Is 120v AC always  around 170v DC? - Page 1
Forum section: electronics / beginners

author: Pcmaker
author_rank: Regular Contributor
timestamp: October 21, 2017, 01:09:12 am
content: On all switch mode power supplies, do all 120v AC power coming in come out around 170v DC after the rectifier?

author: Coceth
author_rank: Contributor
timestamp: October 21, 2017, 01:15:24 am
content: I think so. 120V AC (an rms measurement) translates to a 170V peak voltage (120 x 1.414), so it makes since you are getting 170V after converting from AC to DC.

author: Pcmaker
author_rank: Regular Contributor
timestamp: October 21, 2017, 01:19:35 am
content: Are resistors DC only? Are there no resistors between AC coming in and the rectifier?What usually comes after rectifiers? Resistors?

author: Ian.M
author_rank: Super Contributor
timestamp: October 21, 2017, 01:23:04 am
content: No. due to line voltage variation and type of rectifier circuit. 10% low or high is not exceptional for the line voltage especially in rural areas, and will cause a proportional change in DC out.Also a 115V/230V AC input power supply is likely to have a voltage doubling circuit so will typically give about 330V DC. You can recognise a doubler because there are always two identical big reservoir caps in series and a switch (mechanical or electronic) to short one of the bridge AC terminals to the center connection of the two caps to reconfigure it from a normal bridge (230V) to a voltage doubler (115V),

author: rmacintosh
author_rank: Regular Contributor
timestamp: October 21, 2017, 01:33:27 am
content: the measured 120vac into the rectifier is an RMS value. It has a ~170 positive and negative peak in a single cycle.After the full wave rectifier there are now two positive 170 vpk humps. You would still measure 120v rmsYou measure 170v because the output is filtered out with capacitors to create a smooth DC voltage which sits somewhere 2 diode drops and then some below the Vpk value.

author: ebclr
author_rank: Super Contributor
timestamp: October 21, 2017, 02:04:51 am
content: http://www.learnabout-electronics.org/PSU/psu10.php

author: hermit
author_rank: Frequent Contributor
timestamp: October 21, 2017, 02:13:28 am
content: Quote from: Pcmaker on October 21, 2017, 01:19:35 amThink this one through a little bit. What would resistors do in this case? They would simply waste electricity. It's one thing to burn a few milliwatts off on a PCB but at the wattage an AC runs at the waste would be significant. Manufactures are under the gun to reduce wattage in appliances.

author: Pcmaker
author_rank: Regular Contributor
timestamp: October 21, 2017, 03:01:18 am
content: Quote from: hermit on October 21, 2017, 02:13:28 amThanks. I'm a newbie, so I thought that resistors are used to reduce voltage to whatever voltage they need.I really need to get into the very basics of this. Everything just flies over my head right now. I'm lost as sin wave.

author: Zero999
author_rank: Super Contributor
timestamp: October 21, 2017, 10:13:49 am
content: Quote from: Pcmaker on October 21, 2017, 03:01:18 amA resistor just converts electrical power to heat. The voltage is proportional to the current. Look up Ohm's law.http://www.electronics-tutorials.ws/dccircuits/dcp_2.htmlWhat's after the rectifier is a switched mode power supply. An oscillator converts the 170VDC to a higher frequency AC (higher frequencies mean a smaller, more efficient transformer can be used) and passes it through a small transformer, with a rectifier and filter on the secondary to convert it to a lower DC voltage. An voltage reference and opto-coupler feeds the DC voltage on the secondary, back to the oscillator, which adjusts its duty cycle, to keep the output voltage constant.http://therestartproject.org/wiki/Power_supplies

author: Pcmaker
author_rank: Regular Contributor
timestamp: October 21, 2017, 03:20:47 pm
content: Quote from: Hero999 on October 21, 2017, 10:13:49 amI've known that resistance emits heat, but what is the purpose of converting electricity to heat on a circuit board?

author: hermit
author_rank: Frequent Contributor
timestamp: October 21, 2017, 05:14:32 pm
content: Voltage and current control for the most part. We are generally talking milliwatts of heat 'wasted' so not the same deal as an appliance like an AC.

author: Zero999
author_rank: Super Contributor
timestamp: October 21, 2017, 05:48:16 pm
content: Quote from: Pcmaker on October 21, 2017, 03:20:47 pmUnless the resistor is being used as a heater or to dissipate unwanted energy from an external power source, such as an electric motor breaking, electronic devices, use as little power as possible. Resistors, have many other uses in electronic circuits. They can be used for biasing transistors in amplifiers or as frequency determining components in filters and oscillators. This is the kind of question which you'll be able to answer for yourself, after studying the basics of electronics.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 21, 2017, 06:11:19 pm
content: If it uses active PFC it may get boosted to 300-350 before it gets to the reservoir cap, especially if it's a universal input PSU, which most are these days

author: Brumby
author_rank: Unknown
timestamp: October 22, 2017, 12:34:33 am
content: Quote from: Pcmaker on October 21, 2017, 03:20:47 pmFor 99.99% of electronics, there is nopurposein converting electricity to heat. In fact, heat is theNumber 1enemy of electronics.It occurs as a natural by-product in just about every aspect of electronic circuitry. A fact that we all lament at one point or another.Resistors have, perhaps, the highest profile in this regard because of the way they function ... which brings them straight into the realm of Ohm's Law.One key objective in any circuit design is to minimise the amount of heat generated - and this is done for a number of reasons....- Wasted energy: Why pay for a bucketful of electricity when you only need a cupful to do a job?- Equipment size and cost: Why build a power supply to deliver 10 amps when a more efficient circuit can do the job with 1 amp?- Cooling considerations (again, size and cost): Why build something that requires fan forced cooling and a big heatsink when an efficient design can get away with passive cooling?- Compactness of circuitry: low heat emissions means you can pack more circuitry into less space. A classic example why this is great is the computer - and in particular, integrated circuits like a CPU.

author: amyk
author_rank: Super Contributor
timestamp: October 22, 2017, 02:21:44 am
content: Quote from: Ian.M on October 21, 2017, 01:23:04 amI've always wondered who was the first to come up with that circuit or if there's a patent for it somewhere --- it's quite clever:

author: Old Don
author_rank: Regular Contributor
timestamp: October 22, 2017, 02:35:54 am
content: 110 volt AC voltage varies from 0 volts to about +170 volts and then back to zero and then to -170 volts once per cycle. If it is 50 Hz (cycles per second) then it does this 50 times per second. If 60 Hz then 60 times per second. The 110 or 120 volts measured on a meter is the RMS (root mean squared) or DC equivalent voltage that would produce power as the DC voltage. This is a matter of trigonometry. Area under the curve of a wave vs. constant voltage of DC circuit.Resistors are used for many purposes in circuits and include developing a voltage tap, such as two equal resistors in series to ground will each have half the voltage dropped across each one. Replace one resister with a transistor and the voltage across the remaining resistor will vary with the voltage dropped across the transistor. If you vary how much the transistor conducts then you can make the resistor voltage go from full voltage (transistor is a short) to zero voltage drop when the transistor is cut off and no current flows. Since the power needed to make a transistor is much less than the power the transistor can control then you just made an amplifier with a transistor and a resistor.If you take a LED and place it in a circuit and want the LED to not burn out then you need to reduce the current flow through the LED to some value. Depending upon the voltage of the circuit you can work out a resistor value to limit the current through the LED so it conducts and lights up and yet doesn't exceed its design max current flow.You can use a large value resistor to protest a static sensitive component from static damage by conducting static discharge through the resistor and not the component. Also a million ohm wrist band will "ground" you so you don't damage static sensitive components with your body rubbing against your clothes. that same large value resistor also tries to prevent you from frying yourself as would happen if you used a hand holding a metal pipe in lieu of the grounding resistor in the case of the other hand grabbing if you touch ground and a voltage at the same time.A variable resistor is used to control your sound level when you watch TV, listen to a radio or crank up your stereo.Etc. Etc and Etc.

author: dave_k
author_rank: Frequent Contributor
timestamp: October 22, 2017, 09:41:04 am
content: Quote from: mikeselectricstuff on October 21, 2017, 06:11:19 pm.. even higher than that. 380-410 seems fairly typical.