current through digital pot

[brownt]

Hi, I am using a MCP4341 quad digital potentiometer, and I need to be able to pass 5mA through it for the current loop in a MIDI circuit.
Why? The hardware cannot be altered, but I can do some software switching of signals and pass 5volts through one end of the pot to the wiper and out to a MIDI jack.

However, the data sheet states 2.5mA as the maximum current through either the A, B or wiper terminals. If I switch things in software I can run some of the pots in parallel. Will that allow a higher maximum current?

Or, perhaps 5mA constant is not required by MIDI?

Or, is there something else that can be done?

thanks

 

[KlausST]

Hi,

I don't know how the MiDI signals go to which pot, nor do I want to read through MIDI specifications...
Thus it would be helpful to post a schematic with sufficient information about signal flow and signal magnitudes...

Klaus

 

[FvM]

Sounds like a really bad idea. MIDI won't run reliably at 2.5 mA. If you are constrained to abuse the digipot as MIDI multiplexer, you'll need a current amplifier means, e.g. a transistor or logic gate that restores the 5 mA current level after the mux.

 

[brownt]

Thanks. So, MIDI does need 5mA constant, and not peak?
Does having the digital pots in parallel divide the current, or not?

 

[FvM]

MIDI has 5 mA loop idle current. It's flowing continuously while no data is transmitted. Parallel multiple potentiometers could keep the rating. If you don't manage to change their code simultaneously, you might still exceed maximum current. There should be better solution.

 

[KlausST]

Hi,

I still don't know the aim of it all .... and how the circuit should look like.


Klaus

 

[brownt]

MIDI has 5 mA loop idle current. It's flowing continuously while no data is transmitted. Parallel multiple potentiometers could keep the rating. If you don't manage to change their code simultaneously, you might still exceed maximum current. There should be better solution.

Thanks for that. A better way, sure. But not at this stage. The hardware is set. Other than drilling holes and inserting an additional switch, this is the only way. If the product is produced again, we might look at doing it another way. But still, if this way works, I don't see why it can't be kept this way.

Commands to all wipers are sent over SPI directly after one another, all within the same 10mS interrupt, so their is no opportunity for the wipers to get out of sync. There are four pots, so that will spread the 5mA over them. And, if two do get out of sync somehow then the 5mA will still be spread over the other two.

So, aside from it being unusual, is there a problem with it?

thanks for your assistance.

- - - Updated - - -

Hi,

I still don't know the aim of it all .... and how the circuit should look like.


Klaus

That's OK Klaus, thanks for considering. However, I just needed to know whether I could pass 5mA over the digital pots in parallel, and it looks like I can. I'll set up a circuit and do some measuring.

 

[brownt]

oh actually and the wipers cannot get out of sync. The wipers are set to the top of the pot, before the voltage is switched through, and they do not move again until the voltage is removed. I'll take some measurements and see what I get.