Change a Dc power supply for an AC power supply? - Page 1
Forum section: electronics / beginners

author: Nexo
author_rank: Regular Contributor
timestamp: October 23, 2017, 04:40:14 pm
content: Hello everyone!Today I have a question about concepts and it goes like this: What would happen if a DC power supply in a circuit is changed by an AC power supply with a similar RMS voltage. I would like to understand it. Thank you very much!

author: ovnr
author_rank: Frequent Contributor
timestamp: October 23, 2017, 04:42:38 pm
content: Depends on the circuit. If there's no series diode and given how an AC PSU outputs negative voltage as well, you may expect it to blow up. This is always a nice surprise when you're trying to get something to work.

author: Nexo
author_rank: Regular Contributor
timestamp: October 23, 2017, 05:22:57 pm
content: Thanks.I have two circuit draws, one is a voltage divider and the other one is a current divider. Only resistances. What happens then?

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 05:36:16 pm
content: If the circuit is resistive, then it should work, irrespective of whether it's supplied with AC or DC.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 23, 2017, 05:42:48 pm
content: If the circuit contains only resistors, filament lamps or a universal motor, it will have the same behaviour.Otherwise, it depends of many things...

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 23, 2017, 05:43:06 pm
content: Quote from: Hero999 on October 23, 2017, 05:36:16 pmExcept a resistive voltage divider, when fed with a negative input voltage, produces a negative output voltage. Unless the device this divider is feeding can handle a negative input voltage, you're back to either malfunction or failure being the possible outcomes.

author: Nexo
author_rank: Regular Contributor
timestamp: October 23, 2017, 05:59:24 pm
content: Thank you all!What is the theoretical explanation for this? I mean, why it should or shouldn't be the same in a current divider or a voltage divider. That's what I want to know.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 23, 2017, 06:59:05 pm
content: It's simple. An AC signal (assuming 0V DC bias) rapidly switches between both positive voltage and negative voltage (that's what the "A" in "AC" means, alternating). If attaching a DC supply backwards (so it's a negative supply) would fry your device, then feeding it an AC voltage of the same magnitude would also almost certainly fry it. If, on the other hand, attaching the DC supply backwards would be fine, then attaching an AC supply would likely also be fine. It all depends on whether or not the circuit can handle a negative voltage without malfunction/failure. There's also the issue of peak voltage. You said an AC supply with the same RMS voltage, well an AC supply with the same RMS voltage as DC would have peaks that are 41% higher. So again, can the device handle an input 41% higher than normal? If so, then the peaks from the AC signal would be fine, if not, they may break it.

author: Nexo
author_rank: Regular Contributor
timestamp: October 23, 2017, 07:20:30 pm
content: Thanks, I do get that but I'm no talking about a device, I'm simply talking about a voltage and current divider, two resistive circuits, What's the theory behind the fact of being able two use a DC voltage or an AC voltage with similar RMS?

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 07:59:52 pm
content: Quote from: Nexo on October 23, 2017, 07:20:30 pmThe voltages and currents in the divider circuits depend on the ratio of the resistances. Suppose you have a potential divider, with both resistors the same value. The voltage is always half the input voltage, irrespective of whether it's positive, negative or AC.

author: Nexo
author_rank: Regular Contributor
timestamp: October 23, 2017, 08:42:27 pm
content: I see, so would it be correct to say:1. In a voltage divider, when using an AC power supply instead of a DC one, the output voltage will be a part of the input voltage based on the ratio of the resistors. In the positive semicycle, the output voltage will be positive and vice verse.2. In a current divider, when using an AC power supply instead of a DC one, the output current will be ... how?Will the power (P=V*I) be the same using an AC power supply ?

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 09:10:02 pm
content: Quote from: Nexo on October 23, 2017, 08:42:27 pmIf the RMS voltage or current, of the AC supply, is the same as the DC voltage or current, then the average power will be the same.

author: Nexo
author_rank: Regular Contributor
timestamp: October 23, 2017, 09:12:48 pm
content: Thanks!And what about this point: In a current divider, when using an AC power supply instead of a DC one, the output current will be ... how?Will the flow change? I mean, its direction?

author: Brumby
author_rank: Unknown
timestamp: October 24, 2017, 04:01:16 am
content: To make things very simple - AC is just DC that changes over time (in a cyclical manner).Purely resistivecircuits are easy. They can be analysed at any point in time with a DC inputatthat point in time.When the input is steady DC, then that analysis will apply at all times.When the input is AC, then the analysis will be the collection of (an infinite number of) DC points where each DC point is defined as the input at a specific moment in time.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 24, 2017, 09:37:27 am
content: Quote from: Nexo on October 23, 2017, 08:42:27 pmIn a resistive circuit, the ratio of a divider depends on the values of resistors. So, when using AC instead of DC, the ratio of RMS, peak, mean and instantaneous values remains the same. The resistors do not affect the polarity; it depends on the source. This is the same for voltages and currents.The definition of the RMS value is that it produces the same thermal effect on a resistor, as the same value of a DC.