### Author Topic: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?  (Read 12229 times)

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#### Chris Wilson

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##### Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« on: June 20, 2013, 04:12:41 am »
Basically I understand rectifiers and just about understand regulators, but can modern regulators handle a current draw of say 2 amps at a regulated 6.3V DC, without also using a power transistor? If so, which regulators? How big a voltage delta between AC out of transformer to required DC out of regulator? Thanks. Follow on from my transformer rating thread (VA rating of a transformer). Thanks.
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#### smashedProton

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #1 on: June 20, 2013, 04:22:15 am »
How about two lm317 in parallel?  You need to add some resistance to the output though
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#### Alana

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #2 on: June 20, 2013, 04:36:12 am »
LM350 would be better, 78xx and LM317 are not ment to be connected in parallel.
But there will be like 6W lost to heat so switchmode regulator like L4960 or something alike may be a good idea too. Minimal delta V you can assume for most typical regulators is 2V but some datasheets for like LM317 say 3V.
Datasheets:
L4960 http://www.st.com/web/catalog/sense_power/FM142/CL1456/SC355/PF63199
LM350 http://www.fairchildsemi.com/ds/LM/LM350.pdf

#### Chris Wilson

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #3 on: June 20, 2013, 05:01:52 am »
OK, thanks for the part numbers. I should have asked, this is for a DC supply to valve filaments, two small Russian valves with the filaments in parallel. There will be some inrush current when they switch on. How do these regulators cope with that? I see a data sheet for the LM317 shows a "slow start" circuit, I think it uses a capacitor / resistor charge time "timer". Would that help? Why do I want DC? The circuit is a driver amp for a big tetrode, designed for very low noise, the designer specs DC to reduce chances of hum etcetera. It also allows good voltage control, of course, for better valve life. Thanks. The valve data sheet says 0.55 Amps each filament, so I am playing a bit safe on speccing 2 Amps. Cheers!
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Chris Wilson.

#### Kremmen

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #4 on: June 20, 2013, 05:17:20 am »
As long as you use a linear regulator circuit the voltage you drop will be dissipated in some component, and the magnitude of dissipation will be the same regardless whether you use a power transistor or not. So it might be easier to just use one and have done with it. Any number of circuits out there. Possibly even the 3 component regulator (resistor, zener, transistor) could do but if not, many other simple options exist.
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#### edavid

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #5 on: June 20, 2013, 05:51:42 am »
You shouldn't have to do anything special about inrush current.

If you don't want to find an LM350, you could just use an LM317T for each tube.  That's probably cheaper.

#### Chris Wilson

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #6 on: June 20, 2013, 06:29:56 am »
I have bought an LM350 to have a play with, and for stock, as I am in no rush for them, a very cheap pack of 5 LM317T from China, that will take a few weeks to arrive. Sound like useful regulators to have about.  If the current draw is more than the spec sheets suggest I'll do something else with a power transistor. All good learning experience! Thanks again.
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Chris Wilson.

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #7 on: June 20, 2013, 10:51:19 am »
You should just put two 7805 regulators in parallel on the same heatsink...it will work perfect.

Some people think this idea is poor design and only one of the 7805 will be active...not true. They will quickly balance the load between them and the voltage output will remain constant during this balancing period.

KISS
« Last Edit: June 20, 2013, 10:53:59 am by Paul Price »

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #8 on: June 20, 2013, 10:56:02 am »
Someone says, "LM350 would be better, 78xx and LM317 are not ment to be connected in parallel."

Try connecting two or more 78xx in parallel and test this idea! If you can, mount them on the same heatsink.
« Last Edit: June 20, 2013, 11:45:39 am by Paul Price »

#### edavid

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #9 on: June 20, 2013, 12:35:51 pm »
You should just put two 7805 regulators in parallel on the same heatsink...it will work perfect.

Some people think this idea is poor design and only one of the 7805 will be active...not true. They will quickly balance the load between them and the voltage output will remain constant during this balancing period.

KISS

No, it will not work perfect(ly), and it really is poor design.  The 7805 with the higher voltage output will supply most of the load until/unless it goes into current limit or thermal shutdown.

Now, in this case, the OP needs 6.3V out, so he could trim the voltages on the 2 7805s so they were close enough to share pretty well, especially with a little extra ballast resistance.  However, since he has 2 separate loads, he could just supply them from separate regulators and avoid the issue.
« Last Edit: June 20, 2013, 12:43:25 pm by edavid »

#### Rufus

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #10 on: June 20, 2013, 12:57:32 pm »
Some people think this idea is poor design and only one of the 7805 will be active...not true. They will quickly balance the load between them and the voltage output will remain constant during this balancing period.

If it were good design don't you think some of the data sheets might show it as a suggested higher current regulator and sell twice as many 7805s? Two 7805s can have a much as 400mV difference in output and typically only 10mV of load regulation at 1.5A. The chances are one of the regulators will try to do all the work limited by its SAO and thermal protection circuits.

The other will fill in but who knows what the characteristics of a 7805 trying to current limit or shut down are like. The SAO protection possibly has a negative resistance characteristic. The higher the output voltage the less dissipation in the pass transistor and the more current the SAO circuit will allow to pass. A pair of 7805s heavily loaded might just sit there oscillating.

Who knows, the idea is too naff to be worth investigating.

#### c4757p

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #11 on: June 20, 2013, 12:58:30 pm »
No, it will not work perfect(ly), and it really is poor design.  The 7805 with the higher voltage output will supply most of the load until/unless it goes into current limit or thermal shutdown.

7805 has a negative temperature coefficient of voltage, so as long as the two are reasonably close to begin with, they should end up sharing.

Two 7805s can have a much as 400mV difference in output and typically only 10mV of load regulation at 1.5A. The chances are one of the regulators will try to do all the work limited by its SAO and thermal protection circuits.

I think it's a reasonable thing to do for a hobby circuit, if you take the time to match a pair so that they begin close together. As long as the difference isn't so high that one goes into thermal shutdown before they have a chance to balance, they should equalize.
« Last Edit: June 20, 2013, 01:01:05 pm by c4757p »
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#### peter.mitchell

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #12 on: June 20, 2013, 04:43:18 pm »
I'd still stick a 0.1 ohm or something on the output of each one to ensure some current sharing though.

#### richard.cs

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #13 on: June 20, 2013, 07:10:40 pm »
Is there a particular reason for using a 9V transformer and d.c. heating? Would you consider running the heaters in series?

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #14 on: June 20, 2013, 08:45:19 pm »
Remember what we are talking about here, heating filaments.

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #15 on: June 20, 2013, 08:50:53 pm »
Finding the simplest way to accomplish an electron task can often be the best. All the remarks about output voltage matching an thermal shutdown are immaterial and simply do not happen. They just don't happen with two 7805's in parallel if the output of both total matches the demand.

There is no need to create an electron egalitarian democracy.

The 7805's will always share because one attempting to take more of the load will always drop in output voltage and this drop in output voltage will cause its partner to take up the slack.
It makes no difference for the circuit and it will function perfectly even if one is running a little hotter than the other. The 7805 is designed to work over its range of operating temperature and current sharing will take affect  well before either one approaches it's output current or temperature SOA limit.

« Last Edit: June 23, 2013, 01:56:13 am by Paul Price »

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #16 on: June 20, 2013, 08:54:16 pm »
Edavid says,"No, it will not work perfect(ly), and it really is poor design.  The 7805 with the higher voltage output will supply most of the load until/unless it goes into current limit or thermal shutdown."

I say this is not true, for same reasons I have stated before.

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #17 on: June 20, 2013, 08:56:25 pm »
Rufus says, "If it were good design don't you think some of the data sheets might show it as a suggested higher current regulator and sell twice as many 7805s? Two 7805s can have a much as 400mV difference in output and typically only 10mV of load regulation at 1.5A. The chances are one of the regulators will try to do all the work limited by its SAO and thermal protection circuits.

The other will fill in but who knows what the characteristics of a 7805 trying to current limit or shut down are like. The SAO protection possibly has a negative resistance characteristic. The higher the output voltage the less dissipation in the pass transistor and the more current the SAO circuit will allow to pass. A pair of 7805s heavily loaded might just sit there oscillating. "
---------------------------------------------------

Does not and cannot happen. Have you ever tried such a circuit?

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #18 on: June 20, 2013, 08:59:38 pm »
C4757p says, "I think it's a reasonable thing to do for a hobby circuit"

How can you be sure it not altogether the right thing to do in any reasonable circuit. The characteristics a 7805 under load cause them to share as much as necessary.

A 7805 is not be used to create a laboratory standard voltage reference!
« Last Edit: June 20, 2013, 09:08:32 pm by Paul Price »

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #19 on: June 20, 2013, 09:02:58 pm »
Peter.Michel wrote,"I'd still stick a 0.1 ohm or something on the output of each one to ensure some current sharing though."

Why add components that are unnecessary and make more work for yourself, as I've already said they don't need to exactly match and they will anyway for the reasons I've stated already.

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #20 on: June 20, 2013, 09:05:02 pm »
Edavid says,"Now, in this case, the OP needs 6.3V out, so he could trim the voltages on the 2 7805s so they were close enough to share pretty well, especially with a little extra ballast resistance.  However, since he has 2 separate loads, he could just supply them from separate regulators and avoid the issue."

Firstly, is there really an issue? The 7805's are already matched close enough and there is again no need for the currents to exactly match.

Why create more work for yourself if there is no proven reason to.

« Last Edit: June 20, 2013, 09:07:06 pm by Paul Price »

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #21 on: June 20, 2013, 09:14:02 pm »
cs.richard says, "Is there a particular reason for using a 9V transformer and d.c. heating? Would you consider running the heaters in series?"

Yes, this tampers with an antique circuit and creates more work soldering than necessary with possible damage to brittle old tube socket pins and wires with rubber or fragile cloth insulation that is ready to drop off at the first toggle or breeze.

Why not  try just providing the current you need easily, say with two 7805's in parallel.
« Last Edit: June 20, 2013, 11:52:02 pm by Paul Price »

#### Paul Price

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #22 on: June 20, 2013, 09:17:58 pm »
Why doesn't anyone try paralleling of 7805's to test their current sharing operation before commenting?

Prove me wrong as well, but Is it good to give advice on how to protect a circuit that doesn't need protecting, to match devices that do not need matching, to offer advice without experimental proof, with just fear and ignorance to guide you?

Has anyone forgotten that we are trying to find a simple solution to this extremely delicate and  complex precision design question, heating filaments?

These poor vacuum tubes have managed to cope with unregulated 6.3V AC for enough time for your grandson to enter college, somehow they have survived.
« Last Edit: June 20, 2013, 09:21:34 pm by Paul Price »

#### mikeselectricstuff

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #23 on: June 20, 2013, 09:25:52 pm »
For a fixed load like a filament, you could just use a resistor - it will get hot, but so will a linear regulator
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#### Chris Wilson

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##### Re: Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps?
« Reply #24 on: June 20, 2013, 09:27:48 pm »
Personally I think unregulated 6V AC would be fine, but I am trying to emulate a known REALLY low IMD design, and not experienced enough to want to go off at a tangent from what is known to work. Thanks for the interesting points of view and ideas
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Chris Wilson.

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