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Topic: currrent limiting (Read 2621 times) previous topic - next topic

tim77777

If the jack is one third of the way in, the tip is touching the 0V contact and the ring and maybe the shield are also in contact with 0V. No problem there.

If the jack is two thirds of the way in, the tip is touching the 5V contact. The ring and shield might be touching  the 0V contact. I'm not sure if that is a problem or not. In theory, the PWM pin of the Arduino is high impedance, but on the other hand, it is at a higher voltage than the VCC pin, so current could follow back to the VCC pin, which is connected to 0V, or to the GND pin, which is also connected to 0V. But there is also a resistor between the jack tip and the PWM pin, so it won't be a short.

But I think what you are saying is that just before that "two thirds" position, the tip of the jack can sort the 5V and 0V contacts in the socket? If so, there's nothing the Arduino circuit can do to prevent that, it would happen just the same with the potentiometer. Any protection will have to be done behind the socket in the mystery device, I think
thanks but what i am saying is a user can connect the shield to the ring either on purpose or accidentally by plugging it into something not designed for it or by connecting the two together for whatever reason. TRS is a common socket and theoretically they can plug it into anything. Unlike a ac/dc plug power adapter, both the neg and positive are exposed via the TRS plug.

allanhurst

Perhaps you could consider a different power connector which doesn't momentarily short out....  there are  lots.

Allan

MarkT

Replace that connector - TRS or TRRS is not suitable for power at all.

In fact its not suitable for anything really - an accident of history, made particularly bad
by their common use for audio where the only sensible kind of connector is one that connects ground
before signal - unless you like full-volume mains buzz everytime a connector is plugged and unplugged!
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

tim77777

I see. well thanks for all that. Seems like the only solution is to have a fixed cable with the arduino unit, and a TRS on the other unit. It has to be TRS because of the other things that can plug in to it. The fixed cable is not perfect as it limits a users distance. Perhaps it should be short anyway. What distance can I run the arduino's filtered PWM signal over.

Thanks for all the help, as it helped me work through the issue with the short when inserting the plug. I can remove the resistor in series with the 5V line and adequately run the pro mini and opamp circuit now, and run other things being developed that will plug into this expandable system.

PaulRB

#34
Jul 01, 2018, 11:09 am Last Edit: Jul 01, 2018, 11:14 am by PaulRB
Here is an idea:


Look at the type C socket above.
Quote
This three-conductor jack has two isolated SPDT switches. They are activated by a plug going into the jack, which disconnects one throw and connects the other. The white arrowheads indicate a mechanical connection, while the black arrowheads indicate an electrical connection. This would be useful for a device that turns on when a plug is inserted, and off otherwise, with the power routed through the switches.
You could use the switch activated by the tip to isolate the 5V supply until the jack is fully inserted.

PaulRB

#35
Jul 01, 2018, 11:32 am Last Edit: Jul 01, 2018, 12:26 pm by PaulRB
Jalco js5077





5V to terminal 5. Terminal 6 to terminal 3. This would connect 5V to the ring but only once the plug is fully inserted.

Terminal 8 to A/D. Terminal 7 to ground via 10K. Terminal 9 to Terminal 2. This would replicate the switch in your current socket, giving a steady zero reading to the A/D while no plug inserted.

PS. Sorry if I have given some rather confusing explanations earlier in the thread. I had thought for years that "jack" refers to the plug. But I just found out it refers to the socket!

tim77777

Well that is a great solution. Space is tight, but it might fit.

Any idea on how far the PWM signal from the arduino can be transmitted?

PaulRB

#37
Jul 01, 2018, 02:13 pm Last Edit: Jul 01, 2018, 02:16 pm by PaulRB
Well, you are not transmitting the PWM signal, you are transmitting the analog voltage output by your low pass filter. Assuming that does not change very rapidly, then it should transmit quite a long way. A long wire will have some resistance and capacitance, of course, and that will affect the signal measured by the A/D, but I suspect it will be fine for quite a few metres.

tim77777

back on this again. Going around in circles.

How can this be done.

I have 5 volts coming out of an enclosure and entering another enclosure through a patch cable. The voltage needs to be current limited so that a user does not inadvertently short the supply as the patch cable and the various devices that can attach to it are designed to be plug and play and so it is quite possible that a user might plug something unexpected in, or do something to short the supply.

At times the 5v is simply across a potentiometer, with the wiper feeding back to the other enclosure, and for that application the limiting resistor is not a problem. However, the five volts is sometimes used to power an op-amp and uC circuit (about 40mA total) that doesn't like working through the series resistor.
What actually happens is that the output of the op-amp circuit , which is configured with some gain and a 2.5v swing around zero, starts to pulsate. The uC circuit seems to work ok, though I guess it is unstable.

What about a charge  pump, would that make any difference, or what else can be done.

ChrisTenone

back on this again. Going around in circles.

How can this be done.

I have 5 volts coming out of an enclosure and entering another enclosure through a patch cable. The voltage needs to be current limited so that a user does not inadvertently short the supply as the patch cable and the various devices that can attach to it are designed to be plug and play and so it is quite possible that a user might plug something unexpected in, or do something to short the supply.

At times the 5v is simply across a potentiometer, with the wiper feeding back to the other enclosure, and for that application the limiting resistor is not a problem. However, the five volts is sometimes used to power an op-amp and uC circuit (about 40mA total) that doesn't like working through the series resistor.
What actually happens is that the output of the op-amp circuit , which is configured with some gain and a 2.5v swing around zero, starts to pulsate. The uC circuit seems to work ok, though I guess it is unstable.

What about a charge  pump, would that make any difference, or what else can be done.

The problem is the connector, not the current, right?

Why not use a different connector, one that cannot be shorted?
What, I need to say something else too?

allanhurst

Hear hear! ( along with other contributors)..........

Allan

tim77777

because there is no space for another connector, and it wouldn't make enough difference anyway, because the other end of the cable could still be shorted.

it needs to be current limited.

PaulRB

Some voltage regulators have built-in short-circuit protection. Maybe you could use one of those to supply power to the user's device instead of the arrangement you have now?

tim77777

thanks. but the hardware is already set. There is a AMS1117 that has short circuit protection, but the system will still shut down if there is a short, as it would even with a better suited regulator.

allanhurst

#44
Jul 17, 2018, 03:17 am Last Edit: Jul 17, 2018, 03:28 am by allanhurst
If you won't consider a more appropriate connector, I fear I/we can't help you any further unless you want to go to the extent of foldback current limiting. Lab power supplies have this, but it involves extra complexity, not just a choice of chip.

It could be done.


Allan

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