connecting multiple Korad ka3005p in series?  - Page 1
Forum section: electronics / beginners

author: larkpie
author_rank: Newbie
timestamp: June 21, 2023, 06:39:46 pm
content: Hi all,I'm looking to connect multiple Korad KA3005p power supplies (30V 5A) in series, in order to get higher voltage output ideally up to 120V.I've drawn up a diagram below for how I *think* I should connect two of them, with diodes for protection. I've got a handful of IN5404's, rated at 400V and 3A, which I think should be fine (not planning on going above 3A). I'm not sure if I should also connect the ground & negative terminals of supply B or not?I also drew a diagram of how I *think* I should connect four of them, also with diodes. Same question here about connecting ground & negative terminals of all the supplies.I also read online that I shouldn't connect more voltages than the maximum voltage-to-ground rating of a single supply. I can't figure out what that rating is for this module, so I don't know if 120V is too high or not - it must be in the specs somewhere but I just don't know what to look for.Finally, I'm aiming to control all the supplies remotely from my computer (I've figured out some simple Python code for that). Is it better to "balance" the total voltage across all four, e.g. always make them supply an equal amount of voltage? Or, would it be better to max out supplies at 30V before raising the voltages on the rest of them?Thanks so much!

author: eugene
author_rank: Frequent Contributor
timestamp: June 22, 2023, 01:06:36 pm
content: In both of your drawings you have the load connected between two negative terminals. On top of that, each diode will short the power supply it's connected to.Imagine you had four batteries you wanted to wire in series. How would you do that? Now, ignoring the earth ground and diodes, hook up your power supplies the same way.Having said that, I don't what the Korads can handle as far as having their supp outputs different from ground. I'd expect that if you connected 1000 of them in series something bad might happen, but I don't know what the limit is. I'm sure I've used them in series pairs. I can't comment on the safety of 4 in series. It might be fine.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 22, 2023, 01:56:40 pm
content: Quote from: larkpie on June 21, 2023, 06:39:46 pmYour figures are almost right except they have obvious mistakes (stated by @eugene). You typically can connect PS in series with diodes across them. Of cause, I'm not sure about exactly this type Korad ka3005p. Possibly, there can be something about it in a datasheet to that PS. Sometimes manufacturers state this in a manual like 'You may connect it in series'.About regulating and an equal amount of voltage: I guess, if you need to vary voltage within a small range, e.g. from 100 to 120 VDC range - then it is easier to regulate only one supply. If you need to vary voltage a lot, e.g. 40-120 VDC range - then I guess it is better (easier) to do it synchronously with an equal amount.

author: larkpie
author_rank: Newbie
timestamp: June 22, 2023, 05:41:35 pm
content: Ah, thank you - silly mistakes...I believe I've fixed it now in this newest diagram.I'm still wondering about whether the earth ground terminals should be tied to the negative terminals or not? I usually do this when I'm operating the PS's individually.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 22, 2023, 06:00:36 pm
content: It's better to read the Korad manual.I guess you may ground one of them (and only one). You cannot groundboth(or all of them) because it will be just a short-circuit through a Ground wire.(As for me - I have newer grounded bench power supplies, so I don't see much concern here. I don't say that's good, but that's how I do. One of the reasons - we typically don't have trusted proper Protective Earth wire here, except in new buildings).

author: radiolistener
author_rank: Super Contributor
timestamp: June 23, 2023, 01:36:50 am
content: Since it has halvanic isolation, you can connect them in series. But I don't recomment to use more that 2 PSU, because if there is some mistake you can easy put much more voltage on PSU output that it's designed for... So, for 3 and more PSU result is not predictable...For example if you put 120 V on KORAD KA3005 output terminals, it can damage PSU, because it's output is designed for 30 V.For simplicity analysis, you can replace your DUT with a short circuit and open circuit and see if such circuit can damage any PSU in your circuit...

author: radiolistener
author_rank: Super Contributor
timestamp: June 23, 2023, 01:45:24 am
content: Quote from: Vovk_Z on June 22, 2023, 06:00:36 pmAs I know, KORAD KA3005 doesn't have galvanic connection between ground terminal and +/- terminals. Otherwise it can damage your DUT through ground loops.At least my KORAD KA3005D doesn't have such connection between GND terminal and +/- terminals.

author: Psi
author_rank: Super Contributor
timestamp: June 23, 2023, 02:31:06 am
content: yeah, you just have to be careful you don't accidently pull the input on any of them negative.eg, one psu is on, other is off and you short the series output wires.

author: larkpie
author_rank: Newbie
timestamp: June 23, 2023, 03:55:13 pm
content: Quote from: Psi on June 23, 2023, 02:31:06 amMy understanding is that putting reverse-biased diodes across the inputs will protect against this?

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 23, 2023, 04:29:18 pm
content: Yes, those diodes must protect from load short-circuit. They work at start-up transients too.

author: Psi
author_rank: Super Contributor
timestamp: June 24, 2023, 09:22:37 am
content: Quote from: larkpie on June 23, 2023, 03:55:13 pmUntil the diode smokes itself, yes

author: larkpie
author_rank: Newbie
timestamp: June 26, 2023, 07:08:07 pm
content: Is there a way to prevent the negative voltage? For example, always turning Supply B off before Supply A, or vice versa, etc?I'm confused on how the negative voltage happens in the first place. As long as one PSU is on, shouldn't it always be "pushing" voltage in the correct direction?

author: radiolistener
author_rank: Super Contributor
timestamp: June 26, 2023, 09:28:53 pm
content: Quote from: larkpie on June 26, 2023, 07:08:07 pmPower supply has internal resistance which is very small, almost short circuit. So if you connect them in series and use short circuit as a DUT, you will get one PSU connected to another PSU in reverse polarity. If there is more than two PSU connected in series, it means that high voltage (sum of all PSU in series) in reverse polarity will be applied to each of them. They usually have protection diode on the output, but it can be burned out due to overvoltage or overcurrent...This is why it's a bad idea to use many PSU in series to get high voltage. Two PSU is almost safe for overvoltage, but still not safe for overcurrent. But if you're planning to get 120 V from many KORAD KA3005, if you connect 4 PSU in series it can lead to damage for at least one of them, because KORAD KA3005 is not designed to allow apply 90 V on it's output in reversed polarity.The same you're needs to be careful when charging car battery with a lab PSU. If you connect battery in reversed polarity to PSU, it will lead to overcurrent, because car battery can apply 600-1000 Amps current into PSU. It will leads to burn-out PSU output circuit and so high current can make fire and serious damage. Note, that it will leads to a battery short circuit through PSU output even if PSU output is not enabled.

author: larkpie
author_rank: Newbie
timestamp: June 27, 2023, 06:06:43 pm
content: Quote from: radiolistener on June 26, 2023, 09:28:53 pmWhen you say "use short circuit as a DUT" do you mean having both PSUs on & in series, but not connected to any external load? I'm trying to use the PSUs to anodize titanium (which is why the voltage needs to be so high), but during the anodizing process the circuit is disconnected & reconnected several times, since I'm using it to draw (e.g. the "pen" lifts off the "paper" multiple times). Would this repeated disconnection increase the negative effects of reverse polarity?Does adding an external protection diode help to mitigate the reverse polarity effects? I'm using quite beefy diodes that are rated for up to 400V/3A.

author: radiolistener
author_rank: Super Contributor
timestamp: June 28, 2023, 12:03:35 am
content: Quote from: larkpie on June 27, 2023, 06:06:43 pmI mean thoughts experiment to analyze what happens. And I talked about the case when you connect short circuit instead of your load. Because your load can fault and produce short circuit and it can lead to PSU damage because all your PSU will be connected in series as a circle circuit.Regarding to each PSU, you can assume it as a short circuit for any external PSU.

author: radiolistener
author_rank: Super Contributor
timestamp: June 28, 2023, 12:11:34 am
content: Quote from: larkpie on June 27, 2023, 06:06:43 pmYes, if you install external protection diodes with high enough voltage and high enough current, it will protect your PSU.But 400V/3A is not enough. Since KORAD KA3005 provides up 5 Amps current, such diodes needs to be rated for at least more than 120 V and and more than 5 Amps for a long time (600 Watt power).But note that there is still possible issue with overvoltage damage even with external protection diodes if some PSU will appears connected in direct polarity to a high voltage from exteranl PSU, because it has capacitors on the output and that capacitor has some voltage rating which should not be exceeded. You can add protection by using 5-10 Amps fuse and powerful thyristor which should open when overvoltage happens. In such way opened thyristor make short circuit on PSU output in order to protect it from overvoltage and it leads to fuse blow to protect thyristor from overheating.For example you can get such overvoltage protection from this circuit:If you check all things to make sure that all is connected properly you can use just protection diodes.