SMPS - Dummy load for when main load is turned off, but PSU stays on. - Page 1
Forum section: electronics / projects

author: WanaGo
author_rank: Regular Contributor
timestamp: March 16, 2017, 07:14:51 am
content: Hello,I am building an amplifier which runs off 2x 48V 600W SMPS's.When there is no audio coming into the amplifier, I am switching off the amps, to prevent any speaker hiss etc.Anyway, the issue I have is, when the main load of the amps come off, the PSU's go into some sort of low power mode, and make alot of switching noise, that is very audible. The output stays at the right voltage, but the audible noise is just too much.Basically what I want to do, is apply a dummy load to the PSU's, so they are always under some sort of load, whether its Amplifiers, or resistors, just so they remain quiet at all times.I cant turn them off, due to the nature of the system, as that voltage is used elsewhere, but at very small current, so its not enough to load the SMPS's itself.So I am wanting to put in some large wattage resistors, but I am unsure how you go about calculating what resistance would be suitable.I have seen projects where people use old computer power supplies, and have loads on those, but some of those are as low as 10ohm, and pull about an amp or so.I am just unsure to what level I need to go for 48VDC, to achieve what I want.Any suggestions?I was thinking that when I get my system to shut off the amps when the input signals go away, at the same time switch in the load resistors. That way, I guess the resistors will not be a factor when the amps are actually used.Thanks in advance

author: jeroen79
author_rank: Frequent Contributor
timestamp: March 16, 2017, 09:24:26 am
content: You must first determine the minimum current required to stop the switching noise.Then, if you want to use fixed resistors, you can use Ohm's law to calculate the required resistance and wattage.V/I=RV*I=PYou could also make an adjustable load with a transistor and an adjustable resistor between the base and the PSU's rail.

author: WanaGo
author_rank: Regular Contributor
timestamp: March 16, 2017, 09:31:56 pm
content: Hi,Yes I understand that part. What I am asking is how do I determine this current to stop the noise.Short of buying a ton of expensive large wattage resistors and just trying them until I find the combo which works.Very familiar with ohms law.Thanks

author: edavid
author_rank: Super Contributor
timestamp: March 16, 2017, 10:31:41 pm
content: Quote from: WanaGo on March 16, 2017, 09:31:56 pmOne way is buy or borrow an electronic load or power decade box.Another way is to buy a large bag of 4.7K 1/2W resistors (or derate as you like), and connect them in parallel until the noise goes away.

author: jeroen79
author_rank: Frequent Contributor
timestamp: March 17, 2017, 01:02:49 pm
content: You could use the attached amplifier to determine the needed current.Just turn up the volume until the PSU no longer makes noise and then measure the current.Or you make an adjsutable load from a transistor (BJT or FET) and then turn up the current until the noise stoprs.Then you can just leave it in place.Can you find a datasheet for the PSU?

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 17, 2017, 05:20:41 pm
content: Quote from: WanaGo on March 16, 2017, 07:14:51 amI remember going to an old audiophile's workshop when I was a teenager and he used incandescent light bulbs in his homemade amplifier for loads.. crazy right?

author: stevelup
author_rank: Regular Contributor
timestamp: March 17, 2017, 09:14:14 pm
content: Isn't this terribly inefficient? Why not just shut down the SMPS when not in use and have a small standby PSU if you need it.