Is "Programmable Cable Drop Compensation" something popular? - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 07, 2023, 10:04:27 pm
content: Something like in this parthttps://www.analog.com/en/products/lt3086.html#product-overviewSeems like a generally good feature, no? This can't be a new idea, why aren't more regulators (linear and switching) offering this?IOW I'm missing something, maybe stability and transient response suffer?

author: Benta
author_rank: Super Contributor
timestamp: August 07, 2023, 11:16:14 pm
content: You're not missing anything.It's just a feature to increse ouput voltage when load current increases. Nothing revolutionary.The downside is, that it's not automatic, you need to know the cable resistance to do it. And, you know, USB cables have different lengths and wire gauge (for the power lines)...But clever marketing from AD. Sounds like a lot more than it is.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 07, 2023, 11:59:21 pm
content: Quote from: Benta on August 07, 2023, 11:16:14 pmMy Tek 7613 has a "sense" line that looks at the voltage delivered at the far end of the (quite skinny) interconnecting cables & "tweaks" the regulator output.Unfortunately, the fault is in the regulator board!

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 08, 2023, 12:26:52 am
content: Oh yes well the context is, as usual, my endless tinkering with my Commodore 64. So basically a 3 foot power cord, no sense lines. It would be difficult to find a cable with 4 conductors for the 5VDC/9VAC plus two sense wires and fit into the 7 pin DIN.I just wondered why cable drop compensation isn't more popular. I suppose I could slap something together with a sense resistor and lifting the ground pin of a three-terminal linear regulator.There must be a reason why this isn't done in general.

author: Smokey
author_rank: Super Contributor
timestamp: August 08, 2023, 12:30:11 am
content: I had a customer one time ask us to increase the 5V rail on one of our products to 5.3V to compensate for whatever long cable they were using. Too bad it was a fixed 5V regulator

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 08, 2023, 12:50:07 am
content: Quote from: Smokey on August 08, 2023, 12:30:11 amCommodore did that trick also in the original brick. You lift the ground pin. Frowned upon in polite circles I guess.

author: tooki
author_rank: Super Contributor
timestamp: August 08, 2023, 03:13:55 am
content: Quote from: Smokey on August 08, 2023, 12:30:11 amMany fixed regulators can be used as adjustable regulators with a resistor divider network, with the “fixed” voltage simply becoming the minimum voltage. (Conversely, one could also consider some classic adjustable regulators as “fixed” 1.25V regulators.)See e.g.https://www.eleccircuit.com/7805-5v-voltage-regulator-datasheet/https://components101.com/ics/7805-voltage-regulator-ic-pinout-datasheet

author: ejeffrey
author_rank: Super Contributor
timestamp: August 08, 2023, 04:54:20 am
content: Quote from: Alex Eisenhut on August 07, 2023, 10:04:27 pmIt's useful if you care about cable drop, can't use remote sense, but do know the lead resistance well enough to cancel. USB with it's tendency to distribute 5V at significant current over skinny wires to low margin devices makes it a good candidate, but in general I'd say it's better to avoid that situation.As for downsides, it's a negative resistance supply.Stability is certainly something to watch out for if you try to implement a similar feature with a standard adjustable regulator.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 08, 2023, 08:12:49 am
content: I'm not sure that the cable compensation mode has any advantages over the Kelvin (Force-Sense) arangement.The load voltage isn't being accurately sensed. Rather it's being inferred...."The LT3086 has cable drop compensation (CDC) functionalitythat allows delivery of well regulated voltageto remote loads using only two wires of known fixedresistance." "Known" being the issue.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 08, 2023, 08:55:53 am
content: If it is what I gather from Benta's remark, then it's been around for at least 40 years. Back then there were DIY projects for PCB drills (which had very flimsy motors back then) and in an attempt to keep RPM stable, motor voltage was increased when the motor drew more current under load. But if you increased the motor voltage too much, then the system becomes unstable, and it just tops out at maximum voltage.And for compensating voltage drop over cables it's pretty similar. Assume a variable resistive load. As soon as you start increasing input voltage because of the current draw, the current draw will also increase because of the increased voltage. But that is with a resistive load. With most electronics the load current will remain pretty stable if the input voltage increases. If the gadget has an SMPS, then input current can even decrease if input voltage rises.So you have to know cable resistance to do it properly, or use feedback wires (which is quite common in Lab power supplies).Also, if you are the manufacturer of a gadget that relies on power input via a cable from a power supply made by some other party, then you can't rely on features like this being present. And because the gadget manufacturers can't rely on it anyway, and thus have to design for the gadget to work with the absolute minimum output voltage of the power supply, there is no reason for power supply manufacturers to add such features. It just makes the power supplies more complicated (and expensive) while adding nothing because the gadgets have their own power supply circuit.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: August 08, 2023, 04:44:52 pm
content: Quote from: Terry Bites on August 08, 2023, 08:12:49 amThe advantage of this over Kelvin connections is in places like a power board that then goes through a connector and cable harness to a load board. This might be any of a thousand applications where the impedance of the connections is known but the addition of extra conductors to the cable harness and connectors is not cost-effective (not to mention needing a regulator that has the ability to Kelvin connect).This is almost never done where extremely tight regulation at the load is needed--minor variations in the conductors as well as the resistor setting the compensation mean that you can't get reference level accuracy at the end of the line. That's almost always done with local supplies. But where you want better delivery of power to the final load, you're basically lowering the power dissipation in a part like the LT3086 to compensate for the power dissipation in the cables.

author: Benta
author_rank: Super Contributor
timestamp: August 08, 2023, 08:32:31 pm
content: Quote from: Doctorandus_P on August 08, 2023, 08:55:53 amThat's exactly what I mean.In the mid-80's, one of my first designs was for a constant-speed controller for dentist micromotors, aka "the slow drill" (as opposed to the high speed compressed air turbines). The design used exactly this technique.It is very effective for constant-voltage loads with varying current consumption, eg, USB peripherals or DC motors.For a resistive or constant-current load it makes no sense at all.

author: magic
author_rank: Super Contributor
timestamp: August 08, 2023, 09:11:51 pm
content: I have seen it in some highly integrated USB chargers, i.e. one chip on the primary doing everything including this.It gets weird when the charger has no fixed cable but a USB plug where anything can be connected