Help me with my Dynahi PSU

[flecom]

ok so id rather not spend $140 on two regulated PSU's (oe $280 if i build a balanced dynahi!) and i would rather make it myself anyway...

so a few datasheets later i found the LM338, a whopping 5A regulator

that should be more than enough to run 4 boards no? 10A? 4 of the other PSU's would be 8A so im guessing im safe...

I put this together basically from the datasheets and "borrowing"

some ideas from Tangent's STEPS schematic

is there anything ive overlooked here?

the transformer im using has two secondaries @ 30v (not center tap) so i think this should work as two completely independant PSU's so tieing them to make +30 and -30 should be ok right?

 

[Nisbeth]

Looks allright to me

There's a similar design available here: http://www.tcaas.btinternet.co.uk/jlhnewps.htm

You can use the LDO versions of the LM338 instead (LM/LT1084) for better noise specs. They should be pin-compatible.


/U.

EDIT: One thing. 100 Ohm resistors for the LED is a little low if the output is 30V.

 

[bg4533]

I really don't know anything about PSUs, so bare with me.

Are you planning on using this 1 PSU for all the boards?

Would this put out 5A into 2 boards then?

I would love the Dynahi to power small speakers and Kevin Gilmore said it might be possible if I can up the PSU to at least 4 amps. I am willing to sacrafice a bit of performance for more power.

Are you going to make boards for this? If so I might be interested.

 

[Earwax]

Quote:

Originally Posted by flecom the transformer im using has two secondaries @ 30v (not center tap) so i think this should work as two completely independant PSU's so tieing them to make +30 and -30 should be ok right?

I'm not seeing how you'd get +30/-30 out of that, it looks more like +30/0 done twice.

 

[Garbz]

that's exactly what i'm seeing, but since the transformer secondaries are separate from each other my guess is he'll be connecting 30 - 0/30 - 0 taking ground from the connection of 0/30 and taking -30 from the remaining 0.

I believe this is the same way they are using the pre-made powersupplies in the dynahi thread.

 

[Earwax]

Ah, I see now. I saw the diagram in the build thread, but I glossed over the details http://www6.head-fi.org/forums/showp...&postcount=217
Does that still work if the PS isn't 2 independant supplies but 2 sets of secondaries from one transformer?

 

[flecom]

Quote:

Originally Posted by Earwax I'm not seeing how you'd get +30/-30 out of that, it looks more like +30/0 done twice.

thtats exactly what it is

the dynahi was designedd to be run with two 30v PSU's and tied like Garbz said...

the reason i didnt use a negative voltage regulator is because the biggest negative voltage regulator they have puts out 3A... meanwhile the regular positive voltage regulators go up to 5A...

this is two 30v 5A PSU's so ya it should be able to power small speakers fairly easily... only thing is you will be generating quite a bit of heat both from the PSU's and the amp... so you are going to need some giant heatsinks or some fans and such...

also im not planning on making a PCB, this should be fairly easy to make using breadboard...

 

[kevin gilmore]

make sure you use a stiff oversized power transformer and keep
the unregulated voltage under 40 volts. Dual 28 volt secondaries
should be fine. And put a very large heatsink on the regulator.

 

[flecom]

I'm planning on using a 700VA toroidal transformer...

the load tests are:
1. Primary I = 0.03A, Secondary V = 31.6VAC at 0.0A
2. Primary I = 1.68A, Secondary V = 30.8VAC at 6.2A
3. Primary I = 3.25A, Secondary V = 30.0VAC at 12.0A
4. Primary I = 4.75A, Secondary V = 29.3VAC at 17.6A
5. Primary I = 6.16A, Secondary V = 28.6VAC at 22.9A
6. Primary I = 7.51A, Secondary V = 27.9VAC at 27.9A

i think that should be more than enough no?

also the LT1083 (7.5A!!!) LT1084 (5A) and LT1085 (3A) regulators should drop into that schematic directly replacing the LM338 chip

 

[kevin gilmore]

quote
i think that should be more than enough no?

actually the voltage is going to be slightly high at 600ma.
43.4 volts unregulated which is a bit over the maximum of 40 volts.

 

[amb]

Quote:

Originally Posted by kevin gilmore actually the voltage is going to be slightly high at 600ma. 43.4 volts unregulated which is a bit over the maximum of 40 volts.

For the LM317/338, the 40V rating is the regulator input/output voltage differential, not an absolute. The same goes for the LT1083/1084/1085 family, except the rating is 30V. I don't think the regulator is going to drop anywhere near 30V in the examples mentioned so it'll be ok. Just make sure there is plenty of heat sink, because that poor regulators may need to dissipate 16W each if it's dropping 10V and pumping 1.6A (for four Dynahi boards). Note that the LM317 is only rated for 1.5A.

 

[flecom]

hrmm, perhaps a smaller transformer is in order? i dont want to be dropping 13v... thats going to be quite a bit of heat

600VA toroid

1. Primary I = 0.04A, Secondary V = 28.5VAC at 0.0A
2. Primary I = 1.33A, Secondary V = 28.0VAC at 5.6A
3. Primary I = 2.60A, Secondary V = 27.4VAC at 10.9A
4. Primary I = 3.82A, Secondary V = 26.8VAC at 16.1A
5. Primary I = 4.96A, Secondary V = 26.3VAC at 21.0A
6. Primary I = 6.06A, Secondary V = 25.8VAC at 25.8A

the less voltage drop the regulators have to do the less heat, which is a good thing becuase from what ive heard the dynahi is quite a space heater

*WARNING: probably N00bie question*

how do you calculate the voltage gain after rectification? or is there just a rule of thumb to go by?

 

[MisterX]

Quote:

how do you calculate the voltage gain after rectification? or is there just a rule of thumb to go by?

1.414 × the AC voltage but this does not account for the voltage drop across the diode.

 

[flecom]

Quote:

Originally Posted by MisterX 1.414 × the AC voltage but this does not account for the voltage drop across the diode.

thats the great thing about electronics, its never one, easy, answer