I might want to add a power supply current monitor to a product. The supply is 48 volts and I expect a huge amount of EMI. There are lots of dedicated chips to do this, but just a few that work at 48 volts.
I can do this from parts that we have in stock:
I wonder if that's the best place for C1.
I can put my power LED in series with R2 and save one part.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
INA168 good for 60V in a sot23-5, gain set with a resistor, bandwidth with a cap
Huh, I only 1/2 get it. You need matched transistors? Bob Pease had a current sense circuit with an opamp that looked nice. (I never tried it.)
George H.
INA253? 80V Integrated resistor.
th a cap
TIs page says preproduction and no stock at distributors, so that might be bit young
On a sunny day (Sat, 03 Nov 2018 17:00:41 -0700) it happened John Larkin wrote in :
Interesting circuit. I have somewhere one of those HALL sensors... to measure current. Nice little box, expensive, never used it.
Payed 4 Euro for 2, in army dump...
:-)
Temperature tracking is better if you put an NPN current mirror transistor + diode-connected transistor flipped the other direction at the bottom and then put R3 and R2 at the emitter terminals of those instead.
If you don't mind 0% PSRR.
I used the same circuit in my limiter project,
AD8210 comes to mind. It's pricey but you've never complained about that in your work, that shouldn't be a problem. Good precision, and very good bandwidth (both as signal and PSRR), compared to jellybean current sense amps.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:
My go to part for low amps measurement is the INA270.271. front end is good for 80v and it has an added amplifier with breakouts to add a filter network to get rid of the hash.
With a big shunt drop, hundreds of mV, and modest accuracy requirements, static matching won't be a big problem. At 48 volts, self-heating of Q1 could be. I'd need to keep the signal current low or add a zener or a cascode... more parts.
I suppose I could buy an IC. Adding parts to the PADS library and to the parts database and physical inventory is a mild nuisance. We already have 107 different opamps in stock.
The EMI environment is ghastly (it lights up neon bulbs) so I'd be concerned about an IC. I can't measure temperatures on this board with thermocouples; I have to use an IR imager.
Hey, I'm a circuit designer.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
You should have one or two chips of that type in your set. But one transistor plus an opamp would be a good choice. A sot-23 or sc70 opamp takes no more space than a BJT.
--
Thanks,
- Win
We have several opamps good for 44 volts, but only one higher, OPA552, and it's big and expensive. Creating an opamp v- rail somewhere below
+48 would be a nuisance.I might just buy a current monitor chip, but I thought I'd play with circuits first.
How about this? I need the LED anyhow.
Of course, it needs some caps.
A quad r-pack would help.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Yes, of course. That's much better than an HV opamp.
--
Thanks,
- Win
Cute; I'd like to cascode that PNP transistor, though; if you put three LEDs in series, the '40V' rail could be the negative supply and the base bias for a second PNP. Beta for the top PNP is more stable if the self-heating is kept low, and extra bias voltage could open the op amp options to other jellybeans (even the old LM307).
A volt is more than enough bias for a cascode. Anyway, feedback takes out the Early effect (which is big at 48V), and beta is a second order effect. A Sziklai pair would get rid of that effect pretty well with only a bit more than one V_BE drop (~750 mV), which would fit neatly within the V_F of a LED.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Huh, fun. I didn't know about such low voltage opamps. Can I run it off a single AA battery?
George H.
It's spec'd down to 1.8, but the data sheet suggests that it still sorta works at 1.5.
It also makes a nice slow RRIO comparator.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I have found they're useful for all kinds of things, and especially good for building in diagnostic info.
Recommended.
--
Thanks,
- Win
a long time ago I made this
What part did you use? What part you'd use now? I see the SMA connector, how fast was it? And now?
--
Thanks,
- Win
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