Constant Current Logistics - Page 1
Forum section: electronics / beginners

author: Twistedsnail
author_rank: Contributor
timestamp: September 10, 2015, 01:10:57 pm
content: I've been looking into various constant current circuits and have tested a few and they seem to work great, but only for ridiculously low resistances.Regardless of how fancy a circuit is built, if you want 10mA trough a 10k load you're looking at outputting at least 100v not including the drop over any feedback loops.So how do commercial/industrial lab supplies have a 1A output that supports 100k loads? And do they always generate such high voltages or does it ramp up until the expected current is met?

author: matseng
author_rank: Frequent Contributor
timestamp: September 10, 2015, 01:47:32 pm
content: That would be a big ass unit that would require a bigass fusebox. 100 000 watts - in USA that would be a 1000 amp mains fuse, not something you can find in your average electronics lab or even light industry workshop. :-)

author: Jeroen3
author_rank: Super Contributor
timestamp: September 10, 2015, 02:12:31 pm
content: Unit? Not something I would be having on my desk. Will probably be an oven sized brick on with forklift slots. And not to mention loud fans.

author: vinicius.jlantunes
author_rank: Regular Contributor
timestamp: September 10, 2015, 02:23:15 pm
content: Well, when a PSU has a sticker saying it can do 0-25V / 0-1A for instance, those are the operating range limits - it doesn't necessarily mean it can source 1A @ 25V. That's why power rating is the other important spec to look at when you're buying or using a PSU. If the same PSU from my example above was rated at say, 25W, then yes it would be able to source 1A @ 25V; if it was less than 25W, then not (P = V.i).When you say "how do commercial/industrial lab supplies have a 1A output that supports 100k loads?" - well, I'm thinking they do not say that (if you're talking the usual lab supplies) - they could even be able to output 100V, but at lesser current (i.e. bounded by its power rating).

author: macboy
author_rank: Super Contributor
timestamp: September 10, 2015, 04:17:54 pm
content: A typical lab supply has adjustable current and voltage, with "automatic" switchover constant voltage and constant current operation.In reality, the power supply's circuitry will increase the output current* higher and higher until either the set voltage or the set current is reached. Whichever one is reached defines whether the supply is in constant current or constant voltage mode.A lab supply capable of 1 A constant current mode simply can't supply 1 A through 100 kohm, as to do so would require 100000 V at the output and would require controlling 100000 Watts of power. Let's assume you have a 0 to 50 V, 0 to 5 A supply, and you set it up to output 1 A and crank the voltage. Now you apply the 100 kohm load. The voltage will go up to 50 V (the limit of the supply), and the resulting current will be 50/100000= 0.0005 A,not1 A.* I say that the current is controlled because that is literally the way it works: they have either BJT transistor(s) or FET(s) as pass transistors. Both devices have adjustable pass current based on the control (base current for BJT or gate voltage for FET).

author: jitter
author_rank: Frequent Contributor
timestamp: September 10, 2015, 05:18:46 pm
content: Quote from: Twistedsnail on September 10, 2015, 01:10:57 pmI would be really interested to see such a supply...QuoteAs you already discovered yourself: there's no way around Ohm's Laws. The implication of that law (V = I * R) is that V and I can never be independent from eachother.A laboratory PSU with adjustable voltage and current will work in CV (Constant Voltage) mode as long as the load does not exceed the set current limit. When that limit is reached will the mode become CC (Constant Current).In a sense such a lab PSU is not a current source, rather a voltage source with adjustable current limit. A lab supply will simply switch to CC mode once the current limit is reached and while the load is increased even further, the voltage will drop until the load is a short circuit. Going the other way, it will go into CV mode again.In my work I test measurement instruments used for EIS (Electrochemical Impedance Spectroscopy). These are called Potentiostat/Galvanostat, or short PGStat.In PStat mode it functions as a voltage source, in GStat mode in functions as a current source. Basically CV and CC.Now this is a true current source (in GStat mode) as it will keep the current constant regardless of the load that is connected until the voltage limit is reached (compliance voltage).So e.g. a 20 V/200 mA PGstat could supply e.g. 10 mA into a load in the range from 0 Ohms to 2 kOhms. At 3 kOhms, you would need 30 V to reach 10 mA, which it can't.At 1 mA, the range would be 0 to 20 kOhms. And at 100 mA it would be 0 to 200 Ohms.So now you will probably understand why trying to supply 1 A into a 100 kOhm load is a very tall order.Actually, PGStats can also sink currents, but that not relevant for this topic.

author: retrolefty
author_rank: Super Contributor
timestamp: September 10, 2015, 07:43:44 pm
content: QuoteYes, Ohm's Law prevails.