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Topic: currrent limiting (Read 2615 times) previous topic - next topic

tim77777

thanks, but its not a matter of consideration. The enclosures are drilled, painted and printed, the hardware is set, PCBs done, components soldered. It's all done and dusted except for the final assembly and, except for this significant add-on feature, if it can be made to work.

The software of course can still be changed, and some minor hardware can be added to the supplying enclosure, provided it doesn't take up too much room and it is physically stable enough when soldered and hanging of a pin on the TRS connector. In that scenario, a small current limiting resistor is OK.

It doesn't sound like there is much else that can be done within the supplying enclosure.

There is plenty of scope for hardware changes in the receiving enclosure.

I know this might not be the usual way, but what if I sized the limiting resistor such that half the voltage was dropped across it and used a 40mA charge pump in the receiving device. I don't think the current for the uC and opamp circuit varies much, so things should remain stable. If the uC and opamp circuit is drawing 40mA, then can I assume the input resistance is 125 ohms?

Do charge pumps only double the voltage or are there fractional options.

If so, then I size the limiting resistor at 125 ohms/ 250mW and use a 40mA charge pump in the receiving device. Unorthodox, but would it work.


allanhurst

That's horrendous.

Can't you modify the supply side to be foldback limiting as I suggested? I'll hack out a circuit if you'll give me your psu specs/design.

Allan

tim77777

Horrendous or not, would it work?


I don't know what foldback limiting is.

PSU design. It just a AMS1117 with 9 volts in, 5 volts out. Bypass caps on both sides.

MarkT

#48
Jul 17, 2018, 01:47 pm Last Edit: Jul 17, 2018, 01:48 pm by MarkT
I don't know what foldback limiting is.
Its not a state secret...  Its covered in a wikipedia article.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

allanhurst

Are you using a fixed or adjustable version of the AMS1117 ?

Allan

Smajdalf

Every current limiting circuit will cause a voltage drop. You need either some voltage headroom or complicated circuit. But If you can access the 9V voltage directly it should be easy. You can try adding an additional AMS1117. Since it has dropout less than 2V it should have at least 7V at its input to provide 5V. So you can use the 2V for a current limiting circuitry. A 2V/40mA=50 Ohm resistor between 9V and AMS1117 input should work well to not influence output when current is less than 40mA but limit the current to some reasonable value in case of a short circuit. But ofc. it is quite crude (and possibly another regulator with smaller internal current limit would be more suitable). You may Google for "current limiting" and look on images for plenty of circuits - nearly any should provide voltage drop around 1V and current limiting good enough.

tim77777

Are you using a fixed or adjustable version of the AMS1117 ?

Allan
fixed

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