Is it safe to use a higher microfarad rating for this device? - Page 1
Forum section: electronics / beginners

author: Razor512
author_rank: Regular Contributor
timestamp: February 10, 2013, 05:44:16 am
content: I am planning on using a TI PTN04050C and plan to run a tiny USB enabled router+ a USB powered external hard drive.the PTN04050C offers more than enough power to handle both devices at peak usage, but the power draw can quickly jump around between 600ma to almost 2 amps at 5VI am planning to make a really cheap portable NAS that can still connect to a wifi network by doing a wireless client mode + AP and offering the external drive as a network share.in the datasheet, it explains that a minimum capacitance of 100 microfarad must be used, and the highest that I have seen in the list is 180 microfarad.I would like to know if it is safe to go higher than 180 microfarad especially since the current draw will jump around rapidly (power usage spikes if the hard drive does random reads (and if there are small files being read and written, then the CPU usage on the device will spike, and it's power usage will also fluctuate pretty quickly (faster than the refresh rate on my crappy multimeter)and if so would it be safe or just a really bad idea to use 2 470 microfarad or (probably even) a 2200 microfarad capacitorshttp://www.ti.com/lit/ds/symlink/ptn04050c.pdfI am planning to power the whole thing from a 15,000 mAh 3.7V li-ion battery

author: Lightages
author_rank: Unknown
timestamp: February 10, 2013, 06:17:14 am
content: You can use any input capacitor above 100 microfarad that you wish, within reason. The data sheet goes into some details about high frequency bypass caps, basically low value caps in parallel with bigger caps to take care of filtering higher frequencies that the bigger caps have a higher impedance. Just remember that your power source that supplies the regulator also needs to charge that capacitor at the start and if the capacitor is too big it could draw more on start up than your power source can supply.

author: jaqie
author_rank: Regular Contributor
timestamp: February 10, 2013, 06:26:16 am
content: Lightages is correct. This is one of the reasons I miss the old linear power supplies - they could handle absolutely huge inrush current when plugged in to charge even the most massive of caps. Nowadays you have to be alotmore careful with adding stupidly huge caps for filtering onto things - perhaps a current limiting circuit before the cap bank if you go with stupid huge caps for filtering? It's what I would do, personally. I like sledgehammer-simple solutions like huge caps for filtering.

author: Razor512
author_rank: Regular Contributor
timestamp: February 10, 2013, 06:30:57 am
content: Will a setup using a 16v 100 microfarad on the input and a 16v 330 microfarad on the output work?or at least handle the rapidly changing loads? (mainly trying to keep things stable when the hard drive starts doing random reads/ writes and the power draw starts jumping around like crazy).

author: Lightages
author_rank: Unknown
timestamp: February 10, 2013, 03:45:28 pm
content: Yes it probably will work. The maximum Co value for an electrolytic is 560 microfarad as stated on the data sheet on page 3. Please read the data sheet completely as it really does spell out the answer to your questions.

author: Razor512
author_rank: Regular Contributor
timestamp: February 10, 2013, 05:36:07 pm
content: Thanks, I guess I will test out a range of caps to see which is most stable. ( thinking about installing a socket so that I can easily swap capacitors without soldering and just run the system when the part comes)I read that section on page 3 but was a little worried when I did not see it in any of their other tables and thus had no idea how it would perform (last thing I want to do is kill a external hard drive and fon router during a file transfer)

author: Lightages
author_rank: Unknown
timestamp: February 10, 2013, 10:55:19 pm
content: First thing you should do with this new power supply is test it with things that you don't care about or things that will not break if it does not behave properly. Your first test of a build should not be the mission critical purpose it was designed for. Use small light bulb, or a power resistor, etc.The other thing is that you will never learn anything if everything works first time. You NEED to make mistakes to understand things.

author: jaqie
author_rank: Regular Contributor
timestamp: February 11, 2013, 02:12:51 am
content: I would have suggested a multimeter first and oscilloscope second, instead of a light bulb.

author: Lightages
author_rank: Unknown
timestamp: February 11, 2013, 04:57:17 am
content: I meant to use the light bulb as a test load while observing the output with a multimeter or oscilloscope if available. Thanks for making me elaborate. Those were needed additions to my incomplete message.

author: jeroen74
author_rank: Frequent Contributor
timestamp: February 11, 2013, 01:02:22 pm
content: It's interesting to see how well it handles load transients. You can use a MOSFET with a resistor or two and pulse generator to apply step loads and observe the output with a scope.IIRC Linear Technology has an appnote on this and how to optimize the compensation network, if there is one.

author: Razor512
author_rank: Regular Contributor
timestamp: February 11, 2013, 06:10:53 pm
content: I will work on finding a better way to generate a transient load (that can at least quickly generate an amp of variation) without actually hooking it up to the equipment I ultimately plan to use it on.currently do not have an oscilloscope but plan to buy one (having trouble finding one that is low cost, was thinking of going with an old that could be uses for testing low voltage power supplies and other simple things)