powering op amps - Page 1
Forum section: electronics / beginners

author: m3vuv
author_rank: Super Contributor
timestamp: December 21, 2018, 09:12:45 am
content: hi all,im after using an op amp as a current sense amp on a psu,30v max output,the op amps i have are jrc4558 ic's,the max supply voltage is 18v,i wanted to use them from the same power rail as the psu,obviously i cant do that,now the question is can i power it from say a lm 7824 reg?,the differential voltage is 30v max,can the voltage at the current sense resistor be higher than the op amp supply voltage?hope that makes sense,cheers m3vuv.

author: Ian.M
author_rank: Super Contributor
timestamp: December 21, 2018, 09:51:47 am
content: Its not a rail to rail OPAMP and the datasheet specs the common mode input range as min. +/-12V, typ. +/-13V with a +/-15V supply. That means that, worst case, it will malfunction if either input goes within 3V of the supply rail, and by the time you get to 2V from the rail, you've got 50% odds of it malfunctioning. It certainly isn't rated for operation with its inputs outside the rails.However, you've also misunderstood its supply volttage ratings. Its specced for an abs. max. supply voltage of +/-22V, which means two rails with 44V between them. That would be pushing your luck for normal operation, but I'd certainty expect it to behave with +/-18V supplies, which is 36V between the rails. The only fly in the ointment is the differential input votage rating of +/-18V so if one input is up near the rail, (say 3V below Vcc), the other cant be more than 18V negative of that, which is far above the Vee rail voltage if you are running it from +/-18V rails. At supply voltages above +/-9V you risk destroying it if you ever take its inputs to opposite rails.Its possible to work around these limitations by clever circuit design, and diode clamping to protect the inputs, but that requires multiple matched precision resistors, so its probably better to use a different OPAMP.You'd better provide a block diagram showing exactly where you want to put the current sense resistor, and what supply rails are available, so we can advise you what the options are.

author: rstofer
author_rank: Super Contributor
timestamp: December 21, 2018, 03:59:04 pm
content: Have you looked into 'high side current sense amplifiers'?

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2018, 04:26:02 pm
content: I responded in the other thread. Don't make duplicate threads!https://www.eevblog.com/forum/projects/op-amp-current-sense-amplifier/msg2057128/#msg2057128What are you trying to do: implement current limiting or just measuring the current?

author: m3vuv
author_rank: Super Contributor
timestamp: December 21, 2018, 04:45:36 pm
content: yes its for current limiting with a 723 etc,i want to do it with a low value current sense resistor,ie i need to amplify the voltage developed across the sense resistor,just looking at options to do it,regards m3vuv.

author: m3vuv
author_rank: Super Contributor
timestamp: December 22, 2018, 01:36:26 am
content: im trying to make the current foldback work from about 100ma up to say 20amps adjustable,its on an sg2532 regulator ic,pretty much the same as an lm 723 but only needs 80mv across the current sense resistor to go into current limiting unlike the 723 which needs .6v