Function generator in series with a power supply - Page 1
Forum section: electronics / beginners

author: dadler
author_rank: Unknown
timestamp: July 04, 2015, 02:15:13 am
content: Are there any gotchas when putting a function generator in series with a DC supply?Any risk if the current draw is significant?I'm working on a power supply and trying to simulate common mode noise/mains noise/etc, such as what might be seen after noisy AC rectification.Is there a better way to go about this?I found the following document (http://cp.literature.agilent.com/litweb/pdf/5990-3466EN.pdf):QuoteThis makes it sound like no big deal, but I'm a bit hesitant to put my DC supply in series with my function gen.Edit: Using DG1032Z and DP832.

author: HalfSpace
author_rank: Regular Contributor
timestamp: July 04, 2015, 09:59:23 pm
content: Hi dadler,Most function generators have a 50 ohm output impedance and having it in series with a power supply drawing a large current will damage the generator.Try this linkhttp://gpete.blogs.keysight.com/2012/09/simulating-power-supply-noise.htmlit may be the solution your looking for.HalfSpace

author: Muxr
author_rank: Super Contributor
timestamp: July 05, 2015, 01:14:56 am
content: Yeah I would definitely either build a simple noise gen as HalfSpace suggested, or get one of those single purpose generators from Ebay.http://www.ebay.com/itm/DC-12V-Noise-Source-Simple-Spectrum-External-Generator-Tracking-Source-SMA-/251953200778?pt=LH_DefaultDomain_0&hash=item3aa994be8ahttp://www.ebay.com/itm/ELECTRONICS-SALON-Pink-Noise-Generator-Module-Assembled-/201218811101?pt=LH_DefaultDomain_0&hash=item2ed99364dd

author: edavid
author_rank: Super Contributor
timestamp: July 05, 2015, 04:28:17 am
content: Quote from: HalfSpace on July 04, 2015, 09:59:23 pmThis does not make sense. The current is limited by the function generator output impedance.What can possibly cause damage is if you apply an external voltage to the function generator output, e.g. if the load is shorted while the power supply is on.

author: VK5RC
author_rank: Unknown
timestamp: July 05, 2015, 04:47:18 am
content: The way I look at it the relatively high output impedance of the frequency generator will limit it's ability to put a low impedance 'noise' on top of the DC signal, how about using a 'crappy" smpsu with lots of spikes (prob rel low impedance) and filter them ' a bit'.

author: HalfSpace
author_rank: Regular Contributor
timestamp: July 05, 2015, 11:25:17 pm
content: Quote from: edavid on July 05, 2015, 04:28:17 amHi edavid,Depends on what the power supply voltage is set to and what the load impedance is. If we have a 30V supply and a 30R load and we insert in series a 50R function gen, apart from the load now only getting 11.25V across it the function gen has the rest. 18.75v squared divided by 50R equals 7.03 watts being dissipated in the function gen output circuit which could do it some damage.HalfSpace

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 06, 2015, 10:11:36 am
content: Quote from: HalfSpace on July 05, 2015, 11:25:17 pmThat's right. It pays to understand the output circuitry of your FG; some are a lot more robust than others. My old Racal-Dana / Interstate F43 has a couple of large-wattage resistors that establish the output impedance, so I can get away with series power connections that might damage more modern digital equipment. But in general, FG offsets can be established by connecting a battery or PSU in series with the FG's output and load.The Agilent document attached below may be of interest.