Current Limiting Power Supply in series with other power supplies - Page 1
Forum section: electronics / beginners

author: TechJunkie97
author_rank: Contributor
timestamp: July 10, 2018, 05:59:01 pm
content: Hi,I needed a high DC voltage for a project so I decided to put various (floating voltage) power adapters in series to do that. This made me wonder what would happen if I also put my current limiting power supply in series. Let's consider the following example:Let's say the power supply is set to 1 V and current limit is set to 200mA. The combined voltage of power adapters is 50V and the load wants to draw 500mA (which is more than the current limit) from the combined voltage. Now the power supply will try to decrease its voltage so that the current draw decreases and will eventually set the power supply voltage to 0. But there would still be almost the same current passing through the load (and power supply) because of other 50V in series. I wonder if it will damage the control circuitry of the power supply. What do you think will happen?

author: ArthurDent
author_rank: Super Contributor
timestamp: July 10, 2018, 09:52:04 pm
content: It sounds like what you’re saying is you have an odd pile of leftover power adapters where you plan on wiring the A.C. inputs in parallel and the D.C. outputs in series to get about 50 volts but one of the adapters doesn’t put out the required current. This is not a good idea. The current limiting 200 ma adapter will not only shut down but there is a high probability that the other adapters will try to back feed the remaining voltage into it and destroy it. There are other safety concerns in that any 3-wire input could carry the ground (earth) through to one of the output leads effectively shorting out one or more of the other adapters. This plan is a recipe for disaster.Adapters are relatively inexpensive so just buy one that has the proper ratings. There are some common ones with 48 DVC output on Ebay for $5 to $10 with free shipping. If you really need 50 volts get an adjustable modular 48 DVC power supply. Most can be adjusted +/- ~3 volts

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2018, 07:17:53 pm
content: Good constant current limiting power supplies have a diode across the output so that the output voltage cannot be forced negative far enough to damage the power supply. However once that point is reached, the current is no longer limited.This is also a common feature with bipolar output power supplies so that once side cannot pull the other side past ground.

author: TechJunkie97
author_rank: Contributor
timestamp: July 11, 2018, 08:03:47 pm
content: Thanks AuthurDent for your reply. Sorry, my question was very confusing, I'll try again. When I said current limiting power supply, I actually meant a benchtop lab power supply with constant voltage/constant current mode. I was wondering what would happen if you pass more current through it than the set current limit. One way of doing that is by putting another regular constant voltage power supply in series.Let's say a linear load demands more current than the set current limit. The current limiting power supply will immediately go to constant current mode and decrease the voltage so that the current does not go beyond the set limit. However, let's say the second power supply in series is still providing current more than the set limit of the first power supply.This will definitely confuse the constant voltage/constant current circuitry of the first power supply. what would be the consequences of that?

author: TechJunkie97
author_rank: Contributor
timestamp: July 11, 2018, 08:17:52 pm
content: Quote from: David Hess on July 11, 2018, 07:17:53 pmDavid Hess, it makes sense that there is no current limit at 0V. If another source is passing more current through the power supply than the set limit, then the power supply has no control over the current and will reach 0 V trying to decrease current. There should not be any current limit at 0 V otherwise it would confuse the constant current control circuitry.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2018, 09:56:54 pm
content: Quote from: jaskiratubhi on July 11, 2018, 08:17:52 pmThe current limit should happily work at 0 volts and even slightly negative if the outputs are being driven externally but at some negative voltage, the control circuit will not be able to shut off the power transistor and at higher reverse voltages, damage will occur. Power supplies usually include a diode across the output to prevent damage.