Too much up and down output from small linear transformer!

[Willen]

It is a tiny Linear 50Hz transformer (iron core) rated 9V, 0.2A. Is it used on a simple 3 transistor audio amplifier circuit (PC speaker).

I didn't worked with transformer much. Today I tested its AC output with no load and got around 10.5V AC. I used bridge rectifier with high value caps and got around 13V DC.

I was trying to use it to another device (piano) which need 9V DC input supply. I was sad because tiny transformer gave 13V DC. But when I connected this 13V DC to piano, I got lots of voltage drop. The piano turned ON through this transformer and I got out only 8V DC output. I played the piano with LOUD sound and voltage was just 6V fluctuating. Then I learnt- small transformer (low Amp rated) drops its output voltage according to its load (load current). But 6V to 13V is very large potential difference. So if I didn't calculated Load current Vs Output voltage carefully then I will burn few more sensitive devices by using such power supply.
(piano needs 55mA current according to my poor Digital MM)

Are there any simpler understanding about such load current vs output voltage calculation?

 

[alec_t]

The transformer windings have resistance. The greater the current drawn from the transformer the greater will be the voltage drop across that resistance. Ohm's Law.

 

[dr pepper]

Small transformers have low regulation, this means the transformer is unable to maintain its output voltage under anything other than a light load, a light load being all the trans is meant for.
If you operate the transformer so that the load reduces it output volatge below that it is rated at you'll burn it out.

 

[Diver300]

Small transformers have low regulation, this means the transformer is unable to maintain its output voltage under anything other than a light load, a light load being all the trans is meant for.
If you operate the transformer so that the load reduces it output voltage below that it is rated at you'll burn it out.

I think that should be high regulation.

Transformer regulation is the change in voltage from no load to full load, divided by the full load voltage. It is usually expressed as a percentage.

The lower the number, the less the voltage changes. Small transformers have a high value for this, as their regulation is poor. Large transformers have better regulation, and it might be just 5% or so.

Also some really small transformers are made with such poor regulation that they won't burn out even if shorted out. This is sometimes called "impedance protected". Other transformers are at risk of overheating if the the output voltage is loaded to where it is less than the rating when the input at the rated voltage.

 

[NorthGuy]

When you measure AC, you get an RMS (Root mean square) value, but since this is AC, the actual voltage varies from -Vrms*sqrt(2) to +Vrms*sqrt(2) (in case of pure sine wave). So, for 9V this is -12.7V to +12.7V.

If you rectify this with bridge rectifier, the negative half of the wave gets flipped pver and you get a wave with voltage from 0 to +11.5V (I figured 1.2V diode loss)

When you put a capacitor accross, it gets charged at the peak to 11.5V and, if there's no load it stays that way, so you should measure 11.5V on the capacitor.

When you add a load, the capacitor gets discharged between peaks, may be severely, depends on the load and the capacitor, so the average voltage you measure will be less and less.

At the same time, as others said, when you increase the load the voltage produced by the transformer decreases. That's why you get 13.5V instead of expected 11.5V without load, but it goes down when you apply the load.

Therefore, both capacitor and transformers may be suspect,

If you cannot get 55mA from the transformer rated for 200mA I would suspect the capacitor first.

 

[tunedwolf]

piano needs 55mA current according to my poor Digital MM

I don't know of any small musical instrument having such a low current requirement. Most of the ones I have seen or repaired seem to range from 1A - 2.5A Perhaps when you measured your current being drawn, the keyboard's amplifier was idle rather than being driven? Either way, what is the make/ model of the keyboard/ piano that you are trying to get working?

 

[audioguru]

I have a Chinese 9VDC/200mA wall wart. Its output is 18V with no load and is 9V with lots of ripple at 200mA. It gets hot with a 200mA load.
It is not approved for use in North America.

 

[Willen]

I learnt lots! Thank you!

NorthGuy, did you mean if multimeter says 9V AC then should I have to understand that this 9V AC actually have +12.5V and -12.5V peak to peak sine? (I am a poor student in the case of RMS )

Tunedwolf, Hehe don't be serious, it was just a toy piano. Piano had a power supply same as call phone charger before but it has been shorted. So changed it with this 9V, 200mA transformer.

 

[NorthGuy]

NorthGuy, did you mean if multimeter says 9V AC then should I have to understand that this 9V AC actually have +12.5V and -12.5V peak to peak sine? (I am a poor student in the case of RMS ).

Yes, assuming you have a pure sine waveform. Actually, DMMs often measure peak and then divide by 1.4.

 

[Willen]

Then can I say RMS is- 'peak voltage of sine' devided by '1.414' isn't it?

Like

+12.5v/1.4= RMS value

All devices has AC input voltage ratings like 230V AC, that means can I say this 230v is RMS? And actual peak of this AC is 230v multiplied by 1.414? Wow simple if I am right.

 

[audioguru]

Then can I say RMS is- 'peak voltage of sine' devided by '1.414' isn't it?

Like, +12.5v/1.4= RMS value

Correct.

All devices has AC input voltage ratings like 230V AC, that means can I say this 230v is RMS? And actual peak of this AC is 230v multiplied by 1.414? Wow simple if I am right.

Correct again.