Can the 338 regulator handle 4 Ohms?

[amo]

I have been using the cool little spreadsheet from the National site to help design an amp based on the 3886 chip. I put in 26 volts for the supply voltage and 4 ohms for the load. This yields 62.61 watts of power at 1 % THD, and calculates Vpeak at 22 v and Ipeak at over 5 amps. Is this cutting it a little too close for a 5 amp regulator such as the lm338? Do I have to use the steel package and a massive heatsink? I just hate the idea of the voltage going through the giant heatsink before it gets to the chip, as the steel package has no output pin… Is this totally irrational? I have a feeling that there is a hole in my understanding of how supply voltage relates to the Ipeak/Vpeak. And I am assuming that to get the supply voltage of 26 volts I need (26-3)/1.41 volts from the trafo.

[saabie22]

Amo,

If you use a mica insulator and grease or a silicone rubber insulator with insulating plastic bushings for the screws you will isolate the case from the heatsink.

Here's a Digikey.com link that has transistor mounting hardware:

http://dkc3.digikey.com/PDF/T042/0567.pdf


Mike

[carlmart]

Originally posted by amo
I have been using the cool little spreadsheet from the National site to help design an amp based on the 3886 chip. I put in 26 volts for the supply voltage and 4 ohms for the load. This yields 62.61 watts of power at 1 % THD, and calculates Vpeak at 22 v and Ipeak at over 5 amps. Is this cutting it a little too close for a 5 amp regulator such as the lm338? Do I have to use the steel package and a massive heatsink? I just hate the idea of the voltage going through the giant heatsink before it gets to the chip, as the steel package has no output pin… Is this totally irrational? I have a feeling that there is a hole in my understanding of how supply voltage relates to the Ipeak/Vpeak. And I am assuming that to get the supply voltage of 26 volts I need (26-3)/1.41 volts from the trafo.

The hole in your understanding is that you think current demand will be conitnuous. The 3875/3886 chips are class-B amps, so they will demand current in peaks.

Perhaps if the amp owner listens to rock music all the time at high level there might be a problem, but not all the time. The heat might be a problem, but neighbours will probably complain first.

In any case, any conscious DIY owner will test his amp thoroughly and feel how hot the heatsink is. If it's too hot you may need a bigger heatsink or a lower voltage. IMO the chip protection will kick first.

Why do you say the voltage goes through the heatsink? The chip should be isolated from heatsink, to there's only heat being sinked.

The 5amp mentioned are peak, not continuous.


Carlos

[johnnyx]

On the steel version (TO3 type), the fixing bolt or nut can be soldered to the PCB, or a wire attached to the bolt with a tag (like a washer, but with a solder or crimp lug attached).
Current doesn't have to pass through the heatsink

[carlosfm]

Originally posted by carlmart
The 5amp mentioned are peak, not continuous.
Carlos

Yes.
Another detail: the LM338 gives 12A peak current.

These chips don't get hot if the difference between input and output (regulated) voltage is around 5V, max. 10V.
And this is independent of the PSU voltage, high or low.
I regulate at a voltage differential of around 5V, when possible, and they run .

[amo]

Originally posted by carlosfm


Yes.
Another detail: the LM338 gives 12A peak current.

These chips don't get hot if the difference between input and output (regulated) voltage is around 5V, max. 10V.
And this is independent of the PSU voltage, high or low.
I regulate at a voltage differential of around 5V, when possible, and they run .

Does this rule out the very cool PLITRON LONO 30 Volt trafos for 4 Ohm loads??? By my calculations I will have to drop close to 16 volts on the regulator heatsink to get the rail voltage of 25 volts (30*1.451-3-25=16). I really want to use these trafos, don't know why, but they are sooo sexy. (If my girlfriend would read this post, she would be laughing her @$$ off, calling me a nerd, and I guess I must be, after all, who else thinks a trafo can be sexy?)

Also, I guess it was wrong for me to say that the voltage will flow "through" the heatsink, because the heatsink it self will be isolated from the case. What I meant to say is that the sink will be "live", but no current will flow through it. Does this somehow degrade performance? I would prefer to stay away from isolating the chip. Thanks for your help as always!

[carlosfm]

Regulate for 30~31V, it will work fine for 4 ohm loads.
The more voltage you drop, the more heat you generate and the less current you get out of the LM338.
Oh, and you can use those sexy trafos.

[johnnyx]

A heatsink has a large area, so if it was connected to a noisy source, then the noise could capacitively couple to sensitive parts of the circuit.
For a regulator, I think it will be be solved by moving sensitive inputs away from the power supply, something you'd do anyway.
If it was at a high voltage there are safety issues, but for an amp the worst that could happen is accidentally shorting something with a test probe. Make a "Caution, live heatsink" label to remind you.
I prefer grounded heatsinks myself, but I have had live ones in some projects. I have seen it in commercial products on many occasions.

[amo]

Originally posted by johnnyx
A heatsink has a large area, so if it was connected to a noisy source, then the noise could capacitively couple to sensitive parts of the circuit.

I had a feeling that something like this might be going on, but wanted to hear it from someone with more experience. I will have to reconsider my strategy. I was going to chop up a copper bar of 1.5 x 1.5 inches into 1.5 inch sections, creating tiny cubes weighing over a pound each! I was hoping to mount the chips right to these things (after polishing) and isolate them from the case using PVC bar stock. Some vertical holes could also be drilled in the cubes to promote air movement. But I guess I will have to rethink this plan.

[amo]

Originally posted by carlosfm
Regulate for 30~31V, it will work fine for 4 ohm loads.
The more voltage you drop, the more heat you generate and the less current you get out of the LM338.
Oh, and you can use those sexy trafos.

Ok, I have no idea if what I am about to say makes any sence at all, but here it goes anyway: Would it be possible to put a Zenner diode before the regulator circuit to drop some voltage, to help out the lm338 chip. (Keep in mind I do not know what a Zenner diode really is). I have a feeling that if I put a power resistor before the regulator to drop some voltage it will suck up the current as well. Just shooting in the dark here....