7815 linear reg 5V_PP ripple - Page 1
Forum section: electronics / general

author: king.oslo
author_rank: Frequent Contributor
timestamp: May 22, 2012, 11:37:18 pm
content: Hello there,I am prototyping, I cannot spot any problems, but my 7815 regulator has some serious 5VPPoutput noise. The output is swinging between 10V and 15V. I measured the current going from the regulator, it is 21mA. It has the input and output caps.It was working just fine for weeks, but all of a sudden the voltage has begun swinging. I noticed that at open circuit, there is no voltage swinging.What should I check for? Thanks.M

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 22, 2012, 11:42:15 pm
content: QuoteA small load might stabilise it.1 kOhm

author: king.oslo
author_rank: Frequent Contributor
timestamp: May 22, 2012, 11:42:56 pm
content: Quote from: HackedFridgeMagnet on May 22, 2012, 11:42:15 pmEDIT: The current into the load is 21mA.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 22, 2012, 11:48:34 pm
content: Probably the chip is stuffed. Worth trying a replacement if you have one. It shouldn't do that.Is the load resistive?

author: king.oslo
author_rank: Frequent Contributor
timestamp: May 22, 2012, 11:50:01 pm
content: The loads are a couple of VREF ICs and one resistive load.M

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 22, 2012, 11:53:41 pm
content: I guess the other obvious thing to check is the ground.After that I am out of ideas.

author: Psi
author_rank: Super Contributor
timestamp: May 22, 2012, 11:56:52 pm
content: Quote from: king.oslo on May 22, 2012, 11:37:18 pmSomething in your circuit maybe forcing the chip in and out of limiting/shutdown.

author: Unknown
author_rank: Unknown
timestamp: May 23, 2012, 12:33:34 am
content: What does the input voltage waveform look like? What does the current through the regulator look like? You can use a small shunt resistor instead of a current probe for the latter. Trying another regulator is of course also an obvious troubleshooting step, although 78xx regulators are fairly hard to kill except through excess voltage.If you working with one of those solderless breadboard, be sure to check double check all connections. Measure signals on actual pins to verify connectivity.

author: bingo600
author_rank: Super Contributor
timestamp: May 23, 2012, 05:26:16 pm
content: What PSU are you using to feed the 18+ volts into the 7815 with ?I have just been through a similar thing with breadboard circuit , using a wallwart switcher and a 7815 , driving a AD587KN + a led through 1K (to get some load).If i fed the 18v from a linear PSU i had no troubles , but using the switcher the VOUT of the AD587 clearly dropped.I tried tonzz of things on the breadboard 470uF and lots of 100nF , and even 10uH and/or 100 ohm in series with the supply line.... nothing worked.Finally i tried to put 200nF + 100nF directly in the wallwart output wires ,and VOUT suddenly appeared the same as with a linear psu. So i ended up adding 300nF directly on the wallwart wires , and a 10uH on VCC at the breadboard.Strange that the same 300nF didn't work at the breadboard (even over the inputs).So i'm sure my issue was switcher noise creeping into the AD578 and affecting VOUT , maybe on the GND.I know breadboard isn't optimal , and it might even contribute to the problem , but i learned a bit about EMC ... Thanx to my local EE Guru friend./Bingo

author: king.oslo
author_rank: Frequent Contributor
timestamp: May 23, 2012, 05:28:47 pm
content: Problem solved. The regulator was dropping out because the smoothing cap from the bridge rectifier wasn't big enough.M

author: bingo600
author_rank: Super Contributor
timestamp: May 23, 2012, 05:31:54 pm
content: Quote from: king.oslo on May 23, 2012, 05:28:47 pmWhat size did you use ?My ST's just need 330nF (according to the DS) , but i suppose they expect a clean DC./Bingo

author: king.oslo
author_rank: Frequent Contributor
timestamp: May 23, 2012, 05:39:36 pm
content: Quote from: bingo600 on May 23, 2012, 05:31:54 pmI was using 22uF for a 20mA load. I upgraded to 100uF. It was still dropping out. Now I stick in a .2mF!! Problem solvedM

author: Lawsen
author_rank: Frequent Contributor
timestamp: May 23, 2012, 07:06:20 pm
content: I have built linear power supplies for microscope illumination lamp house. I make a tt filter circuit for the v ripple. It consists of two electrolytic 220 microfarads at 200 V capacitors in parallel and toroid choke in series in between the two electrolytic capacitors with the DC output from the full wave bridge rectifier with a heat sink mounted on top.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 24, 2012, 12:50:37 am
content: "The school of experience is a great school,but the fees are high"!Well,now you've learnt something useful-----there is a lot of difference between a nice steady dc source,& rectified ac.Those of us who were brought up in the days when a regulator had to be built up out of discrete components learnt,often the hard way, that it is always important that the input to a regulator is pretty much free of hum.Most regulators can "dynamically regulate"a small level of hum,so that you can get away with it sometimes,but not every time.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 24, 2012, 01:13:00 am
content: Glad you have found the solution to your problem.I must admit I don't understand how a large cap would help with the switched mode supply.Was the switched mode supply voltage initially coming out at 10-15 volts?Or was the 10-15 volts generated by instability in the regulation circuit?One day I may look into it.

author: free_electron
author_rank: Super Contributor
timestamp: May 24, 2012, 01:27:12 am
content: Quote from: vk6zgo on May 24, 2012, 12:50:37 amexperience is proportional with the amount of parts ruinedexperience is proprtional with the number of projects that don't work right...experience is proprtional with the number of projects that do work right...if something doesn't work it is not a failure. you were just successful at making something that did not work !electronics hobbyists are like little kids. only the price of the toys has gone up ...

author: amyk
author_rank: Super Contributor
timestamp: May 24, 2012, 09:57:55 am
content: Quote from: HackedFridgeMagnet on May 24, 2012, 01:13:00 amSMPS outputs are more noisy and need a lot of bulk capacitance (with low ESR) to filter out the ripple.

author: T4P
author_rank: Super Contributor
timestamp: May 24, 2012, 10:00:45 am
content: Well, what amyk said is completely correct.To compensate for higher ESR more capacitance is required, therefore low ESR caps saves space and not sure if it saves cost when it comes to filtering switching suppliesAnyway, you don't learn much in school, neither do you get to experiment what you want, you start off by them teaching you tagboard and wire wrapping plus stupid stuff. Well, they get this analog part done and over and then move on to digital part.The syllabus i'm looking at is pretty terrible, the furthest they cover is the 555, analog has more then just 555!

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 24, 2012, 10:10:01 am
content: QuoteYeah I know that but they normally have bulk capacitance on the output. I don't think I have ever had to put a large capacitance like 200u on the input of a vreg, but if it works why not.

author: T4P
author_rank: Super Contributor
timestamp: May 25, 2012, 04:09:39 pm
content: Quote from: HackedFridgeMagnet on May 24, 2012, 10:10:01 amI have seen a charger for a Li-on headlamp that have nearly zero filtering i can see the LED's flickering when i connect it to my small cheap magnifying lamp

author: king.oslo
author_rank: Frequent Contributor
timestamp: May 25, 2012, 07:49:21 pm
content: Seems i have confused u guys. This is my circuit:230VAC -> transformer (18V) -> rectification -> smoothing (this is where my value was too low).Thanks.M

author: T4P
author_rank: Super Contributor
timestamp: May 26, 2012, 04:39:01 am
content: Quote from: king.oslo on May 25, 2012, 07:49:21 pmWell pulses can confuse regulators