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help with changing transformer output of 8.5VAC 1200mA to 9VDC 1000Ma. Answered

Howdy folk, 

  The Problem; I need some experienced assistance with changing the output of a transformer from 8.5VAC @1200ma to 9VDC @1000ma. Thanks in advance for any & all assistance.

   The Back Story; The left channel went out in my PC speakers, which is designed for 230VAC, I had to destroy the woofer case to get to the transformer, and discovered that it outputs 8.5VAC @1200ma. I have another set of PC speakers (different manufacturer) that takes US voltage (110v) and steps it down to 9VAC @1000ma. I must first plug that transformer into a 100watt 230>110 step down (because I'm currently in the Philippines) and I want to get at least one transformer out of the "equation". I would just buy another system set up for local voltage, but I'm on a fixed income (until I can return to the States) that prohibits this avenue. I CAN afford some small components, but that's about it.

Thanks again for any help.



3 years ago

I notice that the 8.5 volts is very similar to 9 volts...
It's possible that they're actually equivalent, in terms of electrical requirements, but one is labelled for the phillipines, and the other is labelled for the US.

What's the frequency of the power there? Frequency changes impedance. Impedance is a complex form of Resistance... so it'll probably change the wattage (volts * amps) of the speakers. 8.5volts*1.2amps = 10.2 watts. It's similar to 9 watts (9 volts * 1 amp) if you consider the 10.2 watts might be running at 50 hertz and the 9 watts IS labelled for 9 volts at 60 hertz.

The math to figure out the exact effect of frequency on a transformer hurts my head, but, i can say with some degree of confidence, that 9 watts could be 10.2 watts if there were an increase in amperage for the 9 volt output. Since impedance is a complex form of resistance, changing frequency can alter resistance, and thus affect amps (because R= V*I, and volts are fixed according to the turns ratio of a transformer, which I will get into in a sec).

That all said, if you actually do need to alter things, the easiest and cheapest fix is to re-wind the secondary winding on the audio transformer. That would directly modify the voltage output to what you want, and all you'd need is insulated copper wiring of the correct thickness to accomplish the correct turns ratio.

All transformers conform to a certain ratio which governs their input to output ratio. That ratio is determined thusly:
(Voltage in the primary winding) / (Voltage in the secondary Winding)

This ratio is referred to as the turns ratio and it's called this because it literally means the number of turn in one winding to the number of turns in a secondary winding.

Ehh... noticed the voltage change a bit more... 230 > 8.5 is a bit different then 120 > 9.
They give you turn ratios that are more then double one another. Which is a problem. I think others are better suited to help... but the impedance and turns ratio deal are both valid info...


3 years ago

Since AC voltage is a sine wave, it's constantly changing, and measured as an average--RMS (Room Mean Square). The peak voltage is higher. Once AC is rectified and filtered with "reservoir" capacitors, it's DC value approaches (theoretically) 1.4 times the RMS voltage. An 8.5VAC transformer will result in (very nearly) 12VDC (11.9V approx) after it's conversion.

However--your post mixes AC and DC values (9VDC changes to 9VAC in the second paragraph). Are you asking if a 8.5VAC transformer can be replaced with a 9VAC transformer of similar size?

Probably-- although the less capable replacement transformer (1000mA) might not be up to the task. It's close enough to try it. If it's not a Class A amplifier in the speakers it might draw less current simply by keeping the volume lower...

Doh!,,,Sorry for the careless mistake. The second paragraph should read 9VDC. So, it now appears what I am asking is 3 part;

1) Can I convert 8.5VAC @1200mA to 9VDC@1000mA?

If yes, then,

2) What components would I use to do so?


3) In what configuration would I solder everything together?

Thanks again for your time & attention.

As stated above the voltage should be enough if you add a rectifier and after the rectifier on the DC side some nice caps in the 4700µF range, rated at 16 or 20V.

If you really need the xact 9V you will have to use a voltage regulator as well.

For example the LM7809 - the datasheet will tell you what connection goes where.

Be aware that you also should use a good heatsink for the LM as it is only rated for around 1.5A.

Just to be sure we're all on the same page: two 9VDC 1000mA (1 amp) power supplies are needed, and one already exists. So you need to make one from an 8.5VAC 1200mA transformer. Right?

Yes, that transformer will work. It can safely supply more current than you need. But converted to DC, the output will be nearly 12V.

Using a voltage regulator as Du35m suggests is probably wise. The LM7809 should work in this application. While the datasheet officially quotes a supply voltage of 12VDC (11.5V tested under some conditions), the dropout voltage is 2V, and your supply will exceed that (11.9VDC > 9VDC + 2VDC).

This might be more than you bargained for. But it should be a fairly simple build. At this point, I might be asking "can I find a 9VDC 1A (1000mA == 1 amp) that's cheaper and easier than buying all the components and building this thing?" Could be worth pursuing... You'll need a soldering iron, etc. as well.

A final note: some analog amplifier circuits will run just as happily on 12VDC as 9VDC. You might not want to test that, though. If it's bricked, there's no coming back...

gmoon sez;" two 9VDC 1000mA (1 amp) power supplies are needed, and one already exists."

Well, no and yes; I do have the factory "brick" (Out: 9VDC, 1A) For the Altec speakers, but it's made for 110v input, so I have to plug it into a 50W 230V > 110V step down, which works fine, but it bugs me at some deeper, spiritual/reptilian level such that I am involuntarily compelled to "do it with 1 single transformer". I also have a transformer from a Creative Labs sound system that I liberated from it's entombment in the woofer case that is already set up to convert 230v into 8.5VAC 1200mA. I only need 1 power supply, and I want it to take the local voltage (230V) and output 9VDC, 1A (Or close enough to power the sound system w/o frying it)

gmoon sez;" At this point, I might be asking "can I find a 9VDC 1A (1000mA == 1 amp) that's cheaper and easier than buying all the components and building this thing?" Could be worth pursuing... "

Ya know, you are absolutely right. but it might also be a case of I might just need to tell myself to "Quit moaning and embrace futility". I'm very much out of my element here. My closest big city is Iloilo, it doesn't have the resources & conveniences of a like sized US city, I don't know my way around, and don't speak more than a few words of Tagalog or Illongo.

@Downunder35m Dude, thank you very much for mentioning the LM7809! That tastey lil morsel led me to http://www.circuitdiagram.org/ which has beginner tutorials, what I actually spent several hours failing to find before I decided to lean on the Instructables community. I actually haven't had to deal with this level of electronics since 1990, so I can repair the stuff if it's obviously burnt, but I can't necessarily remember all the info from ac/dc circuits & semiconductors 101. This was super-duper helpful.

I had another thought and there might be an even better way for you.

These new lectronic power supplies we all love for our phones and tablets are available in 9V as well.

1amp output is not too much, so they should be available in your area too.

All you need is to add a suitable connector to the speakers and with that external power supply the input voltage does not matter as those "electronic transformers" work in the US and the modern parts of the world the same way.

Ya know, that is a damn fine idea. My wife has like 5 phones, one of them has a charger that employs a USB cable to get power from the wall brick (9VDC @1200mA) to the phone, which begs the question; "Could I not just get a "Heavy Duty" USB cable (Like for a passport hard drive) and splice the DC cable into that, and juice off a USB port?" If I could do that, I would only be drawing power for the PC & monitor, and THAT idea tickles me greatly!!!

Put a 9V regulator into the 12V line and place it into the airstream of a fan.

Add a standard plug and mount it to a free slot in the back.

If the current transformer provides 8.5AC it should be enough with a rectifier to get to your 9V DC.

Add a nice big capacitor and maybe a toroid coil to filter out the remaining 50/60Hz noise and should be fine.