Double power supply by mistake - Page 1
Forum section: electronics / beginners

author: christerb
author_rank: Newbie
timestamp: July 18, 2019, 08:53:43 pm
content: Hi,By mistake I supplied 5 V DC to a PCB from two different sources at the same time. The sources were a TTi EL183 PSU and an FTDI converter cable (TTL-232R-5V). On the PCB I had connected VCC to the corresponding pin on the cable connector. I was "half-aware" that the FTDI cable supplies 5 V, but since I had used it before in labs without a problem I did not think more about it. I also noticed the low current displayed on the PSU but still I did not react. It was not until I tried to measure the current drawn by the PCB I discovered that current was supplied through the FTDI cable. I even measured a few mA going into the PSU. I had no problems with the PCB, it worked as intended. Perhaps someone could explain the reasons for this behaviour (not mine...).

author: james_s
author_rank: Super Contributor
timestamp: July 18, 2019, 08:56:59 pm
content: I'm not sure I understand the question. If two power sources are connected to the same load, the one with the higher voltage will carry the load. Some power supplies are tolerant of this, others may be damaged but if they both produce a similar voltage very little current will flow.

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2019, 09:41:42 pm
content: when you connect two power supplies together, the one which has higher voltage will be power source and the one which has lower voltage will be load. It may damage the one which has low voltage.For example, if you connect power supply to accumulator, the accumulator will starts to charge. But if you will not disconnect it in-time it will damage your accumulator.Other power supplies can be damage immediately. Some can work in such mode with no issue. So, it depends on power supply if it supports such mode.

author: christerb
author_rank: Newbie
timestamp: July 25, 2019, 09:10:30 pm
content: Ok, thank you. In my case both power sources were at 5 V. I measured a small current (2-3 mA) going into the PSU. It means there was a small voltage difference and the PSU became a load. I don't know if it is desgned to tolerate this, but it seems fine. I will have to ask TTi.

author: MosherIV
author_rank: Super Contributor
timestamp: July 25, 2019, 10:26:26 pm
content: QuoteThey sure are!Bench PSUs are designed to withstand a small amount of abuse.The normal way to do it is with a bypass diode like thishttp://www.learnabout-electronics.org/PSU/images/78xx-protection-diode.jpg

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2019, 08:06:06 am
content: If two power supplies are connected in parallel and the voltage difference between them is small <0.5V, it's unlikely any damage will be done. The power supply with the higher output voltage will simply supply most of the current. Going from the original post, this is exactly what's happening here. Disconnect both power supplies, measured the voltages and you'll no doubt find the one with the lower voltage isn't supply much current.Connecting two power supplies in parallel isn't good practise, as it can result in instability and oscillation and is not recommended, unless the power supplies are specially designed for it.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 28, 2019, 10:00:57 am
content: Under development conditions I often have a lab PSU connected to my mcu circuits plus a VCC from an USB-UART chip or a mcu programmer; I never had any problems - it's surely not good practice and should be eliminated with the use of diodes in the final circuit, but it never caused any damages.