self made power supply

ready961 · Feb 28, 2006

Hello

Im thinking of making a small power supply for a couple sensors I need to power. 1-2amp would be plenty. I was thinking of ordering a transformer, bridge rectifier and capacitor from Allied electronics. I have never have done this before so any suggestions would be helpful, like sizes and such. Or any reason I should not do this.

Thanks

randylud · Feb 28, 2006

Go for it!

If you after a small supply and the fun of building it, then go for it. You can Google and get a ton of ideas plus good workable circuit diagrams. However, if you want a proven product, then these guys http://www.elpac.com/ are a good choice and can be gotten through your local Newark Electronics outlet. The job of doing what you need is pretty easy, so I say try it if you have the freedom to do so!

ready961 · Feb 28, 2006

Ya it an educational thing to. Good idea on google. Not to sound stupid but what do VA stand for on transformers?

burnerman · Feb 28, 2006

VoltAmperes, i.e. Secondary Voltage x Current

rdrast · Feb 28, 2006

If this is for a plant, then there are a couple of reasons not to 'roll your own'...
#1 is that it wouldn't be certified, or listed, under any authority (such as UL).
#2 is that at some point, it will fail, and it's a lot easier for another person to order a Sola/Idec/Other small replacement based on the model number.
#3 is that even at very low ratings (15/30 watt) the new switching supplies are cooler and much more efficient that a brute force regulator.
#4 is consider the cost of the sensor you are supplying... In general, here, a pressure transducer might cost anywhere from $200 to $1000... Just for peace of mind, I prefer to put a decent quality, commercial, $50 supply in front of it

Sergei Troizky · Feb 28, 2006

For a simple transformer-bridge-capacitor power supply do not make a common mistake using 24VAC secondary to achieve 24VDC.
Your transformer must be ~20VAC secondary for this.
The current consumption from the transformer will be 1.4xDCcurrent on the output.

elevmike · Feb 28, 2006

Ready,

I totally agree with rdrast.

However if your going to build your own anyway, dont forget to get a voltage regulator & heat sink for the regulator. The power will flow through the transformer, then on to the bridge & cap, then through the regulator. The regulator will stabelize the output. (cheap insurance for your sensors). You can go to your Local Radio Shack and buy a regulator. On the back of the package it will show a wiring diagram for the entire power supply. I built a variabe power supply a few years ago for my kids slot car track. We could adjust it so the cars wont all whip off the track when somebody stops... Worked great.

randylud · Feb 28, 2006

burnerman said:
VoltAmperes, i.e. Secondary Voltage x Current

Well, yes, but VoltAmperes is Volts Times Amps no matter if it is Primary values or secondary values. The transformer could be a 50VA or whatever, so the primary voltage/current ratio is set by this value as is the secondary. Example, if you have 120VA transformer, for ease of numbers, the primary could be 120V so that would mean a 1A current would flow at full load. Now, if the secondary is 60V that would mean that the secondary could have 2A of current at full load for the same 120VA rating. Cool, huh?

Alan Case · Feb 28, 2006

Dont bother trying to make one yourself. They are cheap to buy. Also the voltage will fluctuate with load, especially with a capacitor. I have seen over 30 volts DC on a 24V AC transformer, rectifier and capacitor setup with no load.
Regards Alan Case

rsdoran · Feb 28, 2006

Sergei Troizky said:
For a simple transformer-bridge-capacitor power supply do not make a common mistake using 24VAC secondary to achieve 24VDC.
Your transformer must be ~20VAC secondary for this.
The current consumption from the transformer will be 1.4xDCcurrent on the output.

I think the secondary output needs to be 25.8vac for a 24vdc output, this allows removal of the voltage drop across the diode(s). If you use a bridge rectifier expect a 1.4 drop to approx 24.4vdc.

I am not sure what is meant by "being certified", any transformer or rectifier will have its own certification. I have seen and used bridge (and other) rectifiers for many years...at one time it was very common, but today does not look as pretty.

For most devices as I stated a 25.8v step down transformer rated for the necessary current with a bridge rectifier rated at 50WVDC (working volts dc). If you want to lower the voltage ripple then a 1000uf electrolytic capacitor rated at 50wvdc.

keithkyll · Feb 28, 2006

Ron: The 24vac rating is RMS. The caps will charge to peak. Multiply rms x 1.414. 20vac will give 28v unloaded. This is a good range for 24v unregulated.

ready961: Check the specs for a LM337 or LM350. This is an adjustable 3 terminal regulator from National and many others. It is the simplest wat to create an adjustable supply. Check the datasheet for circuit examples.

Doug_Adam · Feb 28, 2006

I have made some of my own sypplies in the past, but they are now so cheap and good quality that I would not any more.
Even for home hobby projects, I use commercial plug packs, they are cheaper and faster.

However, if you have never done this before, it would be a good education. So go for it if you want.

darrenj · Feb 28, 2006

Go for it!! I just wish some people didnt post Cap sizes ot transformer voltage.. It would be interesting and more benifitial if the poster came back in a week asking why he had to much voltage..Or not enough..Then tell him why..

O.K. we saved a few bucks for the guy by telling him now..i think he would of learnt a little more had we not told him..

CaseyK · Feb 28, 2006

Make one for the test bench or plc wall.

For best reliability.....buy one, and a spare, for a machine or panel.

I used to manufacture power supplies and emergency lights in a basement lab in the early seventies. Lotsa time consumed, lotta fun, not much profit, a lot or filament transformers still laying around (1.2 amp CT).

regards.....kc

self made power supply

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