lm317+22uF tantalum output == 8mV ripple?? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: February 21, 2016, 11:35:12 pm
content: Hi guys,I noticed that my lm317 gives huge ripple of 8mV peak-to-peakThe output capacitor is 22uF tantalum (2-3Ohm EST). What could be wrong? (scope picture is attached). The load was just 100mA. It looks to me the internal reference is not stable. Or may be I did something wrong? I tried to increase dropout with no luck. Only at ~30mA output the ripple becomes lower 3mV (which a noise floor of the "scope").The part in question is "ON SEMICONDUCTOR LM317MTG"http://www.farnell.com/datasheets/1878384.pdf.PS R4 on the scheme is 240 Ohm, R5 is 5k pot (set to ~3k).PPS it's avoltage doubler, that's why the input ripple is so high (about 6V). It supplies LT3080 (Vcontrol pin) that needs to be a few volts higher than the input voltage.

author: c4757p
author_rank: Super Contributor
timestamp: February 21, 2016, 11:47:38 pm
content: What type of circuit is drawing the load, and what does the input voltage look like?Edit: Ripple rejection in that configuration is specified about 65dB. That implies the input voltage ripple is (8mVpp)(10^(65/20)) = 14.2Vpp - that seems within the realm of possibility, to me; I haven't calculated what to expect from your rectifier there (honestly with that little charge pump configuration I CBA).

author: DrGeoff
author_rank: Frequent Contributor
timestamp: February 21, 2016, 11:49:19 pm
content: What is the ripple on the input?I'd suggest adding 0.1uF ceramic caps between input and ref pin, and output and ref pin, as described in the data sheet. You might also need a capacitor across your trimmer (ie ref and GND). A 1uF-10uF works there.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 22, 2016, 12:06:26 am
content: 8mV is not a great deal of signal level and it is clearly related to the mains frequency. I would be looking closely at the layout of the ground, in particular the path of the current that charges the 470uF. Is the measurement made on the output of the regulator or does it include a ground loop?

author: Audioguru
author_rank: Super Contributor
timestamp: February 22, 2016, 12:31:47 am
content: The ripple is at 50Hz. Then one of the rectifiers is not working because a fullwave rectifier should produce 100Hz then the filter capacitors will filter much better.With the loaded LM317 connected, please post a photo of the input ripple and its DC voltages. Is the output actually 17V? Then the pot is set to about 3024 ohms, not 5k ohms.If you built the circuit on a solderless breadboard then the resistance of the contacts might be causing a lot of the ripple.

author: Whales
author_rank: Super Contributor
timestamp: February 22, 2016, 12:50:27 am
content: Out of curiosity: hook up your scope leads to nothing but a 100K resistor, keeping them in approximately the same spot as when making your voltage measurement. Is there still ripple?

author: exe
author_rank: Unknown
timestamp: February 22, 2016, 09:24:43 am
content: Thank you guys, you are awesome! Now answersQuote from: c4757p on February 21, 2016, 11:47:38 pmYeah, as you mentioned, it's a charge pump and the input ripple is about 6V. So, that is not that far from the specs, just I expected much better results... The load was an electronic load, something like this:http://dev.emcelettronica.com/basic-design-of-an-electronic-load. I think I also measured with a plain resistor with the same results.Quote from: DrGeoff on February 21, 2016, 11:49:19 pmRipple is about 6V. There is plenty ceramic capacitance in the input. I also tried to add some ceramic to the output with no changes in the results. I'll try to add a capacitor accross the trimmer... Just I don't have many high-ESR capacitors.Quote from: Andy Watson on February 22, 2016, 12:06:26 amHmm, it may be a ground issue, but then I'm screwed because I did my best to provide good ground: I chose protoboard with a ground plane and tried to make every ground connection as beefy as I could. The 470uF could be better connected, I thought this is not important, is it? I ensured connection only for LM317 and reference/output sections.I have to check if that ground plane is really good, the ground connection is 2cm away from the LM317.Quote from: Audioguru on February 22, 2016, 12:31:47 amIt's a charge pump. This is not obvious from the scheme, sorry. The output is set to 15V (although I tried a few volts lower as well). Pot is indeed set to 3k, sorry for confusion. On the scheme 5k specifies the pot itself, not the value it is set to (should I add a note to the scheme?). The scheme is build on a protoboard with a "ground plane". It does not ensure too much confidence, though (twin industries 8100-45). Breadboards "proved" several times they don't work for my power stuff(. I'll post more photos.Quote from: Whales on February 22, 2016, 12:50:27 amNo, in this case no ripple. It only starts visible (>3mV p-p) after 30mA. I thought that my electronic load could create problems so I tried a plain resistor of as well, no ripple. But I didn't try to give it a real load because I already burned a few 1/4W resistors. Now I'm quite confident that the problem is not with the load, but with a regulator or my board. Or ground resistance higher than I thought.Another idea: I'll try load more to see if the ripple scales linearly with the load. But with 470uF charge pump it can't give much more than 100mA. I designed it to give just up to 100mA, this is the worst-case expected current consumption.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: February 22, 2016, 09:33:10 am
content: Quote from: exe on February 22, 2016, 09:24:43 amBe sure to put those 0.1uF ceramic caps at the pins of the LM317. They are of no use further away.

author: exe
author_rank: Unknown
timestamp: February 22, 2016, 10:07:42 am
content: Quote from: DrGeoff on February 22, 2016, 09:33:10 amYeah, they are at most 2.54mm away and I use 1uF+0.1uF, but... I just re-read the datasheet and found that the line regulation is about 0.1%. That is 6mV for 6V input variation. So... I'm afraid it works as expected, doesn't it?Should I keep fighting with this beast or just leave it alone? It supplies two LT3080 (that supposed to attenuate this ripple further) and a cheap Chinese panel meter that is known to be imprecise anyway.Just... I feel a little bit unsatisfied with the performance. On the paper, 0.1% line regulation looked good to me, but when it showed up as almost 10mV ripple under a small load (what to expect under rated current??)...

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 10:40:17 am
content: Considering the input ripple and the ripple rejection spec of the LM317, ripple you get is a spot on with what I calculated. Use proper rectifier, not a half bridge crap. Also use a capacitor on ADJ pin, it should reduce the ripple by about 7dB.

author: exe
author_rank: Unknown
timestamp: February 22, 2016, 11:52:13 am
content: Quote from: wraper on February 22, 2016, 10:40:17 amSorry, It's a voltage doubler. I'll edit the topic to stop confusing people. Concerning ADJ cap, it can be any capacitor, right? ESR is not important? Because right now I have only MLCC's of 1n, 10n, 10n, 0.1u, 1u, 4.7u and 10uOkay, I'll make an experiment, anyway, thank you for help.

author: wraper
author_rank: Unknown
timestamp: February 22, 2016, 02:50:42 pm
content: 10uf should be fine.

author: exe
author_rank: Unknown
timestamp: February 22, 2016, 07:41:48 pm
content: Quote from: wraper on February 22, 2016, 02:50:42 pmYeah, it helped a lot if expensive 10uF used. Also, there is a glitch captured on 10uF image. Don't know where it comes from. Hope it's from mains or there is a bad contact to the load...Input ripple is also attached, if someone is still interested in it.BTW, Chinese meter creates 20mV p-p disturbance. I think Ineedwant something better than lm317. But what...

author: exe
author_rank: Unknown
timestamp: February 22, 2016, 08:48:18 pm
content: Hmm, with Cadj the output noise induced by the meter has gone...