MFOS Adjustable Power Supply

[STremclad]

Hi,

Just purchased a 2A 24v (12V center tap) transformer to feed my Adj Power Supply.

First question: Is 2A to much? The MFOS adj. power supply is meant to deliver 1.5A max, the only issue I see is that a 2A transformer will be less forgiving if there is a problem with my modules or the MFOS power supply board itself(?)

Secondly: Could I actually use this 2A transformer to feed two adj. power supplies? 1A per adj power supply would probably be sufficient for my needs anyway.

Thank you.

[ashleym]

I am just building one of those too. Have a look at all of Ray's notes and the thread "LM317/LM337 Power supply KIT " below for some help.

A 24V 2A supply will be a little under powered or marginal at best. Think Power=IxV watts, 24x2=48. For your supply 2 sets of 12V at 1.5A, 2x12x1.5=36W. There will be some losses in the regulation (the heatsinks on the regulators are there for a reason) and so you soon get close to the transformer maxing out. The difference in price is minimal between a marginal and a well specified transformer and as this will be your pride and joy- do it some justice!! I understand you might not be expecting to draw too much current but save yourself the worry and build a tank. Then again if you really dont want to build in any spare capacity get a lower rated set of regulators. I think there are 1A or 500mA versions.

The other thread recommends something like a 150VA transformer and that is what I am going for. I am also getting a 15+15v transformer. Again there are some losses in the regulation process and any voltage loss will drop you below the required 12v a side.

[STremclad]

Thank you for the insight. Makes a lot more sense now. And yes, I'm aiming for a tank, as of now, per my calculations I have a rough requirement of 1.2A worth of modules without padding and I'll be expanding of course so I better go for the kill right away.

Any special transformer make/model you would recommend?

Thank you.

[ashleym]

More than happy to help.

Try this

http://search.digikey.com/ca/en/products/VPT30-3330/237-1340-ND/2090078

it gives you the required 2x15v outputs, you have to put one end of each of the 15v outputs into the CTAP input on the PCB. The specifications show 6.66A output with both 15v outputs paralleled or 3.33A per 15v output.

Yes that sounds like you have got twice the current you need but you can still think in terms of pre-economic crisis cheap less efficient car engines (phew) think of a big V8 cruising or a tiny 4-cylinder revving away, one is relaxed and the other is working hard. The difference here is that the circuit will only draw what it needs so you dont have the petrol consumption problems of a big V8. So having 3A available when you need 1.5A keeps everything running well.

The same goes for the components you put in. For the main capacitors they might only see 12V but you always add at least 1/3 to their specification, ie 16V. But when you look through a catalogue you might see capacitors rated at 16, 25 and 63V and for this project I would recommend the 25V rated caps.

[STremclad]

Thank you mon ami for your help, I'll be ordering that exact transformer in a few days.


Take care

Pat