Why do so many CCCV PSUs allow setting to milliamps but not to millivolts? - Page 1
Forum section: electronics / beginners

author: NeverDie
author_rank: Regular Contributor
timestamp: May 14, 2023, 02:09:36 am
content: Why one but not the other? Most often the volts can be set to the nearest 0.01v, whereas amps can be dialed in down to 0.001a.I apologize for the vagueness of the question, because I do realize that higher end PSU's do allow dialing in to 0.001v, but, for instance, when I look at the vast majority of power supplies for sale on amazon or aliexpress, most of them only allow dialing the voltage in at a 0.01v resolution--yet many/most of those nonetheless allow setting the CC at a 0.001a resolution.

author: TopQuark
author_rank: Frequent Contributor
timestamp: May 14, 2023, 02:16:19 am
content: Say you have a 30V 3A power supply, 10mV in 30V would be 0.33% of full range, and 1mA in 3A is also 0.33% of full range. In multimeter lingo, both are 3 1/2 digits of resolution.Most of these power supplies probably use DAC of the same performance for CC and CV control, so the amount of resolution they can reliably provide is very similar.

author: NeverDie
author_rank: Regular Contributor
timestamp: May 14, 2023, 03:03:31 am
content: You may be on to something with that. There are some curious anomalies, though. One would be this:where on the PSU it shows voltage to the nearest 0.01v, whereas the PC which is linked to it appears to show voltage to the nearest 0.001v.Then there are others which will sneak in a resolution of 0.001v if the displayed voltage is <10v:which is probably disingenuous given the specs on that particular PSU:

author: NeverDie
author_rank: Regular Contributor
timestamp: May 14, 2023, 03:49:56 am
content: Well, heck, it looks as though even Sigelent may be guilty of the same sort of thing:

author: JustMeHere
author_rank: Frequent Contributor
timestamp: May 14, 2023, 04:10:03 am
content: It's impractical. If you wantmilliampmillivolt resolution, then the load must be next to the power source. Like an inch or two at the most. Most of the time your environment will cause too much noise in the leads.EDIT: Typo fixed.

author: NeverDie
author_rank: Regular Contributor
timestamp: May 14, 2023, 04:58:20 am
content: In the case of millivolts, would not some kind of 4-wire or perhaps differential voltage sense over twisted pair compensate for that?edit: remote voltage sense

author: wasedadoc
author_rank: Super Contributor
timestamp: May 14, 2023, 05:32:35 am
content: Quote from: JustMeHere on May 14, 2023, 04:10:03 amSurely that should be "... want millivolt resolution ....".

author: MrAl
author_rank: Super Contributor
timestamp: May 14, 2023, 05:32:57 am
content: Quote from: NeverDie on May 14, 2023, 02:09:36 amHi,This is a good question and if you want to find the answer just do some testing. Use a multimeter to measure your circuit while you power it with 1.000 amps as set by the meter on the power supply. That might come while setting the volt meter on the ps to say 10.00 volts.Now your multimeter reads 9.900 volts. Why? Because the voltage drop in the two wire leads drops 0.1 volts, and that means higher resolution in the volt meter on the ps is not going to help this situation if it reads instead 10.000 volts as that is not any more accurate.To get a good reading on the ps itself you would have to have two more wires for voltage sensing. Lookup "Kelvin Sensing" for more info.Note the current meter on the ps does not have this problem because your ps and your actual circuit will always be in series, and current in a series circuit is the same throughout the circuit, so the meter on the ps would read the same as an external current meter without any extra leads.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: May 14, 2023, 05:38:45 am
content: Quote from: wasedadoc on May 14, 2023, 05:32:35 amYes, should have proof read better!

author: RoGeorge
author_rank: Super Contributor
timestamp: May 14, 2023, 08:51:43 am
content: Quote from: NeverDie on May 14, 2023, 02:09:36 amBecause wrong model, I guess. Choose one that has the needed resolution. For example, the Rigol DP832 power supply can have (with all the upgrades unlocked) 1mV and 1mA resolution. You can generate 1mV limited at 1mA, if you need that.If you need even finer settings, you may want to search for a SMU (Source Measure Units). Most SMUs will be capable of setting uA and uV easily, either as a source or as a load. In fact, SMUs often work in all 4 quadrants, not just in quadrant I and III (I/III == source/load).

author: JustMeHere
author_rank: Frequent Contributor
timestamp: May 14, 2023, 11:39:41 am
content: Quote from: karpouzi9 on May 14, 2023, 07:56:42 amI'm blaming autocorrect on the second one

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2023, 11:40:08 am
content: Quote from: NeverDie on May 14, 2023, 04:58:20 amRemote sense solves the problem but is not commonly available except in some higher end power supplies. Most applications do not require load regulation that good.

author: mawyatt
author_rank: Super Contributor
timestamp: May 14, 2023, 01:08:32 pm
content: The GPP-4323 has higher resolution voltage readback, altho 1mv for voltage setting. Current readback and setting resolution is 100ua.These are more Lab Grade Supplies, and thus more expensive.Best,

author: MrAl
author_rank: Super Contributor
timestamp: May 14, 2023, 04:14:00 pm
content: Hi,The real problem is not the model number it's the way that the voltage drops in the leads that connect the power supply to the circuit under test. The voltage drop will vary with current output so the meter will never read the correct voltage. You can have a volt meter with 1uv resolution it STILL will not read the right voltage because the circuit will have a lower voltage due to the test leads that connect the power supply to the circuit under test.The only way around this is to see if you can find a power supply with Kelvin voltage sensing for the voltage meter readout. That's the only way. Of course you also need the meter to have the right number of digits. If it goes up to 30v then you need it to read 30.000 volts at the top end, unless it has auto ranging and you don't need 1mv resolution at voltages 10v and up.Usually the best way is to use an external quality volt meter on the circuit itself, not on the very output of the power supply, if you really need 1mv resolution that is.There is one other little problem that comes up however. That is the resolution of the potentiometers that set the voltage output. It seems like you should get continuous adjustment of the output voltage with the pots, coarse and fine, but it's not that simple. Sometimes it is very hard to adjust the output to an exact voltage like 5.003 volts. This is required when doing repeated tests that require setting the voltage to some known value and then taking a reading of the current or some other parameter. If you set it to 5.020 volts for one test, you want the next test to have the same voltage, so you have to set it to 5.020 volts the next time too. The next day you may want to test another unit, so you have to set it to 5.020 volts again. It gets very tricky trying to turn the pots a very very tiny amount clockwise or counter clockwise so you can get to exactly 5.020 volts every time.

author: artag
author_rank: Super Contributor
timestamp: May 14, 2023, 10:23:02 pm
content: Quote from: David Hess on May 14, 2023, 11:40:08 amAnd there you have it. These are all cheap power supplies. See the pictures posted earlier in the thread. They don't have sense terminals. Remote sensing costs more in circuitry, terminals, and users who don't understand how to use them.Note that the current measurement, whilst it needs less in terms of hardware, may not actually operate to the indicated precision.

author: NeverDie
author_rank: Regular Contributor
timestamp: May 14, 2023, 10:36:20 pm
content: Thanks, everybody!