- I need small shunt resistor due to power dissipation limit under
1/2W, but that reduces the drop across the shunt resistor. Hence Vbe technique fails.
- I can use an opamp differential comparator across the shunt, but the problem is that the high side of the shunt is close enough to the supply voltage and unfortunately there are no truly rail-to-rail input comparators I could find (Even those CMOS ones have 0.2V offset from rail).
Yeah, kinda. instead of wiring the comparator directly across the shunt you use a divider string to bring both sides down just enough to keep it within the rail limitations of the comparator.
Shunt is in the high side line somewhere. One (relatively low value) resistor from each side of the shunt forms a voltage divider with one each relatively high valuer resistor to ground. The junction forms the input to the comparator - all you've done is removed the comparator from the need to sense close to the supply rail by droping the inputs down a little.
You do sacrifice a little gain that way but the open loop gain of the comparator is on the order of 200K so that shouldn't be a problem. I don't see why a pair of diodes or diode strings to just offset the voltage by .6 volts below the high supply rail wouldn't work also if you are just limiting current and don't care about measuring it. Same as the above example but diodes in place of the low value resistors connected to the shunt.
I did something like this awhile back to add an electronic ammeter to a motorcycle. It lowered the signal into my op amps and I had to recalculate the gain but it worked like a champ, Didn't try using diodes though.
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The simple way is to hang an isolated +/- power supply across the + rail and run all the electronics from there. Those little 1W DC-DC converters are cheap enough. john
The LM101 op amp has an input voltage range that includes V+. I've used it in power supplies I designed a long time ago to form part of the current-limiting circuitry. Can you still get them>
Last time I did this, I used a small shunt resistor and an LM7301 r-r opamp as the "comparator". It directly drove the gate of the current-limit fet, although you may need some logic if the current-limiter fet is also a controlled switch. You can power the amp from the + rail and ground, up to 30 volts or so, I think.
Yep, but have a look at linear's LTC6101, it's a high side current monitor, that may be useful to you. You will need a comparator as well, to wiggle the fet
Absolutamente, as long as the limit doesn't have to be very precise. I'd do it with only one PNP up there and pre-bias it a bit so that the shunt only drops a couple hundred mV or whatever is acceptable. Then a resistor from its collector to a FET, NPN or whatever down at logic level, with a bleeder resistor to GND.
If really hard pressed on the BOM budget there is the option of, gasp, running the resistor from that PNP's collector right into a CMOS Schmitt input and let it ease into the VCC substrate diode upon conduction. Needs a bleeder as well.
Don't know about the LM101 - which was specified for the military -55C to +125C temeprature range and always came in the TO-5 can - but Farnell still stock the LM301AN for a euro (in small quantities) - at the moment that's $1.28 in God's own currency.
There is also the Burr-Brown INA159 which is a difference amplifier aimed at this sort of job.
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