MNats PSU overheating problems- SOLVED : )

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Purusha

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Jan 14, 2006
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I am using Mnats PSU board for my solid state Neve/Pultec DIY project.
I noticed that the negative side of the PSU output is not stady. You
can also notice this on the LED cause it blinks. The voltage drops to -6V
occasionally and stays there for a few seconds and rises back up to -12V
for few seconds and so on. It looks like the LM337 transistor is overheating
and shutting down from time to time cause it can't handle so much current.

I wired the Pultec by using one side +12V and another -12V to get the 24V
across and didn't use the middle gnd. I am powering 3 LEDs along with 4x24V 5A
relays (SMI-S-224L ) plus two Fabios Neve output driver boards (BA283).

Those two transistors are cooking although I have heat sinks on them so it's obvious
I need to change something in regard those two transistors (LM337, LM317) but what?

1. I could mount another PSU board in parallel after the toroid and split the power
consumption from audio boards to one PSU and relays and LEDs to another. Would this help
to reduce the over heating of the transistors?

2. Another thing is to mount the transistors on the chassis directly to cool them down even more
but I am afraid this would cause a short. Would there be any shorts between the back of those transistors and GND?

3. Or should I find some transistors which can handle 5A? ...any suggestions?

Appreciate any help.
 
2. Another thing is to mount the transistors on the chassis directly to cool them down even more
but I am afraid this would cause a short. Would there be any shorts between the back of those transistors and GND?

OK, this will not work as the back of the transistors has some voltage running through.
Maybe with TO-220 KIT this could work...
 
What about a piece of mica between the transistor and chassis?

Sounds like you just need more heatsink.
 
Huh, now you got me :shock:

I was not in the school when this was explained :green:

Sounds simple but not for my brain. I like soldering but not doing the math kind of stuff... makes me dizzy :roll:
 
5 AMPS?! Those 317/337's are designed for 1.5 amps max...much better to use a 44 or 48 volt center tapped transformer (or a 24 volt ac wal wart hooked up like the schematic on Joe Malone's site). I don't think there is any way to get to 24 volts bipolar using a 12-0-12 transformer and those regulators on Mnats PSU board...I have had no problems using a 24 volt wal wart and his board fot +/- 24 volts...

cheers,
Ray
 
OK, I have some progress. I mounted double heat sinks and so far it's stable although
the transistors are hot like anything. I can't touch those heat sinks not even for a moment...
 
Hi Purusha,

these 5A relays don't need this current for their coil, it's their max switching ability before contacts burn up. A piece of mica is no cap but a thin (fragile) isolator to thermocouple your regulator to a heatsink or case. Use some -less is more- grease between to increase this coupling effect. You most often have to use an isolating washer for the mounting screw as well, as there may/will be voltage on the regulator's metal sheet. :wink:

What are the output voltages of your centertapped transformer ?

If you feed your psu board with 12-0-12 AC transformer, you'll get about 16V DC raw voltage across each loading cap. The voltage regulator will need a margin of min 3V between input and output to operate as desired. All voltage difference between input and output of the regulator multiplied by your connected load will be transformed into heat. If your transformer for instance is a 22-0-22v, you'll get about 30V DC between your unconnected ground and pin 3 of lt317 (pin 2 of lt337) and each chip has to burn additional 18V (30V input - 12V output) multiplied by the current, your load draws. Ohm's law.

I guess, your input buffer is also fed by this +24V single rail. (If you used an op-amp, powered as in steffen's design with +/- voltage from the same Mnats psu board, you simple have shorted out your negative part. ..max current .. max heat)

All the best
Harpo
 
[quote author="Purusha"]I wired the Pultec by using one side +12V and another -12V to get the 24V
across and didn't use the middle gnd.[/quote]

What did you do with the middle ground? How's your grounding scheme?
Do you only need 24v for this unit? In that case..why use a Mnats PSU?
It's much easier to make a 24v PSU yerself.....les parts less costs...

EDIT: I did'nt read Harpo's post very carefully. I think he just answered your question...
 
Do you only need 24v for this unit? In that case..why use a Mnats PSU?
It's much easier to make a 24v PSU yerself.....les parts less costs...

Yes, I only need 24V for this Pultec.

Can someone point me to a 24VDC PSU layout or schematic? I am using 24-0-24 30VA toroids, just for the info.

You need a seperate 24VDC PSU for the neve clone and then use the bipolar supply for the pultec clone.

Neve and Pultec are the same unit in my case. I am not sure what you mean with two different PSUs?
 
Purusha said:
.

Can someone point me to a 24VDC PSU layout or schematic? I am using 24-0-24 30VA toroids, just for the info.

I used this in my Lawo Pultec to power the Lawo card. Wire the secondaries parallel....

2iszjhe.gif
 
radiance said:
[quote author="Purusha"]
.

Can someone point me to a 24VDC PSU layout or schematic? I am using 24-0-24 30VA toroids, just for the info.

I used this in my Lawo Pultec to power the Lawo card. Wire the secondaries parallel....

unfortunatly you can't wire a center tapped transformer in parallel and with yours you can only pull <15VA from one half. (looking at some pics arround, CJ could do it, but your transformer may appear a bit frightened afterwards :green: ) Best will be to get a 22V toroid as described in previous posts and perfboard the 5 components. On Mnats psu board you would have to cut and change some traces to get the rectifier and ground working.
 
I did not know that. So when you have one of these 110v/220v tranformers you only use one primarie when set up for 110v, and both primaries in series when set up for 220v right?

I remember now that I did use a 2 x 12 v transformers with the secondaries wired in series to get my 24v.

So when it says 24 - 0 - 24 it's a center tapped transformer and when it says 2 X 24 it's a ...yeah how do we call these then? Or are all common transformers like this center tapped?
 
I was using my Pultec the whole day and it survived the heat :green:

I will just mount the transistors to the chassis with TO-220 KIT and that's it for now.
This should cool them down even more. I have too many Mnats boards and 24-0-24 toroids
at home to buy different ones. :wink:

Thank you guys for all the nice tips :grin: :thumb:

I just wish I had more brain to use some of them in practice :green:
 
You can't easily parallel voltage sources. You'd need current sharing resistors.

I rather suggest you check the voltage at the input pin of the voltage regulator. If it's above 30 V, I'd use a series resistor at the input (preferably between transformer and rectifier diodes as this will reduce magnetic hum at the same time) to drop it and to share the dissipation. Make sure you use a suitably rated resistor with sufficient heatsinking (e.g. mounted on the chassis).

Samuel
 
Re-reading this thread I realised that you're regulating 2x 24 VAC to 2x 12 VDC, right?

The highest voltage on LM317 is 30 VDC.
So then use a pair of series resistors for the transformer secondaries to get things down to about 18 VDC at the input.

Alternatively you could set the regulators to +/-24 V and not use the negative supply. Makes about half the heat.

Samuel
 
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