Resistors Question

On Mon, 4 Sep 2006 19:24:27 -0500, I said, "Pick a card, any card" and "biff" instead replied:

Yes. It is both the equivalent and the same value with the added advantage of being center tapped should you need it. Be careful of the wattage ratings on the resistors you choose. A bad choice can turn both into fuses that blow rather than resistors.

-- Ray

Thanks!

In this case the wattage of each resistor only needs to be half of the desired wattage for the 1120 ohm resistor. So, if you need 1120 ohm, 1 W resistor, you can use two 560 ohm, 0.5 W resistors. But if you start using non-equal resistor values, the wattage will no longer be split equally between them.

Peteski

Yes. Resistors in series add. Resistors in parallel follow a more complicated rule:

1/Rparallel = 1/r1 + 1/r2

Both rules can be derived from Ohms law:

I = V/R

Which is worth committing to memory if you are going to do electrical work.

David Starr

On 4 Sep 2006 19:30:54 -0700, I said, "Pick a card, any card" and "Peter W." instead replied:

I have no idea of his use so I deferred offering specific advice but this is almost correct. The heat dissipation is the factor based on whether or not both are of the same composition and so forth. Wirewound, carbon film and etc. will all have an effect on how fast a part will go if the wattage is exceeded by use. Without knowing all the relevant details it's impossible to advice him beyond to state that he should be careful.

-- Ray

On Mon, 04 Sep 2006 22:45:08 -0400, I said, "Pick a card, any card" and David Starr instead replied:

Be warned that I got a ticket once from the engineering police for breaking Ohm's Law. Cost me 30 days in the front office and a hefty fine. I never forgave the guy who turned me in.

-- Ray

On Tue, 05 Sep 2006 11:34:50 +0800, I said, "Pick a card, any card" and Ray Haddad instead replied:

Advise him - not advice him. Sorry.

-- Ray

I've got Tomar signal instructions that suggests a 560 ohm resistor for a signal LED assuming a 12.5V DC power supply. My transformer puts out 24 V, so based on the formula Tomar supplied, I figured double the 560 Ohms, hence

1120 Ohms.

This isn't correct. The full current flows through both resistors when they are connected in series. There's only one path, going through both. So if you're making up an 1120 ohm, 1W resistor from two 560 ohm resistors in series, they both need to be rated at 1W. You're thinking of parallel resistors- in that case, the wattage is indeed split between them, as there are two paths for the current to flow through.

Dale

Sorry Dale, Ray was right for the circumstances he specified. You are wrong, watts are volts times amps, although you are correct that the same current flows through both resistors the voltage across each is only half the total hence the wattage is halved. If you made your 1120 ohms from two 2240 ohm resistors in parallel exactly the same result is obtained except that this time the voltage is the same and the current is halved. Keith

You're absolutely right. I apologize for the misinformation...

Dale

On Mon, 4 Sep 2006 22:59:48 -0500, I said, "Pick a card, any card" and "Biff" instead replied:

The formula for LED current limiting resistors is Voltage divided by Current or 24 divided by .015amp, which is the current for an ordinary LED (not an UltraBright). That means you should use a 1600 ohm resistor or 1.6k. They can be purchased for less than a dollar a pair from an electronics shop. As long as the LED doesn't short, you needn't worry about the power rating for this application. A 1/8 or

1/4 Watt resistor will do just fine.

Their reasoning for selecting a resistor of the value they did escapes me. Perhaps they want to sell more signals.

-- Ray

OK, your resistance computation is correct. If they call for 560 resistor on "12 Volts" it is reasonable and proper to double that for "24 Volts". One complication: You said "Transformer". Real transformers only work on AC current and the furnish AC voltage. LED's don't like AC and as a matter of good practice they should be driven with DC. If your "transformer" is a heavy iron core device from radio shack, it is giving you AC rather than the DC you want for LED's. If you find you need a real DC power supply you can use a model railroad power pack intended for two rail operation. Or you look for an electronic surplus store and see what they have for DC power supplies. Old 5 volt computer power supplies will light a lot of LED's using resistors in the 180 to 220 ohm range.

Then you are in good shape.

David Starr

On Fri, 08 Sep 2006 20:52:51 -0700, I said, "Pick a card, any card" and Clark Martin instead replied:

I am an expert. Thanks for your concern.

-- Ray

[This followup was posted to rec.models.railroad and a copy was sent to the cited author.]

I am not sure why we need to confuse the poor guy. If he has the correct wattage resistors to begin with then there is no crime in connecting the resistors in series. In series the wattages will be twice what is required and therefore will not cause any harm.

The effective increase in wattage is reduced if the resistors are not the same Ohmic value.

Ken Harst> >

I heard about a guy who got pulled over by the Information Highway Patrol for trying to use a 1200-baud modem. He couldn't go fast enough to keep up with the traffic.

Ray Haddad wrote:

in article GbmPg.5953$ snipped-for-privacy@newssvr14.news.prodigy.com, Jay Cunnington at snipped-for-privacy@sbcglobal.net wrote on 9/17/06 5:59 PM:

Come on; neither of those jokes is current.