Understanding what's going on with my PS - diyAudio
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Old 24th October 2012, 04:25 PM   #1
Carlp is offline Carlp  United States
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Default Understanding what's going on with my PS

The background:
I've been running the amp on a SMPS capable of 5-6 amps with no problems, but wanted to play around with a standard, well filtered PS. I'm just trying to learn all I can.

So, I built a Pi filtered PS not unlike the PS in the amp design (but CRC instead of CLC) with a 25amp bridge into a 470uF/50v cap parallel to a 8200uF/50v cap. That goes into a 5 ohm/50w resistor then another 8200uF cap. PS output goes to the V+ input for the ZCA amp. After that it goes past some more caps, through a 15ohm load resistor and then to the mosfet/output. Simple enough. I'm trying to get about 24v for B+ at the "top" of the load resistor and drop that to 12v at the mosfet.

Right now it's running at 22.5v at the end of the PS/top of the load resistor. At the output of the rectifier it's 30v. That means the 5ohm resistor is dropping 7.5v, and therefore passing 1.5A. What concerns me is the 5ohm/50W resistor is getting quite HOT. Within a few minutes it's too hot to touch.

1.5A means it's dissipating less than 12W, and that doesn't seem to me to be a big deal for a 50W resistor. So why is it getting so hot? Is there danger of frying it? Should I heatsink it? Something else?

Also, at 1.5A, that 15ohm load resistor SHOULD be dropping 22.5v, correct? But I'm reading 11.6v or so at the bottom end of the load resistor, so dropping about 11v through the load resistor and running 0.73A. How is that possible? If the PS is running 1.5A, I thought that had to go through the load resistor, and according to ohm's law, that would mean that resistor should burn 22.5v, right?

I'm so confused!

Also, I'm hoping to play around with a LM317 regulated supply using virtually the same filtering and a tip3055 to up the current rating (something like this: Build an Adjustable 0-34 volt power supply with the LM317, with the filtering preceding the regulator). Is there anything I should know about doing this?

Thanks so much!
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Old 24th October 2012, 09:16 PM   #2
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I'm guessing you're measuring with a multimeter and that the multimeter is getting confused by ripple---if so, check whether the meter's true RMS and what its bandwidth is. Measuring with a higher bandwidth tool (like an RTA or oscilloscope) or simulating in Spice will give you more insight into what's going on.

Check the thermal derating on your resistor's datasheet. 50W parts often derate to zero at 250C so, assuming the usual derating starting 25C, the expected Trise for 12W would be (250C - 25C) * 12W / 50W = 53C. One usually wants to design for a Trise of 40C or less.
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Old 25th October 2012, 12:02 AM   #3
dmills is offline dmills  United Kingdom
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Also, that 50W is usually on a specified heatsink if the resistor is one of the common gold coloured ally cased ones, it will not do 50W without a heatsink.....

Actually, too hot to touch is not automatically all that much of a problem, that usually corresponds to 60C or so, no big deal for a modern power component (If the solder melts in use you might have a problem!)....

Regards, Dan.
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Old 25th October 2012, 12:10 AM   #4
AndrewT is offline AndrewT  Scotland
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the R in the CRC is far too high.
Expect the R to drop <<10% of the total available voltage. A target value for reasonable filtering may be around 1V of Vdrop for a 35Vdc single polarity supply to a ClassA single ended amplifier.

If you are passing 1.5A then 0r68 would be about right.
Pd would then be about 1.5W and a 5W resistor would be adequate.
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Old 25th October 2012, 01:33 AM   #5
Carlp is offline Carlp  United States
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Thanks, everyone. I'm using a Fluke 12B so I doubt it's getting confused by ripple. I don't think the meter is true RMS, but since we're talking basically sine wave until filtering, then DC, does it really matter? It's bandwidth is up to 600v but I have it set to "auto" range. No problem there.

Dan, yep, it's one of those gold cased power resistors. So otherwise, I shouldn't really worry too much about the resistor. It may be too high R (I'd rather not drop quite so much v anyway...) but heat sinking should help. I may do a longer test with the resistor bolted to a heatsink.

I used the 5 ohm b/c that's what I had on hand - and was basically guessing anyway. Hmmm, maybe try 2 5Rs parallel since I have them on hand? Right now I want to see if there's any sonic advantage to this type of PS over the SMPS so I don't want to spend anything on it yet (all the PS parts are from my parts bin). Note that I'm also trying this amp as a head amp with output pulled straight from the speaker outputs and the SMPS just puts out tons of noise through my 32 ohm Grado SR60s. That was part of my motivation to try a filtered linear PS.

So AndrewT, are you saying I should aim to drop the PS voltage from 30v (what's coming out of the bridge despite the roughly 35v unloaded transformer voltage) to 29v? If so, I'd probably aim for 15v at the drain. I think the 2sk1058 can handle those conditions (the gate to source v might be close to the max) but I'm not sure and I'm new to transistors. I'd be burning 15w on the 15R load resistor, but it's 50w and heat sinked, so shouldn't be a problem.

Hmmm. So much to learn. Thanks a ton for all the great responses.
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Old 25th October 2012, 01:36 AM   #6
Carlp is offline Carlp  United States
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So one other question. I'd like to try a regulated PS and was thinking of trying this: Build an Adjustable 0-34 volt power supply with the LM317. I know the LM317 is supposed to be a good regulator and with the TIP3055 the current shouldn't be a problem. Would this schematic on it's own provide a pretty ripple-free output, and how would it compare to a brute-force CRC PS like I'm working on right now (sonically)? Would there be any advantage to adding CRC filtration to the regulated supply?
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Old 25th October 2012, 01:58 AM   #7
Yooper is offline Yooper  United States
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Why anyone would use a resistor as opposed to an inductor in a staged filter arrangement is beyond reason.

Why burn power inside a power source!?

Makes no sense.

Also, Fluke multimeters are notoriously bad in noisy measurement environments.
Especially hand helds.
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Old 25th October 2012, 02:22 AM   #8
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Quote:
Originally Posted by Carlp View Post
Would there be any advantage to adding CRC filtration to the regulated supply?
Depends on the PSRR of the amp's control loop and your desired SNR on the output(s). This is an individual assessment, both on a per design and per designer basis , but in most cases the answer is that the regulator provides sufficient ripple rejection that additional filtering's not particularly useful if the control loop is well implemented and the bypass caps in the supply are reasonably sized. The corner to pay the most attention to in the analysis the low power limit as that's where the ratio of output power to total power the lowest.

The other thing to check is the peak input voltage to the LM317 if the mains run high. Back of the envelope I'm guessing the odds of blowing the part's 40V are acceptably low. The LM338 and LM350 may also be interesting to look at.

Quote:
Originally Posted by Yooper View Post
Why burn power inside a power source!?
It's class A so the dissipation's invariant of whether the power's burned off in a resistor, inductor, regulator, output device, or driver. Yeah, it costs some output swing to burn it off a CRC but the OP stated this is a deliberate design choice to knock 30V down to 24V. Plus since the amp pulls 1.5A it'd take a 16 ohm speaker for a 24V swing to be required. Not as efficient as aligning the trafo with the desired output voltage but, well, this is class A...

(Full disclosure: I have 20V 100VA trafos for sale in the swap meet forum for cheap. )
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Old 25th October 2012, 02:23 AM   #9
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Have you thought of using an LC filter on your noisy SMPS?, inductors which effectively block SMPS frequencies are small and common these days. I have used filtered SMPS supplies with class A amplifiers and got good results. The high frequency noise that an SMPS produces does require proper layout and attention to signal routing if it is to be effectively supressed
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Old 25th October 2012, 02:38 AM   #10
DUG is offline DUG  Canada
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Yooper:

1. doing a rough search in Digi-Key for >1mH and >6A gives us the cheapest at 1mH / 10A and this would give us about 0.75 ohms reactive at 120Hz for $19 US.

the next one is 2.8mH / 10A for $28 US (2.1 ohms)

a 1 or 2 ohm 20W resistor (even at Digi-Key prices) is between $3 and $6

so price would be a factor

2. "...but wanted to play around..."... so Carlp is experimenting and learning.

Makes a lot of sense to me.

Keep playing Carlp
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Last edited by DUG; 25th October 2012 at 02:39 AM. Reason: add post reference
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