Load testing a dual rail P.S.? - Page 1
Forum section: electronics / beginners

author: gsrokmix
author_rank: Contributor
timestamp: December 19, 2017, 05:41:38 pm
content: Hi,I am building a power supply that will power a microphone pre-amp using an op-amp. I would like to test the p.s. with my DC electronic load. The instrument has only 1 channel, and I was wondering if I can just put a load on the + rail, without putting a load on the - rail? There is also a third rail (+48V) for phantom power, but that rail is normally switched on or off via external circuitry.The P.S. is a linear supply w/ a 120V to 28V transformer. I am using LM7818 + LM7918 regulators. The secondary of the transformer says it will deliver 500mA, which may be cutting it close with 2 pre-amps (its intended use) hooked up, which is why I wanted to load down the rail at 500mA for awhile to make sure the supply is stable.I appreciate any and all advice.Thanks very much!George

author: Ian.M
author_rank: Super Contributor
timestamp: December 19, 2017, 06:04:17 pm
content: A transformer secondary, full wave rectified, feeding a reservoir capacitor can only deliver a DC current of 62% of its RMS current rating. At best you will get a little over 300mA without overloading the transformer. That's assuming two 28V 500mA RMS secondaries.If the transformer secondary is center-tapped, (14V-0-14V) there will be insufficient headroom for 18V regulators at full load, so they will drop out on the ripple troughs. You may not even get 150mA before they start to drop out.If you are using a half wave rectified voltage doubler configuration to get positive and negative rails from a single untapped secondary, it will only be good for a DC current of 31% of the RMS current per rail, so you'll only get 150mA per rail.Also, LM78xx and LM79xx regulators are only rated for an absolute maximum of 35V input, therefore the secondary voltage at minimum load must never exceed 25V. Assuming a mains supply voltage that's 10% high, and a transformer that has 10% regulation, that limits the nominal secondary voltage to 20V RMS with almost no safety margin.IMHO you've got an unsuitable transformer and unsuitable regulators for this project.

author: floobydust
author_rank: Super Contributor
timestamp: December 19, 2017, 06:08:49 pm
content: If you don't have reverse-diodes on the outputs of the Vregs, they will get damaged during this test, i.e. when you connect load from 7818 to 7918, one always is weaker and loses.It was shown here but Fig. 1 seems missinghttp://sound.whsites.net/project05.htm

author: Ian.M
author_rank: Super Contributor
timestamp: December 19, 2017, 06:21:29 pm
content: If your DC load can take 36V, and either the load or the PSU is fully floating, test from the +18V rail to the -18V rail.

author: gsrokmix
author_rank: Contributor
timestamp: December 19, 2017, 07:02:21 pm
content: Ian,I was thinking that the transformer was insufficient, but wasn't sure. Here is a schematic of the P.S.Thanks again!George

author: gsrokmix
author_rank: Contributor
timestamp: December 19, 2017, 07:12:52 pm
content: There are diodes across the regulators. I have attchaed the schematic below.Thanks again!George

author: Ian.M
author_rank: Super Contributor
timestamp: December 19, 2017, 07:22:03 pm
content: Well, that's certainly not going to work - if there's enough secondary voltage to get 48V regulated from the LM317, you'll grossly exceed the maximum input voltage ratings of the LM7818 and LM7918. You haven't shown the transformer - how is it wired to the AC in connector, what's its nominal secondary voltage, and (if you've already bought it) what's its actual measured unloaded secondary voltage?

author: gsrokmix
author_rank: Contributor
timestamp: December 19, 2017, 07:41:08 pm
content: Hi Ian,Yes I see that now.....The transformer I have is a single winding secondary rated at 12VA. What kind of transformer will satisfy the requirements of both the fixed regulators, and the LM317?The values for R1, R2, and R3 are calculated based on the secondary voltage, current requirement, and final regulated voltage. These resistors (3W) are supposed to "waste" some extra power from the transformer to make the circuit more flexible AFA the power transformer goes. That's how I understand it's supposed to work.Thanks again!George

author: Ian.M
author_rank: Super Contributor
timestamp: December 20, 2017, 12:54:54 pm
content: The design is fatally flawed - there isn't enough margin between the minimum input voltage to avoid dropout and the absolute maximum input voltage of the regulators, so unless you grossly over-size the transformer to reduce the difference between its unloaded output voltage and its output voltage when loaded by a bridge rectifier passing your 500mA max. DC load current, you'll run into trouble: If the transformer voltage at full load is enough for 3V headroom for the regulators when the mains supply is at its lower limit , the unloaded output voltage will be high enough to destroy them.The dropper resistors R1, R2, R3 make the situation even worse because the increase the differential between the max no load input voltage, and the fully loaded voltage at the trough of the ripple. However you may still need something there to reduce the regulator max dissipation at full load current. See below.I recommend redesigning using LM317 and LM337 regulators for the main rails, as their 40V abs max input to output differential gives you enough margin for a 20V-0-20V transformer to be practical. For the capacitively coupled 48V phantom power supply, you'll need to stack it on top of the +unreg rail to get enough input voltage. However under high line, minimum load conditions there could then be up to 100V on its unreg rail so you cant use a LM317 regulator there. ASupertex LR12looks to be possible. If it doesn't provide enough current for the number of mikes you want to run, split up the mikes and use multiple regulators, one for each pair of mikes, or even one for each mike for better isolation. You'll probably want a transistorcapacitance multipliercircuit feeding the 48V phantom power regulator(s) to reduce the ripple and avoid hum breakthrough.You may also want a capacitance multiplier feeding each main regulator to reduce ripple breakthrough - depending on how good a PSRR your preamps have. If you Zener clamp their bases at say 28V from the 0V rail, you can also use them as preregulators, to split the full load dissipation between them and the regulators. That would also limit the no-load input voltage to the regulators, allowing you to keep the LM7818 and LM7918 regulators you were originally proposing to use.The average DC current you can draw without exceeding the transformer's RMS secondary current rating depends on the rectifier topology, and whether or not it directly feeds a reservoir capacitor. See Hammond (transformer division)Design Guide for Rectifier Use.You'll need at least a CT 20V-0-20V or 2x 20V, 35VA transformer for 500mA out, (with about 1.6VA margin) and more VA if you need a lot of phantom power current. The effective secondary voltage for calculating the phantom power contribution to the VA is 60V, so you'll need 1VA extra for each 10mA load at 48V.To get enough headroom for the LM337 at full load current and minimum supply voltage you'll need at least 1500uF reservoir capacitance on each main rail. You can ease that requirement slightly by using Schottky 3A diodes in place of silicon 1A diodes in the bridge.LTspice sim of 20V-0-20V 35VA transformer + bridge rectifier attached. N.B. the sim assumes transformer losses can be modelled as pure resistances transferred to the secondary. This isn't actually true except at zero load and the max rated load, but gives 'ballpark' figures that are near enough as long as you design with reasonable safety margins

author: gsrokmix
author_rank: Contributor
timestamp: December 20, 2017, 09:31:10 pm
content: Ian,Thanks so much for taking the time for the very detailed explanation, I appreciate it.I understand most of what you saidGeorge