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 Forum index » DIY Hardware and Software » Thomas Henry designs
Att - Thomas Henry: Heatsinking regulators...
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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Sun Aug 12, 2007 8:06 am    Post subject: Att - Thomas Henry: Heatsinking regulators...
Subject description: ..in the psu from "The drumsynth cookbook"
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Hi Thomas, been wondering about this for a while:

In your drumsynth cookbook, you describe a psu using LM7815/7915 regulators. You recommend to plan for 1A per side, and then go on to describe how to diy a heatsink for the regulators. Are these heatsinks, in your experience, big enough to draw 1A from a 78xx regulator?

Just wondering, since the heatsinks I have lying around didn't come with datasheets, either because they're from futurlec, or from scavenged equipment.

Also, looking at other psu designs, somewhat bigger heatsinks seem to be recommended, for somewhat lower current (up to 800mA per side).

Confused? Yes I am! Laughing

Any insight?

C
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Tim Servo



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PostPosted: Mon Aug 13, 2007 8:42 pm    Post subject: Att - Thomas Henry: Heatsinking regulators...
Subject description: A little info
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Hey, here's a little info on your heat sink question:

From http://www.paia.com/KRUKits/K83-5/k83-5.pdf

"The 78xx and 79xx series can provide up to 1A load current and have on-chip circuitry to prevent damage in the event of overheating and excessive current. That is, the chip simply shuts down rather than blowing out.

The unregulated input voltage must always be higher than the regulator's output voltage by at least 3V in order for it to work. If the input/output voltage difference is greater than 3V, then the excess potential must be dissipated as heat... Without a heat sink, 3 terminal regulators can dissipate about 2 Watts. A simple calculation of the voltage differential times the current drawn will give the Watts to be dissipated. Over 2 Watts, a heat sink must be provided. If not, then the regulator will automatically turn off if the internal temperature reaches 150°C. For safety, it is always best to use a small heatsink even if you do not think you will need one."

The gist of all this is: if you get a 78xx too hot, it will shut down. Even a modest heat sink will help a lot. Also, look at the heat sinks available for the TO-220 package that the 78xx comes in and you'll get a good idea what sort of mass is required.

Hope this helps!

Tim (current events) Servo
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Coriolis



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PostPosted: Tue Aug 14, 2007 2:39 am    Post subject: Reply with quote  Mark this post and the followings unread

So basically I should look at the biggest heatsinks for TO220, and go with those, to get the max amps from a 78xx?

I guess that's a very useable rule of thumb.

Thanks Tim!

Btw: I wonder why even in his new "21st Century" psu schematics, Henry holds on to the 7815/7915 circuit, when everybody else seems to be into 317/337's or LM723's? Ray Wilson too...

I guess they must both feel 78xx779xx is good enough in real life situations...? Does it really make much of an audible difference?


C
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Pehr



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PostPosted: Tue Aug 14, 2007 2:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Coriolis wrote:
Ray Wilson too...


in which schematic? Confused

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Coriolis



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PostPosted: Tue Aug 14, 2007 2:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Eh..I meant that he uses 78xx/79xx too, not the other chips... Embarassed
But I wonder what all the fuss is about Power One supplies (LM723), when these two gurus use the cheap stuff and seem to be happy with that.. Question

C
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Etaoin



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PostPosted: Tue Aug 14, 2007 6:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Does it really make much of an audible difference?


I guess it might if the VCO core is referenced to the power supply.

My own modular performs markedly better since I switched from a 7815/7915 based PSU to one based on CGS65 (LM317/337). Ken Stone seems to prefer the LM317 based one for his VCO's.

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Etaoin



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PostPosted: Tue Aug 14, 2007 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

For what it's worth, National Semiconductor give 0.01%/V line regulator for their LM723 and 0.005%/V for their LM317A. Which suggests the 317 is the better regulator.

Sadly, the 7815 isn't spec'd in the same units; it just says "4mV", it doesn't say relative to what. If it's 4mV per Volt that would be 0.004% which would make it better than the LM317, which I somehow doubt, especially since Ken Stone explicitly says his LM317-based PSU is far better than his 7815-based one.

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Last edited by Etaoin on Tue Aug 14, 2007 6:22 am; edited 1 time in total
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Coriolis



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PostPosted: Tue Aug 14, 2007 6:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting. I thought the 723 was the better regulator?
Still curious tho: Did your old psu cause actual, audible issues with VCO's for instance? Did you hear a lot of noise?

78xx's supposedly generate a lot of noise...but is it actually audible?

I notice in Ray Wilsons psu schem, there are 3x4700uF caps per side - perhaps that amount of capacity is meant to compensate for shortcomings of the regulator. I've also seen some designs, where the usual small-value cap after the regulator, is then followed by one more large-value cap (like 1000 uF). That should help too, I'm told...


C
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Etaoin



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PostPosted: Tue Aug 14, 2007 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Interesting. I thought the 723 was the better regulator?
Still curious tho: Did your old psu cause actual, audible issues with VCO's for instance? Did you hear a lot of noise?


No noise, but I got LFO and other stuff to bleed through into the VCO's.
That's all gone now after I replaced the regulators. And I only changed the 7815/7915 board for the LM317/337 one. I'm still using the same transformer.

So I'm not sure what made the difference, but the LM317/337 seem to be able to handle my modular far better. I originally intended to get the VCO's their own power supply, but that hasn't been necessary yet.

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Coriolis



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PostPosted: Tue Aug 14, 2007 6:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, very interesting to know that.
Perhaps 78xx just..pick up noise or something.

Well, nothing beats hearing about someone's actual experience - except for gaining that knowledge firsthand of course! I'm off to do some building...

Thanks!

C
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Etaoin



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PostPosted: Tue Aug 14, 2007 6:41 am    Post subject: Reply with quote  Mark this post and the followings unread

It might just be that the 7815 needs a bigger voltage drop than the LM317, I haven't checked that. Regulation on the 7815 gets flaky if the voltage drop is too low, and I might be too close to that (I think I have about 19 or 20V on the input of the regulator).
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fonik



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PostPosted: Tue Aug 14, 2007 8:15 am    Post subject: Reply with quote  Mark this post and the followings unread

besides all that regulator IC stuff you may want an PSU which can be adjusted.
as soon as you come into the situation where you use more than one cabinet of modules and each cabinet comes with it's own PSU, you want to have exactly the same voltages for all three cabinets. otherwise you may run into troubles with the tuning of different VCOs running from different PSUs... at least you want to exactly balance the PSUs of the cabinets.

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Coriolis



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PostPosted: Tue Aug 14, 2007 9:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Good points fonik - that's also in favor of using 317/337's (or 723's).

C
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Tim Servo



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PostPosted: Tue Aug 14, 2007 11:00 am    Post subject: Att - Thomas Henry: Heatsinking regulators...
Subject description: Regulator choices
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Coriolis wrote:
So basically I should look at the biggest heatsinks for TO220, and go with those, to get the max amps from a 78xx?

I guess that's a very useable rule of thumb.

Thanks Tim!

Btw: I wonder why even in his new "21st Century" psu schematics, Henry holds on to the 7815/7915 circuit, when everybody else seems to be into 317/337's or LM723's? Ray Wilson too...

I guess they must both feel 78xx779xx is good enough in real life situations...? Does it really make much of an audible difference?


C


I'd say the biggest reason to go with the 78xx/79xx is simplicity. A simple package, a couple of caps and a diode and you're ready to go. They'll pass a fair amount of current even without a heat sink, whereas the 723 is only good for 150mA max. To get more current, you have to add external "pass" transistors. Not sure about the 317 off the top of my head.

As far as whether the 78xx/79xx is "good enough," I'd have to do an A/B test. Several people have said they've noticed a difference, so it might be worth checking out. Also, my (t)rusty Odyssey with its nice stable tuning does feature a 723-based supply (with the aforementioned pass transistors), so there's another point for the 723.

Tim (nice, stable, but not tuned) Servo
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Thomas Henry



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PostPosted: Tue Aug 14, 2007 1:26 pm    Post subject: Re: Att - Thomas Henry: Heatsinking regulators...
Subject description: ..in the psu from "The drumsynth cookbook"
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Coriolis wrote:

In your drumsynth cookbook, you describe a psu using LM7815/7915 regulators. You recommend to plan for 1A per side, and then go on to describe how to diy a heatsink for the regulators. Are these heatsinks, in your experience, big enough to draw 1A from a 78xx regulator?


Probably. I've never worried about it, just sticking on whatever heatsink I had handy. Even my monster synth of the the 1980s chugged along without any problem. The reason I don't worry is that a typical module draws 30 mA per side. At even half-an-amp total for the rack that's seventeen modules! In other words, I'll run out space before I run out of current.

As for the 7815/7915...well...how can I put this politely? In the synth world (just as in the worlds of cars, computers, gardening, cooking, poker playing---you name it) there's an awful lot of fussing about things that barely matter, even superstition. In my designs, there are all sorts of things going on that are far more important than any possible side-effects of a regulator. I like the 7815/7915 and simply have never noticed any problems using them.

In a nutshell, if a new VCO circuit loses tracking, the last place I would look to is the regulator. There are tons of other things that matter way more...

A little bit of Ockham's Razor goes a long way in synth design.

Thomas Henry
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Coriolis



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PostPosted: Tue Aug 14, 2007 1:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Thomas, great to hear your opinion on this!

I always did wonder about that whole vco tuning-stability-thing:
If your guitar goes out of tune, you...tune it, right?

I seem to remember reading somewhere on this forum, that when Don Buchla designed the Music Easel, he and Morton Subotnick came to the conclusion, that it should have the tuning stability of a concert violin, IE not perfect!

It's good to remember: We're not making lab equipment here!

Well, since I've got the 317/337's already, I'll use those, but I have been curious about this subject for a long time, and you guys shed somelight on it.

Thanks!

C
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