By Steve Bush 19th August 2021
Seeking something-or-other, I came across a high-side current sense circuit that I had never noticed before.
Here it is (right), in an app note for the OnSemi NCP3020A buck dc-dc converter controller, driving a bunch of leds.
It is used to turn the normally constant-output-voltage chip into a constant current chip.
Q2 to Q5 convert the voltage across Rcs (referenced to the positive rail) into a voltage across R3 referenced to the 0V rail.
I had a search around to make sure I knew how it worked, and a Zetex (now Diodes) app note and data sheet cropped up describing ZDS1009, a monolithic equivalent to Q2 – Q5 (left – I say monolithic, but it might not be, as Zetex was a dab hand at ‘system-in-package’ before that was a thing).
Looks like this one works from a handful of volts up to 30V, and can take a bit of a pounding as, despite normally working at milliamps, maximum current is 1A (forced through the sensing circuit, I think, and much more though the sensing resistor).
Max output offset voltage is 4mV across R4, although apparently 70% of chips have an offset below 300μV.
The app note describes operation in detail.
And the app note also includes Zetex’ clever circuit for speeding up the turn off of a bipolar high-side switch (right).
To highlight a extremely clever current sense circuit, that can be powered from 2.7 – 18V while sensing current through a resisistor at −16 to +80V, take a look at Texas Instrument’s INA19x family (INA193 – INA198) (left).
OnSemi NCP3020A design note (DN05002/D)
Tagged with: current EinW LED drivers sensor
Ah, the OC71: which turned into the far more expensive OCP71 if you scraped the paint off.
Until Mullard started putting opaque silicone in the case rather than clear.
Morning zeitghost. Indeed, by the time I scraped the paint off one, it was opaque 🙁 Grey or pale purple, in my faint recollection – so asthetically tastefull 🙂
I was once vastly amused to see 45MHz oscillations from a 7805 in an Esteemed Customer’s project (student project).
So that’s what happens when you forget to put the caps in the circuit.
Mind you half of them couldn’t understand how a breadboard works, so a 7805 was like magic.
45MHz – mist be some fairly fast transistors in there 🙂 Did the student worship you as a god when the oscillations magically went away?
With enough phase shift you only need a gain bit more than 1 to oscillate 🙂 An OC44 or AF127 could just about manage to honk at 45MHz.
Morning Mike I have not heard those part numbers for a while 🙂 BTW: the first circuit I ever made was germanium – I seem to remember OC71s featured. And indeed, I was being far too excitable…
It was probably a 78L05 since those fit a breadboard without deforming the contacts within.
It was quite impressive seeing the oscillation on a scope.
It certainly oscillated better than the superreg receiver circuit I was playing with at the time.
Er, that’s “superregen” receiver for a 27MHz remote control, fresh out of the 1966 Practical Electronics that didn’t work in 1966 and still didn’t work in 2006 when I had enough kit to see what was (or more to the point, wasn’t) going on.
Morning Zeitghost Superregen – surely the cleverest rf circuit of all time, and the fussiest? Not that I know much about RF – except connecting 13GHz blocks together, which was mostly plumbing followed by staring dumbly at a spectrum analyser…
Yes OC71s for me too (or in fact the TI equivalent 2G371 my dad found cheaper). A bistable multivibrator from Practical Electronics Nov 1964 to flash two bulbs. Definitely a life-changing moment for me when they (eventually) starting flashing on and off.
Mine was a ‘white line follower’ – I made the mechanics too. Sadly, it never followed a line as the circuit I was dumbly using drove the motors with germanium triple Darlingtons. I now suspect leakage turned them on full-time, bypassing the ORP12 photo-resistors. (now, there was a good use for cadmium 🙂
For not-too-critical applications I just used a voltage regulator (e.g. 7805) and a resistor. The 7805 output was connected to one end of the resistor, the other end of the resistor to the load and the 0v terminal on the 7805 (which was not connected to 0v). The voltage across the resistor was always 5v (for a 7805) so the resistor value determined the current output from the 7805. A suitable positive voltage from a PSU was connected to the 7805 input. The load was connected between the resistor / 0v terminal junction and the 0v terminal on the PSU. For very low currents the resistor needed ‘tweaking’, or you could use a preset. Be careful when fast-charging Ni-Cad or Ni-MH ‘D’-cells for 7 hours at twice the C/10 current, things can get quite warm… Many years ago I built a selectable-current (using a 12-way rotary switch) constant-current source with a timer for charging Ni-Cads from single cells up to packs of ten, of various capacities. Although this two-component ‘circuit’ may not be accurate to the pico-amp it did (and still does) very well for portable radio transciever batteries and the kids’ radio-controlled cars, which I enjoyed ‘testing’ now and again.
Hi Luke Hear, nice to hear form you. The magnificent 7805 🙂 Another great design that has stood the test of time. BTW, Marco Reps tested one and dicovered remarkably good noise performance (19:45 in this https://www.youtube.com/watch?v=XpbDMo8an5w). Also BTW, how about an LM317 for charging NiCds with less voltage drop?
Yes the 78xx series were always good designs. But the 79xx not quite so. Some people even assume they are PNP versions of the 78xx circuit, but in fact the output is from the collector of an NPN transistor and when feeding some loads, instability has been seen.
Hi Mike Thanks, I had assumed exactly that, that the transistor was npn in the 79xx. Heck, it is a Darlington too (just looked it up) Will try to remember that. I wonder if it has trouble with ceramic decoupling caps like low-drop-out regs can?
Hi Steve, Luke Hear again, and yes, the LM317 would have made a cooler-running current source at the expense of one more component. But I was a poor broke student in those days, and had aquired a small collection of 7805’s for a nominal fee just before the LM317 and other LDO regulators made an appearance. Parting with more beer vouchers and motor-cycle spares tokens wasn’t an option though – the 7805’s had to get used up first!
I just took a look at the LM317 and there are lots of ready-made LM317 circuits available on PCBs, some for about two quid each. They’d be very tempting if I need one or more for a future project.
Ah ha, Luke Hear, now I understand your entertainment-based design decision 🙂 BTW, I just bought some interesting (Vin=2.8-6V) constant current modules from ebay They have LDO6AJSA written on the pcb and the chip is a CN5711, possibly from Consonance (http://www.consonance-elec.com/pdf/datasheet/DSE-CN5711.pdf) They are sold as led drivers, and are about 10 x 17mm with a trim-pot that can set them up to 1.5A. Not sure if they reverse-block, nor how much power they can cope with – I bought them for a torch rather than charging. I was looking at the circuit in the blog to measure current in something else, with a very wobbly voltage rail!
Your email address will not be published. Required fields are marked *
Get our news, blogs and comments straight to your inbox! Sign up for the Electronics Weekly newsletters: Mannerisms, Gadget Master and the Daily and Weekly roundups.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Electronics Weekly teams up with RS Grass Roots to highlight the brightest young electronic engineers in the UK today.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Tune into this Xilinx interview: Responding to platform-based embedded design
Tune into this podcast to hear from Chetan Khona (Director Industrial, Vision, Healthcare & Sciences at Xilinx) about how Xilinx and the semiconductor industry is responding to customer demands.
By using this website you are consenting to the use of cookies. Electronics Weekly is owned by Metropolis International Group Limited, a member of the Metropolis Group; you can view our privacy and cookies policy here.