Ripple current / voltage, what is too much? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 24, 2018, 10:05:09 am
content: I am trying to learn how I determine max allowable power supply ripple for flawless functioning of a component, out from a datasheet, because I can't seem to find out.Please help:-)Here are two of the component I can't find out:Opto PC619 component:https://www.datasheets360.com/pdf/-3876812092323290973Opto TLP3543Ahttps://www.google.dk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwi_kfXq6J7eAhXBlIsKHWFPCiIQFjAAegQICRAC&url=https%3A%2F%2Ftoshiba.semicon-storage.com%2Finfo%2Fdocget.jsp%3Fdid%3D60317%26prodName%3DTLP3543A&usg=AOvVaw3BW5ay3ild_na7FThE9tP6

author: r0d3z1
author_rank: Regular Contributor
timestamp: October 24, 2018, 10:21:53 am
content: it's a good question, the LED don't like ripple but it will be hard to find such a parameters in the DS

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 24, 2018, 10:24:57 am
content: Ok so I am not blind:-)But what would you say I shall go after, except NUL ripple? :-)Are we talking of m, u, n or p ?

author: Mario87
author_rank: Frequent Contributor
timestamp: October 24, 2018, 10:32:30 am
content: I would try to aim for approx 5%, so on a 12v PSU that would be between 11.4v - 12v, and on a 5v PSU it would be between 4.75v - 5v, but it depends on your load, as you may require some fairly hefty capacitors to be able to filter to within 5%, ultimately though, if you want to eliminate ripple you'll need to use a voltage regulator.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 24, 2018, 10:40:17 am
content: I do not know the exact voltage or current but 2-10V at 500mA is what we are talking aboutI have thought about something like this to filter the voltage and current:

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 24, 2018, 12:26:00 pm
content: Sorry forgot to say that, I have still not got my first Oscilloscope or any other gear, so I am relaying on your expertise:-)Therefore my questions that a scope easily could answer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2018, 02:58:15 pm
content: Why would an opto be concerned about ripple? Their purpose is to communicate signals, presumably if anything they would be rated not by a maximum, but by a ripple transmission spec?Quote from: r0d3z1 on October 24, 2018, 10:21:53 amThe LED can be subjected to any level of AC or DC that is within the bounds of the absolute maximum ratings. If that counts as maximum ripple, then sure, but that would be a very special-case way to rate a very general component.Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 24, 2018, 08:28:28 pm
content: As I understand it, is an Opto made of a LED and a LDR or LDFET. Therefor do I think that a slight dimming or flashing LED can mean a lot for the function and the signal in the receiving part of the Opto.

author: not1xor1
author_rank: Frequent Contributor
timestamp: October 25, 2018, 10:38:12 am
content: Quote from: r0d3z1 on October 24, 2018, 10:21:53 amthen may be they like tea with both lemon and milk like the young Mr Feynman

author: not1xor1
author_rank: Frequent Contributor
timestamp: October 25, 2018, 10:47:12 am
content: Quote from: FriedMule on October 24, 2018, 10:40:17 amlike others have stated: LEDs have no problem with ripple or even short pulses of current and unless you plan for at least 10V of dropout and a lot of power waste a mosfet as a capacitance multiplier makes little sense (unless you supply the gate voltage from elsewhere)

author: cowana
author_rank: Frequent Contributor
timestamp: October 25, 2018, 12:24:31 pm
content: Quote from: r0d3z1 on October 24, 2018, 10:21:53 amLEDs are perfectly happy being driven with PWM, which could be considered as 100% ripple.The datasheet will state the maximum allowed current (usually in continuous, average and pulsed cases) - from there it is just a matter of considering the IV characteristics to convert that into a ripple specification.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 25, 2018, 01:57:54 pm
content: My knowledge is wary poor so I'd trust the smart people, like you on this forum and Mr. EEV:-)My circuit is inspired from this EEV video:https://youtu.be/wopmEyZKnYo?t=633Since I am new and don't have any lab gear yet, could anybody please tell me if my circuit will work at all and if so how much ripple it delivers?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2018, 02:07:00 pm
content: You've drawn it with a MOSFET, which doesn't multiply capacitance at all. Or, actually, following the same design motivation, it still is, but the ratio is nearly infinite; but that's absurd, and in practice, that means the description breaks down and it's better called just a filtered source follower.You aren't apparently filtering anything the LM317 won't handle, so I don't get the point in the first place.How does an opto fit into this?Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 25, 2018, 02:19:34 pm
content: Dave it talking about the advantages of using a N-FET in that circuit in his video, don't know if I have misunderstood.The Opto arrives for two reasons, first I can't find out how to read max ripple for a component, don't know what to look for and the Opto is something I want to use in a smps.

author: mvs
author_rank: Frequent Contributor
timestamp: October 25, 2018, 05:54:29 pm
content: Quote from: FriedMule on October 25, 2018, 02:19:34 pmLarge gain of MOSFET is advantage for this circuit, but it comes together with a large voltage drop.MOSFETs are usually enhancement mode devices with VGS(th) >2V. Single silicon BJT or Sziklai pair will drop only 0.6V.QuoteYou can get some SPICE simulator (LT-SPICE, PSPICE, TINA-TI, etc.) and simulate your circuit or at least parts of it.LT-SPICE calculates around 1.2mV peak-tp-peak ripple after your capacitor multiplier for 10V 60Hz AC input and 10 Ohm load (please see attachment).

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 25, 2018, 06:58:02 pm
content: Wow thanks a LOT!!I have tried to use LTSpice but it do seem hard to use I think.Sorry for my questions but a drop of 2V or more, that will go t heat, right?Dave tells that the FET solution mean that you can get wary high caps-"multiplication" since it don't use any current on the gate and therefor the resistor can be wary high.So the resulting ripple is about 1.2 mV?If I drop the FET and use two BJT in one of the configurations, that should give less heat, but also the same ripples?Can I get even better data in both heat and ripple with other solutions?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2018, 06:59:40 pm
content: Note that the output resistance is 1/Gm, or around an ohm for typical MOSFETs at typical load currents.Output resistance is defined as a change in voltage for a change in (load) current. So, you will get ripple voltage produced from a ripple-current-drawing load.Tim

author: mvs
author_rank: Frequent Contributor
timestamp: October 25, 2018, 08:53:49 pm
content: Quote from: FriedMule on October 25, 2018, 06:58:02 pmYes, its linear circuit, P=U*I.QuoteNo, it is only after cap. multiplier.LM317 does some ripple rejection too... Voltage regulators have PSRR typicaly at least 60dB (1:1000) at 100/120Hz, meaning you do not even need this cap. multiplier stage to get ripple down to few mV.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 25, 2018, 11:41:45 pm
content: Okay I se but how low do ripple have to be if I maybe want to power a sensitive chip?Sorry I am a noob:-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2018, 12:35:18 am
content: What chip is sensitive?Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 26, 2018, 01:00:53 am
content: It could be chips like ADCs and DACs, but also other kinds.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 26, 2018, 08:19:52 am
content: Is the common mode choke placed correct or where would you place it?

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 26, 2018, 12:12:16 pm
content: I have tried to simulate my PSU in LTSpice, what have I done wrong to get that strange result?Have never simulated before!

author: mvs
author_rank: Frequent Contributor
timestamp: October 26, 2018, 03:54:38 pm
content: Quote from: FriedMule on October 26, 2018, 12:12:16 pmGround connection is missing at lower R1 terminal.To simulate common mode choke you need to flip one inductor and add following SPICE directive (Edit->Spice Directive):K1 L1 L2 1Start with normal resisistor as load. Pulsed current source is a bit complex.Quote from: FriedMule on October 26, 2018, 08:19:52 amCommon mode choke is typicaly used in mains input filter of switched mode power supplies to limit EMI.If you are designing power supply with 50Hz transformer, you do not need it.

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 26, 2018, 04:52:41 pm
content: Thank you a lot:-)It changed it but how can the ripple be so low, where is my error now?

author: mvs
author_rank: Frequent Contributor
timestamp: October 26, 2018, 05:25:26 pm
content: You need to add some load. Try to set R4 to 10 Ohm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2018, 05:27:24 pm
content: For a 976k (5uA) load?For a circuit that looks like it's made for, I guess a few amperes maximum load?Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 26, 2018, 05:32:59 pm
content: I am trying to reach 500mA-1MA 2V-10VOhm law, right?10V / 0.500 = 20Ohm?It looks much more realistic:-)What do you say?

author: not1xor1
author_rank: Frequent Contributor
timestamp: October 27, 2018, 07:00:12 am
content: Quote from: FriedMule on October 26, 2018, 05:32:59 pmyour circuit just doesn't make sense1) the source voltage is too low (5V peak)you must take into account:a) drop through the rectifier and ripple at the levelling capacitors (how much depends on the current)b) gate-source voltage must be higher than the Vgs threshold (how much depends on the current)c) drop through the regulator (how much depends on the current)just use any voltage regulator IC for you first experiments, but read other people posts, study and experiment before trying to design any complicated circuit

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 27, 2018, 07:03:38 am
content: Thank you a lot, I'll happy for all your help:-)

author: cowana
author_rank: Frequent Contributor
timestamp: October 27, 2018, 07:07:52 am
content: Your current flowing through R4 is absolutely tiny - which means the capacitors are all basically sitting at full charge, and having to supply significant energy during the 'off' portion of the input waveform.How does it look if you change the value of R4 to 10 ohms?

author: FriedMule
author_rank: Frequent Contributor
timestamp: October 27, 2018, 07:11:17 am
content: If I change the R4 to other then 20Ohm, I only get a flat line to the picho Ohm.I am trying to change inV to 20V and see what happens:-)

author: r0d3z1
author_rank: Regular Contributor
timestamp: October 30, 2018, 10:15:39 am
content: Quote from: not1xor1 on October 25, 2018, 10:38:12 amoh yes, and maybe they like play bongos like Mr. Feynaman

author: spec
author_rank: Frequent Contributor
timestamp: October 31, 2018, 08:15:43 am
content: Quote from: FriedMule on October 24, 2018, 10:05:09 am+FriedMuleAs some of the other members have indicated, afraid you are asking the wrong question here. What you need to determine is what noise plus ripple (N+R) does my particular application require.For example, if you were just turning a 12V relay on, a 12V supply line with a N+R of 2V would be acceptable.But if you were building an an audio preamplifier using an operational amplifier you may need a 12V supply line with no more than 100uV N+R.There is one component where the ripple voltage is fundamentally important. The ripple voltage across a capacitor creates a ripple current through the capacitor. The ripple current causes internal heating and if the ripple current rating of a capacitor is exceeded the capacitor can be damaged. This aspect is most important in the case of power supply reservoir capacitors.In electronic engineering there are basic design rules (BDR), which everybody uses. For example, to decouple an opamp you would use 100nF low ESR capacitors. You do not generally work out what value capacitor to use because in most cases 100nF will be fine.Likewise with power supplies. In most cases a mains transformer-bridge rectifier- reservoir capacitor raw supply is adequate for the input to a three terminal regulator, like the LM317 or LT1086. And providing that you use the components specified in the datasheet, you will have a stabilized supply line more than adequate for most circuits, especially if you add the extra capacitors to increase the regulators performance as shown in the data sheet.Typically, the output supply line from a medium price linear bench power supply would have a maximum N&R of 10mV, so that gives you an idea of the amount of N&R that is generally acceptable.If you do have a circuit that requires very low N&R supply lines, an often used approach is to fit another low noise regulator running from the supply line of the first regulator. Also local decoupling is used.There are many factors that affect N&R: layout, especially the 0V line, component type, screening ...That ends the cracker barrel advice. But I hope you can move on and enjoy your electronics without worrying anymore about ripple.