DC power adapter used insted of a AC power adapter, cause issues?

[Strikerj]

Hi everyone, Just wanted to know if its ok to use a 9DC power adapter on a 9AC device (in this case a Digitech RP1000 AC power version)
As far as i can tell there is a internal bridge rectifier circuit for AC to DC and looking at the power supply schematic it looks like no AC power is needed to run any other part of the board.
Am i wrong to assume this?
Just got parts in and repaired my damaged RP1000 mainboard (not by me) and wanted to find a cause for failure.
ATM the unit was supplied with a 9.5v DC power adapter putting out 9.85v DC. (AC adapter is DC regulated using a LM317 which I regulated now to 9v from 9.85v)
Im wanting to test it but before installing the audio processor I wanted to run the power supply's first to test all other repairs and power outputs.
Any advice or info will be helpful.
Thank you.

[The Dude]

If the unit requires AC it needs AC. You yourself said it has an internal bridge rectifier, so why would we think a DC adapter would work?

AC adapter is DC regulated using a LM317 which I regulated now to 9v from 9.85v

I don't know what that means. How can an AC adapter be DC regulated?

[Strikerj]

Just to clarify (now talking just about the power adapter and not the RP1000)
Its a 240v AC to 9v DC adapter that was given to me with the unit. Inside the adapter it has a 240vAC to 13.8vAC transformer feeding a bridge rectifier into a LM317 voltage regulator. Not the OEM adapter for the RP1000.
There are x2 main types of DC power supply to my knowledge.
Regulated (Use a voltage regulation circuit to maintain a stable voltage)
and unregulated (direct transformer output voltage with no regulation)
My 9vDC adapter is a regulated type, is what I should have said.
Sorry if I was not clear.
Thanks





I don't know what that means. How can an AC adapter be DC regulated?[/QUOTE]

[Enzo]

AC means alternating positive and negative voltage. In a 9vAC adaptor, it puts out 9vAC. That means alternately it goes up to a peak of a little over 12v positive, then goes over to a little over 12v negative. Over and over. Run that through a rectifier, you get DC, about 12v of it once filtered.

A 9vDC adaptor includes the rectifier and filter so it puts out 9vDC, and you can pick which side of it to ground and select polarity.

If your unit asks for 9vDC, then it expects 9vDC of the proper polarity, VERY VERY few devices include a bridge rectifier just so you can use a DC adaptor of the wrong polarity. All the companies making such units expect you to use the proper adaptor.

Now what if your unit says it wants 9vAC? It is expecting 9vAC, which means it expects both negative and positive voltages. If you put a 9vDC adaptor to it, it will go through the bridge rectifier just fine, and power up that polarity circuit in the unit. But what about the other polarity circuits? They will get nothing, because you didn;t use an AC adaptor.

A typical old pedal might use +5v for the computer/digital stuff, then +12v and -12v for the op amps of the audio path. It makes all those from the 9vAC. If you stick a +9v adaptor on the power jack, then that 9vDC comes through and powers up the +5, and at least partly powers the +12. But the -12 sees nothing. And you get no audio, The +5 is working so all the controls and display work, the pedal APPEARS to be working, but alas only the digital part is running.

[Strikerj]

Thanks Enzo, good answer.
That was just what I was thinking but wanted to be sure.
Ive only got the schematic for the power supply and from what I see there is no alternate feed of AC power between the power socket and the bridge rectifier.
Is there a chance AC is being used on the other circuits but not shown on the power schematic?
Also could that be a cause of power supply and audio processor to have burnt out? (using 9.85vDC in a 9vAC device or something other?)
Here's the schematic.
Thanks again.RP1000 Power Supply.pdf

[Jazz P Bass]

My take on this is the Vdc adapter will simply not function.
There is not a ground reference.

I am not at all clear what you mean by "Ive only got the schematic for the power supply and from what I see there is no alternate feed of AC power between the power socket and the bridge rectifier.
Is there a chance AC is being used on the other circuits but not shown on the power schematic?"

The internal (to the RPS1000) power supply converts the externally supplied 9 Vac to a positive voltage.
This is then sent to U38, which converts that positive voltage to your plus & minus dc voltages that the pedal requires.

Kind of a long way around the block.

[Strikerj]

Whats your take on it?

My take on this is the Vdc adapter will simply not function.
There is not a ground reference.

I am not at all clear what you mean by "Ive only got the schematic for the power supply and from what I see there is no alternate feed of AC power between the power socket and the bridge rectifier.
Is there a chance AC is being used on the other circuits but not shown on the power schematic?"

The internal (to the RPS1000) power supply converts the externally supplied 9 Vac to a positive voltage.
This is then sent to U38, which converts that positive voltage to your plus & minus dc voltages that the pedal requires.

Kind of a long way around the block.

What I meant by my comment was I have no evidence that 9vAC goes anywhere else on the board or feeds any other circuit apart from the bridge rectifier AC to DC 9v.
You mention the U38, it just so happens that when I got the MB it had the U38 missing (someone had taken it off)

My take on this is If no circuit on the MB needs 9vAC then a 9vDC power should work.

I think that power will only pass through x2 diodes as it is DC.

or am i wrong?

That's everyone's thoughts?

[J M Fahey]

Thoughts?

Use the device as intended.

FWIW fitting a DC jack for an external wall wart and nothing else (ok, maybe 100uF in parallel, add 5 cents to the cost ) *IS* cheaper than a bridge rectifier, regulator(s), diode bridge, filter capacitor(s) but SPECIALLY having to use a non standard wall wart , so THERE MUST BE A REASON FOR THE EXTRA EXPENSE.
We can argue here until the cows come home, *maybe* solve it or not, or simply do things the Factory way.

[Enzo]

OK, I see what you mean. Schematic in post #5. This is more modern than what I was thinking. In the older units, the AC went to linear supplies to make both polarities.

In this unit the incoming AC is immediately rectified and filtered and THEN that DC is used to generate the other voltages of both polarities using little switcher ICs.

So then yes, technically a 9vDC of either polarity ought to work. At least partly. AC adaptors are not regulated, and most DC adaptors are not either. They should put out the proper voltage at the rated current. if you use less current, the voltage will be higher. But even so, At load, your 9vDC adaptor puts out 9vDC, as you would expect. The 9vAC adaptor puts out the 12v, and a tad more, peak twice a cycle, and that is what the bridge will put out into the switchers. That means your 9vDC adaptor will be starving the power supply circuits by 3v - a 25% reduction. They may work, they may not, or worst case: they will mostly work but unreliably

[J M Fahey]

^^^ That's the point.
9VDC (what the non original adapter puts out) and 12VDC , what you get with the original 9VAC original supply is NOT the same.

[Jazz P Bass]

What I meant by my comment was I have no evidence that 9vAC goes anywhere else on the board or feeds any other circuit apart from the bridge rectifier AC to DC 9v.
You mention the U38, it just so happens that when I got the MB it had the U38 missing (someone had taken it off)

My take on this is If no circuit on the MB needs 9vAC then a 9vDC power should work.

I think that power will only pass through x2 diodes as it is DC.

or am i wrong? [ATTACH=CONFIG]27991[/ATTACH]

That's everyone's thoughts?

You are missing the point of the whole circuit.
The 9Vac goes through diodes, yes.
But not in a dual Vdc way.
The reason for the 9Vac is to supply U38 with enough positive polarity current so it (U38) can make the +15Vdc & -15Vdc voltages.

A 9Vdc adapter will do nothing, as there is not a ground reference for it to work.

[Enzo]

Well, yeah there is, through the bridge. Think of the AC as flipping a battery over and over, applied to the two terminals of the power jack. Now a DC adaptor is like the same battery just not flipping. It is + and - at the bridge, then they ground the negative bridge terminal. it isn;t a dual polarity like a split winding supply, but it will pass the DC on through the bridge as if it were half a cycle. You don;t get both positive and negative with respect to ground, you only get positive, but that is the circuit's fault. All the voltages of both polarities are generated by little switchers.

[g1]

Take a hammer to the shell of the wall wart. Remove diodes and caps etc. and wire it up so it puts out AC. Glue or duct tape as required to put it back together again.

Edit: Guess that would not work for SMPS wall wart.

[Jazz P Bass]

And don't forget to replace U38.

[km6xz]

It will work but it would help to connect the - side of the power adapter to ground so there will be less voltage drop that would increase the current pulled by the boost regulator. It will run on 1.5 volts but efficiency drops a lot. That circuit will supply the bi-polar voltage needed for the analog stages just fine.