PS in series and parallel - Page 1
Forum section: electronics / projects

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 18, 2018, 02:00:54 am
content: Ive built 2 of these linear PS (attached)I would like to put them in series and parallel for different projects.It seems that when in parallel, they are designed such as one goes to constant current.When in series the only thing Ive found is that I should put a reversed bias diode accross the output of both.Also, most of my PS projects that use a relatively large capacitor bank take several seconds to turn off until they are fully discharged. How should I adapt the circuit so by powering off it will cut the output instantly?Any tips to help me out?PS: there are some small capacitors Ive omitted but nothing major, just to avoid oscillation

author: Sylvi
author_rank: Regular Contributor
timestamp: December 18, 2018, 02:15:41 am
content: HiYou cannot put two of these supplies in series unless one is completely disconnected from ground. The ends should be labelled as simply "plus" and "minus" unless you designate one unit to always be the grounded one. When in series, the minus of the upper supply ties to the output plus of the grounded supply. In this way, the total output is regulated.In parallel, the supply with the higher voltage will support the load and the other supply will "loaf" doing nothing. At worst, the lower supply might actually load the higher-voltage one, stealing away some of the potential current to the load.As far as having the output caps drain quickly, there are some active ways to do that but it is is best to drain the caps slowly, and this is easily achieved with an appropriately sized resistor across the output: not too low a value to waste significant power and heat, but not so high as to have essentially no effect on discharge time. There should be such a bleeder resistor across the input filter cap. This also protects the bridge from cross-conduction failure were the filter to open.The reverse output diode is a good idea. it is also good to have another across the pass element of each supply, from output cap to input cap. This protects the active devices if one cap discharges faster than the other.

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2018, 02:31:11 am
content: Quote from: AngraMelo on December 18, 2018, 02:00:54 amThe objection to putting them in series seems to be that you have shown a ground symbol on the schematic on the secondary side of the transformer. This is ambiguous and could indicate an intention to connect to a common chassis ground.To put two such power supplies in series you need the output side of each one to be completely isolated and floating. So it's best not to use a ground symbol and just draw the positive and negative rails as lines on the schematic.As long as each supply is floating you can put two in series.QuoteThis is normal. Each supply will be trying to control to the specified output voltage. Since there is one output voltage and two control loops there are not enough degrees of freedom. One control loop will saturate.What you could do, if your load demands more current than one supply can deliver is to set one supply to maximum output voltage so that it is current limited, and then control the voltage with the other supply. You will end up with one supply delivering maximum current and the other supply delivering just enough current to achieve the required voltage. For this to work you need that the supply be able to operate indefinitely in current limiting mode at maximum output current.

author: Audioguru
author_rank: Super Contributor
timestamp: December 18, 2018, 02:42:31 am
content: The power supply circuit has no error amplifier, it just has three emitter-followers in series. Then the resulting voltage regulation will be awful.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 18, 2018, 03:43:09 am
content: Quote from: Audioguru on December 18, 2018, 02:42:31 amIt was initally a simple regulation just to limit the maximum output voltage. I would use it to power audio amps and most of their PS are never regulated anyway. It would just be a bonus feature.But now I might pursue a better load regulation with the error amplifier you mentioned, I dont know of jellybean opamps that go to those voltages and importing specific parts is really a concern for me (crazy taxes and fees)

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 18, 2018, 03:43:52 am
content: The earth is just tied to the chassi, both supplies are floating, it was a mistake on the schematic

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2018, 03:47:05 am
content: Quote from: AngraMelo on December 18, 2018, 03:43:52 amThat's not necessarily a good idea. Suppose each supply is in a metal enclosure and you have connected the output side of each to the enclosure. Then if the enclosures happen to touch while connected in series then sparks will fly.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 18, 2018, 04:39:51 am
content: If both the PS are floating and only their enclousers are earth grounded, how can there be sparks?

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 18, 2018, 04:42:10 am
content: Quote from: IanB on December 18, 2018, 03:47:05 amI see what you mean. The enclosures are made of metal but the part where the output plugs are mounted is plastic. Unless an output wire gets loose and touches the enclosure everything should be fine, correct?

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2018, 05:25:48 am
content: Quote from: AngraMelo on December 18, 2018, 04:42:10 amYes, as long as no part of the circuit on the secondary side of the transformer touches the enclosure. This could include the heat sinks on the transistors. All should be insulated and isolated.