custom circuit which will do similarly rectify, smooth and regulate this current, this will be fed into the high voltage pins of the ULN2803 mounted on the dout. 

1) The current provided by the second transformer is being fed to lamps via the ULN2803.  Given these are just lamps, can I do away with the rectifiction, smoothing and regulation on this supply.  I am assuming this won't effect the ULN2803s, but will it have an impact since the ground will be shared across the first transformer (this may relate to the second question)?

2) At what point in the circuit should the ground be shared? 

3) I've heard about ground loops, is this something I should be concerned about in the scenario described above?

Thanks for your patience,

James

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If the supply is really 5 volts then it won't be sufficient for the MIDIbox. The Core includes a voltage rectifier and regulator. It needs more than 5 volts to produce a regulated 5 volts. This from the description of the Core at uCapps:

J1 Connection to the powersupply. Either the output of a 7V-10V transformer, or a wall adapter can be used. 500 mA is recommented, especially if a backlit display is used, but MIDIfilter and MIDImerger work also with ca. 100 mA. AC or DC doesn't matter, since the rectifier behind J1 converts to DC anyhow. Also the polarity has not to be taken into account.

The lamps don't care about AC or DC but the 2803s do. You have to power the lamps with DC but it can just be rectified AC. I'd suggest using full-wave rectification. Connect the negative terminal of the 12v rectifier to the systemwide DC ground. Connect the positive terminal to one terminal of the lamp. Connect the other lamp terminal to the outputs of the 2803 which will connect the lamps to ground when ON.

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The ULN "inputs" (really outputs) where you connect the lamps, are solid state switches to ground. Everything else on the ULN is connected either to ground, regulated +5v, or a signal that moves between ground and +5V. If you measure the voltage where the lamp is connected to the ULN when the ULN pin is OFF, you'll see the unregulated +12v. But it is not connected to anything by the ULN. The ULN is open switch at this point. When the ULN closes the switch, there will be a low resistance connection to ground and you will only see a low voltage at the pin.

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  • 3 weeks later...

I’m using the DOUT+ULN to test Wurlitzer devices. My 12V supply is a car battery charger. I connected the 12V ground and the +12V to connectors J3-J6 on the DOUT module. The battery charger ground is the ground reference in the system.

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Thanks guys.  Well this is frustrating, I have uploaded midiox128 and connected in the described way, I am using a multi-meter to test the lamp voltage, since these cannot yet be connected, but I'm not getting the result I expect. 

The PS ground is connected to Vs on J3, all the output pins on J3 read the output voltage (12V) when the unit is powered up. Sending midi messages does not seem to change this state (90 30 7F/90 30 00).  Note that the output on J4-J6 are

Any thoughts on what might be going wrong/things to check?

James

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Some further info that may help.  I have connected the PSU ground to both the Vs pins on the Smashtv dout J3, however looking at the board this takes one ground as input into pin 9 (which is ground) but also pin 10 of the ULN2803, which according to the datasheet is "free wheeling diodes".  Is this correct, I'm not sure what free wheeling diodes are?

James

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Thanks guys.  Well this is frustrating, I have uploaded midiox128 and connected in the described way, I am using a multi-meter to test the lamp voltage, since these cannot yet be connected, but I'm not getting the result I expect. 

The PS ground is connected to Vs on J3, all the output pins on J3 read the output voltage (12V) when the unit is powered up. Sending midi messages does not seem to change this state (90 30 7F/90 30 00).  Note that the output on J4-J6 are

Any thoughts on what might be going wrong/things to check?

James

The outputs are open collector outputs. When they are turned ON the output pins are connected to ground. When the are OFF they are disconnected from anything. You'll need to connect a resistor, 10K would be a reasonable choice to the output pin you want to measure. Connect the other end to some positive voltage, 5 volts would be a good choice for the initial testing. Now the output should read 5 volts when OFF and 0 volts when on.

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Thanks for your continued help.  I'm going to do as you suggest, I'll get a physical lamp and connecting it.

In the meantime I would appreciate it if you could make sure I have understood correctly.  Ground of the PS is connected to ground on the ULN2803, the outputs of the ULN2803 switch ground on and off.  Therefore, if I connect the multi-meter to the positive of the PS and an output pin of the ULN2803 then I would expect to get a + V reading if the ULN2803 has ground switched on and a 0 reading if the output is switched off?

If this is the case, I am always getting a + V reading regardless of whether it is switched on/off.  I can't understand why having a physical lamp/resistor in place of the multi-meter would lead to a different reading?

Thanks again,

James

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I've got hold of a 12V lamp and done some further testing.  Here is a photo of my configuration:

cimg0757gz7.th.jpg

The PS is fed into the connection block on the right.  The positive terminal of the PS goes to the lamp (red wire), and the negative terminal of the PS goes to the ground pin of J3.  The negative terminal of the lamp (black wire) is connected to D0 of J3.  As can be seen from the photo the lamp is off, even though I have sent messages to switch it on.

There are two ground pins on J3 labelled vs, looking at the tracing one goes to common ground, this is the one currently connected in the photo.  The other vs goes to "free wheeling diodes" (whatever that means), if this is also connected to ground then the bulb comes on continuously regardless of messages sent or even if the core has power.  Should this be left disconnected?

Thanks,

James

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I was running a Wurlitzer Chrysoglott just the other day. Here’s how I connected the magnets (in my case, bulbs should behave the same way).

Look at J3. Apart from the D0-D7 there is the Vs and one unmarked (note: this is not another ground! It’s connected to ULN pin 10, which is the +Com).

I have +12V connected to the unmarked (+Com) and to one side of the magnet (or bulb). The other en of the magnet goes to one of the D0-D7.

Ground is connected to Vs. See http://www.ucapps.de/midio128/relay_example.pdf

Also make sure that the orientation of the components are OK

Note the Pin numbering. Pin 1 in the INI file corresponds to D7, Pin 2 to D6 and so on. I think this is how it is, I don’t have access to my setup right now.

I don’t know how your INI file looks like but you can throw lots of Note On/Note off using MIDI-OX and its keyboard function.

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Thanks JimHenry - I'll get one of those ordered.

Per S - That info was really useful - thanks.  I hadn't realised that pin 10 requires +12V as well.  I've now wired this as you describe, although this still does not work - it does answer my question about what should be wired to "common free wheeling diodes".  I'm going to get an LCD display and start diagnosing from there.

James

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