9 VDC from 9 VAC

[LyleCaldwell]

I've found this project: http://generalguitargadgets.com/index.php?%20option=content&task=view&id=192&Itemid=217 which converts 12 VAC to 9 VDC.  However, I have a 9 VAC feed to my MIDI controller and I'd like to "steal" 9 VDC from it to power a Korg tuner on my pedalboard.  The supply I'm using has enough spare current to add the tuner.

My question is - will the L7809CV in the circuit above take the 9 VAC and say "oh I just have less work to do" and spit out 9 VDC, or does it require a 12 VAC input?

Or is the above more complicated than I need just to go 9 VAC to 9 VDC?

Thanks for all responses.

[Dirk_Hendrik]

It's on the edge but could work. In general when rectifying the AC voltage (with a Graetz 4 diode rectifier (which is pretty standard)) the DC output wil be 1.4 times the input AC voltage. This means 1.4*9 is 12.6 volts. Since the regulator requires a difference of 3 volts you're 0.6 volts on the safe side. Otherwise take a low drop type like a LT1085 which is satisfied with 1.2 volts difference.

hope this helps.

[Mike Burgundy]

You could also use the fact that a transformer that's rated at 9V@xxAmpere will put out more voltage if it has to supply way less than the xxA rating. Depends on the transformer. I've built a spider based on "9V" transformers that put out 11V at the current that I'm drawing.

[LyleCaldwell]

Dirk - do you mean use the linked circuit but use an LT085 in place of the L7809CV?

Mike - I'll measure what's actually coming out of the transformer in a day or two - I haven't built in the split that will let me tap into the 9 VAC power yet (waiting for some paint to dry).  Thanks.  That might affect my plans.  Question, though - if it is putting out, say, 11 VAC with the two current devices powered, how can I predict what it will put out when a third device is powered by the supply?  If it drops down to 10 VAC with the third connected, I don't want to go in with the assumption that it's putting out 11 VAC.

[Dirk_Hendrik]

Dirk - do you mean use the linked circuit but use an LT085 in place of the L7809CV?

Yep

LT1085's come in a fixed voltage as well as a variable breed. (www.linear.com). Apart from the relatively high prize they're definitely easy to use.

[Sir H C]

National Semi has a bunch of LDOs with dropouts much lower.  1.2 V is pretty high for an LDO.

http://www.national.com/VCatalog/view.cgi/?command=toggleSort&attr1=Dropout+Voltage%2F%2Fascending&attr2=0&a2=Dropout+Voltage%2F%2Fv%3A8&a3=Quiescent+Current%2F%2Fv%3A8&a4=Output+Current%2F%2Fv%3A8&a5=InputMax+Voltage%2F%2Fv%3A8&a6=On+Off+Pin%2F%2Fv%3A8&a7=Multiple+Output+Capability%2F%2Fv%3A8&s0=PNumber%2F%2F1&a1=SubCategory%2F%2Fv%3A8&a0=Category%2F%2Fv%3A8&m1=SubCategory&t=0&m2=Bpn&m0=Category&as=0&render=1&domains=PNumber&q=25&i=PNumber&c1=e%3A0%2F%2FSubCategory%2F%2F%3Aeq%2F%2FLinear+Regulators+-+Low+Dropout+Voltage%3APositive+Voltage+-+Fixed&c0=e%3A0%2F%2FCategory%2F%2F%3Aeq%2F%2FAnalog+-+Regulators&tId=84

[LyleCaldwell]

OK, after reading the info on those regulators and the ones that seemed likely candidates at Mouser and at Digitech, I am more confused than ever.

The spec sheets all seem to indicate 15V or 16V are needed for the regulators to put out 9V.  Yet the schematic above uses 12V.  I am far from understanding all this.

Unless someone says "no wait don't" I'm going to try the LT085 Dirk suggested.

[Sir H C]

Headroom.

With unregulated supplies, you assume that the wall voltage can vary from say 100 to 125, so what is 12V out at 120 is 10V out at 100.  For a 78xx regulator you probably need 3 volts to have it regulate (3 Vbe drops on NPN transistors + the Vce on a PNP, all cold).  You want some extra room so that you can chug along with no problem.  Now if you go to that National site that I link with the huge link, they sell some LDOs that need about 150mV across them to regulate, So with 12V in you are set to get 9 out even if the wall voltage drops to 100.  Drops to 90 and the LDO will act as a small resistor in series with the supply, not regulating, but keeping the voltage as high as possible.

[LyleCaldwell]

Headroom.

With unregulated supplies, you assume that the wall voltage can vary from say 100 to 125, so what is 12V out at 120 is 10V out at 100.  For a 78xx regulator you probably need 3 volts to have it regulate (3 Vbe drops on NPN transistors + the Vce on a PNP, all cold).  You want some extra room so that you can chug along with no problem.  Now if you go to that National site that I link with the huge link, they sell some LDOs that need about 150mV across them to regulate, So with 12V in you are set to get 9 out even if the wall voltage drops to 100.  Drops to 90 and the LDO will act as a small resistor in series with the supply, not regulating, but keeping the voltage as high as possible.

I keep looking at that site, and I can't find any that will clearly work.  I will have 9 VAC in.  What regulator on that page will give me 9 VDC out?

I appreciate your help - this is one area that I'm just not very experienced with.

[nelson]

Headroom.

With unregulated supplies, you assume that the wall voltage can vary from say 100 to 125, so what is 12V out at 120 is 10V out at 100.  For a 78xx regulator you probably need 3 volts to have it regulate (3 Vbe drops on NPN transistors + the Vce on a PNP, all cold).  You want some extra room so that you can chug along with no problem.  Now if you go to that National site that I link with the huge link, they sell some LDOs that need about 150mV across them to regulate, So with 12V in you are set to get 9 out even if the wall voltage drops to 100.  Drops to 90 and the LDO will act as a small resistor in series with the supply, not regulating, but keeping the voltage as high as possible.

I keep looking at that site, and I can't find any that will clearly work.  I will have 9 VAC in.  What regulator on that page will give me 9 VDC out?

I appreciate your help - this is one area that I'm just not very experienced with.

You need to convert the AC to DC with a bridge rectifier, then regulate it. A bridge rectifier can be seen on nearly every audio circuit that uses an AC transformer it is 4 diodes in a diamond shape. Now when you rectify the 9VAC to DC you times it by 1.41 to get the resulting DC voltage, the resulting DC voltage will be enough for a standard 1 amp 7809 regulator to easily regulate 9vdc.

Does that make sense?

[Sir H C]

Okay, as stated by others, you can expect to see 1.4 times 9 volts on the output of the rectified output.  That means, about 13 volts.  Now any of the regulators that have under a volt drop-out would work so long as their current handling is within your needs.  As an added bonus, the lower input voltage means les power dissipated in the regulator.

[LyleCaldwell]

Thanks very much! 

So the LM2940-9.0 on this page: http://www.national.com/VCatalog/view.cgi?command=eq&attr1=Output+Voltage&attr2=9.0&q=25&m0=Part+Number&i=PNumber&s1=PNumber%2F%2F1&s0=Dropout+Voltage%2F%2F1&a3=Quiescent+Current%2F%2Fv%3A8&a1=SubCategory%2F%2Fv%3A8&a0=Category%2F%2Fv%3A8&s=910332289532&a2=Dropout+Voltage%2F%2Fv%3A8&a5=InputMax+Voltage%2F%2Fv%3A8&t=0&a7=On+Off+Pin%2F%2Fv%3A8&a6=InputMin+Voltage%2F%2Fv%3A8&a8=Multiple+Output+Capability%2F%2Fv%3A8&c1=e%3A0%2F%2FSubCategory%2F%2F%3Aeq%2F%2FLinear+Regulators+-+Low+Dropout+Voltage%3APositive+Voltage+-+Fixed&c0=e%3A0%2F%2FCategory%2F%2F%3Aeq%2F%2FAnalog+-+Regulators&c2=e%3A0%2F%2FBpn%2F%2F%3Aeq%2F%2FLM2940&a4=Output+Current%2F%2Fv%3A8&as=0&render=1&c=&domains=PNumber
would be a good choice, right?  9V Output Voltage, .5V Dropout Voltage, 1A Output Current, 26V Max Input Voltage, no Min Input Voltage, in a TO 220 form.

[Sir H C]

Sounds like a good one.  Should be good.

[LyleCaldwell]

Very much appreciated.

[gez]

Recommended reading:

http://www.amazon.com/gp/product/0900162961/qid=1136627336/sr=8-1/ref=sr_8_xs_ap_i1_xgl14/104-9590757-9896745?n=507846&s=books&v=glance

[Paul Perry (Frostwave)]

WARNING!!!!!!
You say you are going to "steal" voltage from the 9v AC supply to another unit.
That means, the supply you make, may well (definitely, if you use a full wave bridge rectifier) have a different ground to the ground of the original unit that the AC supply is being shared with.
If this is the case, then when you connect the grounds of the two units (via signal leads either directly or via other equipment) then, ther ewill be a flash & the smoke will cme out, and you will be very unhappy.
You can get a wart from www.jameco.com for $4 or so.......

[Peter Snowberg]

PLEASE READ PAUL'S WARNING ABOVE!     (and the one below too)

The ground potential will vary by the difference in the forward voltage of the diodes if both supplies use bridge rectifiers. That means you will be making a current carrying loop out of the ground connection and that's not desirable or good. Depending on the diodes used, you may see up to a large part of a volt difference between the two grounds. Which ever diodes have the lower forward voltage will end up conducting ALL of the current going through ground. You can always put super heavy duty diodes in your new supply, but you may burn out the diodes in your other gear. If you used high current Schottky rectifiers in your supply, you can "force" both supplies to use your new ground path to the transformer, but now you're drawing current through a signal ground loop! That's very bad.

If one of the pieces of gear uses a half-wave rectifier to get +/- supplies and the other uses a bridge to get a single supply, connecting the two "ground" lines will short out one of the diodes in the bridge rectifier, causing the other diode in that bridge-pair (the diodes connected to - on the bridge,) to be connected across the transformer. That WILL smoke the bridge rectifier in about 1 second.  

Either get another transformer with a 1:1 ratio to isolate your new DC supply (a split secondary transformer of the same voltage will be fine, just tape off the primaries really good !), or just spend a couple dollars and get another wall wart. The life you save will be your power supply's.

[LyleCaldwell]

Hmm.

The 9 VAC supply is currently going to my Axess FX1 via 2 pins on the MIDI cable.  It also powers the GCX back at the rack.  I have a Korg DT-7 tuner on my pedalboard next to the FX1, and it is the only other thing on my board that takes power.  I don't want to have to run a separate AC cable to the board just to use a wall wart for the DT-7.  I was hoping to share (safely) the existing supply, which I know has been done by others (see below).  But maybe a 9V battery is a better option - I don't want to kill the FX1 because I have too shallow an understanding of this subject.

[Peter Snowberg]

I'm guessing the Alesis box has a bipolar supply made up of two half-wave supplies (a simple diode+cap). In that case, one of those AC leads will be the DC ground lead too. If things are hard wired and you know which wire is also ground, you can safely add another half-wave supply for the tuner.

Can you just open the FX1 and tap DC power from in there? That would get things running, but the tuner power would end up in a loop or running over your signal lines. At least a tuner is a safe one to do that with (no oscillators to worry about).

The ultimate option would be to get a little 9V transformer with dual secondaries. Connect one secondary to your 9VAC input and the other secondary can now go to a bridge rectifier and a cap to power the tuner. Tape up the primaries because they'll try to deliver 120V to anything they touch.

If you don't do one of those options, you'll be passing DC through your signal connections.

[LyleCaldwell]

I'm guessing the Alesis box has a bipolar supply made up of two half-wave supplies (a simple diode+cap). In that case, one of those AC leads will be the DC ground lead too. If things are hard wired and you know which wire is also ground, you can safely add another half-wave supply for the tuner.

Can you just open the FX1 and tap DC power from in there? That would get things running, but the tuner power would end up in a loop or running over your signal lines. At least a tuner is a safe one to do that with (no oscillators to worry about).

The ultimate option would be to get a little 9V transformer with dual secondaries. Connect one secondary to your 9VAC input and the other secondary can now go to a bridge rectifier and a cap to power the tuner. Tape up the primaries because they'll try to deliver 120V to anything they touch.

If you don't do one of those options, you'll be passing DC through your signal connections.

The supply is actually the GCX supply.  I can't open the FX1, as it's a $1000 box and I don't want to void the warranty.  I can tap the 9V supply between the GCX and the FX1 though, at a MIDI (5 pin XLR actually) jack.  The GCX and the FX1 share the same supply, with current to spare, so I was thinking I could just tap into it, like branching circuits in home wiring.

I'll ask Mario at Axess about the power coming into the FX1 to see if the AC negative is also the DC ground.  Maybe he'll have other ideas.