Need help building 21VDC, 900ma transformer

Hi,

I need to build a 110VAC to 21VDC, 900ma power supply.

(This is to power an ACT Labs gaming steering wheel for which I lost the power brick)

Would someone be able to provide me a schematic of the circuit I need to build and the parts?

I am new to the power supply world, how would I control the output voltage 21VDC while also limiting the current 900ma?

Thanks!
Sharad

You dont limit the current to 900mA, you just make sure that the Power Supply is capable of supplying at least 900mA without the voltage dropping, or without the power supply overheating.

I'm thinking you do not what to build a transformer; rather starting from a commonly available transformer that you can buy, you want to build a regulated power supply.

Frankly, I would look at the power supplies and transformers which are available from a surplus electronics dealer, like this one.

Thanks for the reply.

So if I buy a power supply that has an output 24vdc (with an output amperage greater than 1 amp) which is higher than the 21vdc that I need, knowing that the consumer will only suck up up to 900ma, can I just put an appropriate resister in series with the consumer to bring it down to 21vdc?

Thanks
Sharad

You cannot use a resistor to drop the voltage unless the current is always the same. The current probably varies so then the voltage will be too low and too high sometimes if you use a resistor instead of a voltage regulator.

Use a 24VDC/1A unregulated power supply feeding an LM317 adjustable voltage regulator IC. the LM317 needs an input capacitor, an output capacitor and two resistors to set its output voltage. Its max current is 1.5A to 2.2A. It might need a heatsink.

Thanks. Sorry I have another related question..

I found a power supply that outputs 21vAC and max 900ma. Note that the output is AC. I'm assuming I can attach to that a rectifier, smoother and regulator to provide a smooth DC signal of the same voltage and allowable amperage? If so would I be able to preserve the 21V and 900ma max amperage?

This would be another option.

Thanks

When 21VAC is rectified and filtered then its peak voltage (minus the 2V loss of the full-wave rectifier) is produced which is 27.7V. It is too high.