Suitable smoothing capacitor

[RobSmith]

Hi all,

Yesterday my son and I made up a velleman 'delux Christmas tree' kit.
It is great but flattens its battery in a very short time. I rummaged around in the shed and found a mains to 9vdc 250mA power supply.
It looks lke just the job except it has the wrong plug on the end of the wire. I had another power supply with the right kind of plug so opened up the power suplies to swap the leads over. I was surprised to find how cheap and nasty it is inside and also has no smoothing capacitor.
The only components in there are transformer, 4 diodes (bridge rectifier), a resistor and an LED. Presumably the resistor is for the LED.
Can anyone recommend a suitable value capacitor and discharging resistor to go in there?
I was going to pop in a 2200uF 16v electrolytic capacitor and a 5K resistor as I have these.

Rob

 

[GonzoEngineer]

What is the 5K resistor for?

 

[RobSmith]

What is the 5K resistor for?

Just to discharge the capacitor after the power is turned off.
It is maybe not needed on such a small capacitor?

Rob

 

[GonzoEngineer]

I wouldn't bother with it, just wasted power (albeit small).

It's not like 9 volts is going to shock anybody.

 

[RobSmith]

Would the 2200uF 16v capacitor be suitable?
I have others but that fits nicely inside the original case.

Rob

 

[Resqueline]

Probably.
But remember that the 9V is referenced to an unsmoothed voltage. When you smooth it it will rise to the peak voltage (12.6V).
In addition comes a voltage rise due to any load of less than 180mA. With no load the output voltage could exceed 16V.
You could use the LED and perhaps an additional discharge resistor to make a minimum load to pull down the peak voltage.

 

[RobSmith]

Probably.
But remember that the 9V is referenced to an unsmoothed voltage. When you smooth it it will rise to the peak voltage (12.6V).
In addition comes a voltage rise due to any load of less than 180mA. With no load the output voltage could exceed 16V.
You could use the LED and perhaps an additional discharge resistor to make a minimum load to pull down the peak voltage.

Thanks for that. I will have a play later and see what the output sits at with a few different loads. I have a 2200uF 30v capacitor which I might use to clear the 16v. I just need to solder some legs on that one as it came from an old circuit.

Rob