Rectifier diode assistance

New to this forum and here's my situation;
I have a transformer rated at 115vAC at the primary and 12-15vAC at the secondary.
I want to use rectifier diode to convert the secondary voltage from AC to DC with the DC reading going no greater than 12. I want to supply power to a 12vDC electronic timer card.

I have looked at Digikey's website, but get lost really quick.
What diode will work for my project and keep me from smelling fried plastic? The timer board cost $45.

Many thanks and if more info needed, please ask.

Roland

 

What is the power rating of the transformer and what is the power required from the card?

 

The transformer is 115vAC imput currently reading 124vAC.
The timer card is 12vDC max input.

 

Is the transformer secondary winding center tapped? (12v-0v-12v)
Or is it just two leads?

If it's two leads, you will need a full-wave bridge rectifier, a large electrolytic cap rated at least 50v, and a voltage regulator IC with a cap on the output.
[eta]
See the attached for one way to build a linear regulator supply.

 

I am talking about the watt ratings. Look on the devices, on the transformer it will say VA and not Watts. If you can't find the watt ratings tell us the current ratings.

 

This is a GRS (General Railway Signal) transformer with no wattage ratings on the nomemclature plate.
The transformer is center tap with various voltage rating tap possibilities. I can get readings anywhere from 1.2-17.4 vAC depending on where I place the voltmeter probes.

 

Ok, you could just use the tap where you measured the 17.4VAC and connect it up like the circuit I threw together for you.

If you don't have a rectifier bridge, you could build one out of 1N5400 series diodes.

 

Ok, SgtWookie, I'll give that a try. Does that D1 bridge in your schematic have anymore specifics; like maybe a Radio Shack no. or some identifying no's?

 

Just happend to have 1999 RS cat. at hand;1N5402 200 V @3 A ,276-1143; FWB 100 V @4 A, 276-1171; 12V Reg. @ 1 A, 276-1771; Adj. reg. @ 1.5 A 276-1778 Hope nos still same.

 

What your local RS might have in stock can vary widely.

This bridge rectifier will do the trick:
http://www.radioshack.com/product/index.jsp?productId=2062580

But it may not be stocked at your local RS.
Their online diode/rectifier catalog is here:
http://www.radioshack.com/family/index.jsp?categoryId=2032269
In a pinch, you could just buy an assortment of 1N4000 type diodes, but you'd be better off with rectifiers that can handle at least a few amps.

 

mik3 and Bernard,

Thank you for your inquiries and imput concerning my project.
Roland

 

SgtWookie,

I have the Radio Shack bridge rectifier no. joted down and will check its availability soon.
Now, to be sure I have things correct, I will need to attach a short pigtail to each of the four (4) leads coming from the rectifier component. These leads will then be used to make the connections to both the transformer at their + and - posts and the + and - contacts on the timer card. I can even check the current voltage from the rectifier after making the connection at the transformer to be sure I am receiving DC current coming from the rectifier.
Your response when convenient and thanks again.
Roland

 

Roland,
I'm afraid you've misunderstood me.

The rectifier bridge is but one single component in a simple linear regulated power supply:


If you attempt to power your timer card directly from the output of the rectifier bridge, you will let all of the smoke out of it. You're measuring 17.2 VAC from the transformer output, which is an RMS value. When rectified to DC, the unregulated output will be around 23 VDC!

You must use a filter capacitor, a voltage regulator, either an LM317 with the two values of resistors on the output that I have specified, or a fixed-voltage 7812 regulator (without the 120 Ohm and 1k Ohm resistors, but the GND terminal tied to ground) and the 10uF and 0.1uF caps on the output to surpress transients (voltage spikes) and improve regulation.

You will also need a heat sink on the tab of the LM317 to dissipate the heat it will generate.

Your transformer does not have + and - outputs; they are all AC outputs. You will connect them up to the AC inputs to the rectifier bridge. These are usually marked using tildes (~) to represent sine waves.

Note that the first item on the primary "hot" side of the transformer is a fuse. This is not an optional item; it is for safety. If you mis-wire something, very high current may flow, and burn things up.

S1 is an optional on/off switch.

Note that on a 3-prong plug (in the States) the narrower of the two blades is the "hot" connection (black wire).
The wider blade is "Neutral" (white wire), the return path for the 120v "hot".
The round pin is ground (green wire), and for safety must be connected to the chassis of your project. That way, if the transformer winding insulation should fail on the primary side, the current through ground will cause the fuse to blow.

 

SgtWookie,
Whew!! We don't want to let the smoke out, that's for sure. I have been studying your schematic and trying to find the components needed using the Radio Shack online catalog.

Being electronically challenged, I will no doubt have some more questions. I was a BT (boiler technician) and not an ET (electronic technican) in the Navy.

 

Don't be impressed by ET's, it was the DS's who knew their stuff (from ex DS2).

Radio Shack has a puny line of stuff. Mouser Electronics or Digi-Key Corp. have every part you might need, but big catalogs to crawl through. I think their prices are generally better, too.

 

Ok beenthere, enlighten me, what was a DS? I wasn't acquainted with many ET's anyway. There hasn't been any BT designations since the 80's?
I have stumbled my way through a Digikey catalog and it's mind boggling.

 

That was Data Systems technician - the rate got canned in the 80's as well. We kept up the old Univac computers in the first NTDS setups.

 

SgtWookie,
In the schematic drawing you submitted to me, C1 is designated as 2200uF, 50v. Is this the same as what Radio Shack calls their filter capacitor p/n. 272-997; 2.2 uF 50v Non-polarized Electrolytic Capacitor?

beenthere recommends a higher quality supplier/dealer for electronic components, such as, Mouser or DigiKey. What has been your past experience?

Many thanks,
Roland

 

Thanks for helping him out, Bertus!

I'm afraid that Radio Shack doesn't carry a suitable capacitor for this project any longer.

The particular capacitor you're asking about is 2.2uF, the one I specified is 2,200uF, or 100 times as large. It would not be sufficient. The one you found is intended to be used in audio applications, in particular for speaker crossover networks.

Beenthere is absolutely right.
Both Mouser and DigiKey are first rate suppliers, and are authorized distributors for a number of semiconductor and component manufacturers. This means that when you order parts from them, you can be assured that the components will perform to meet or exceed the specifications in the manufacturer's datasheets.

This is quite different from Radio Shack; you can never be sure who actually made the components you're getting unless you bring a magnifier with you and are able to determine the manufacturer by reading the tiny printing on the part itself through the bubble pack or plastic bag.

You will also discover that most parts at DigiKey or Mouser are considerably less expensive than if you purchased them at Radio Shack.

There is one catch; the shipping costs.
In the case of VERY small orders, DigiKey wins out, because they will ship small and lightweight orders via USPS 1st Class mail, which can save you money.

I prefer Mouser's part search screens to Digikey, but I use them both.

They both ship quickly, and package parts so that they are well-protected.