Using 24v 0.4A to power up a 20w 12v halogen bulb

[X3msnake]

Greetings

I'm a complete newbie in the electronics realm, I wonder if you guys can help me figuring out how I can fix this problem I have.

I have this 20w 12v halogen bulb that I´m directly connecting to a 12v power. So far I have manage to burn 2 power supplies, one 12v 0.8A and one 12v 1A, My guess after investigating a bit, according to one amp calculator that can be found in the web this lamp needs 12v 1.6A at least to run without overheating the Power supply.

Makes sense since one could fry an egg at the time of death of those poor power supplies.

I went trough my stash of power supplies and found a 24v 0.4A, My belief after pluging it directly to the bulb is that it will hold but the lamp seems to heat up more than it might be normal.

Hope someone can point me in the right direction as to what must I do to use this 24v 0.4A supply without burning the lamp and specially the Power supply.


Thank you

 

[audioguru]

24V x 0.4A= 9.6W. Its voltage is too high and its current and power ratings are too low for a 12V/20W light bulb.

When the 12V light bulb overloads the transformer then both get extremely hot. One will quickly burn out and the other will be strained to almost destruction.

 

[mvs sarma]

Greetings

I'm a complete newbie in the electronics realm, I wonder if you guys can help me figuring out how I can fix this problem I have.

I have this 20w 12v halogen bulb that I´m directly connecting to a 12v power. So far I have manage to burn 2 power supplies, one 12v 0.8A and one 12v 1A, My guess after investigating a bit, according to one amp calculator that can be found in the web this lamp needs 12v 1.6A at least to run without overheating the Power supply.

Makes sense since one could fry an egg at the time of death of those poor power supplies.

I went trough my stash of power supplies and found a 24v 0.4A, My belief after pluging it directly to the bulb is that it will hold but the lamp seems to heat up more than it might be normal.

Hope someone can point me in the right direction as to what must I do to use this 24v 0.4A supply without burning the lamp and specially the Power supply.


Thank you

24V 9.4A can deliver at the mx 9.6 watts
As your bulb is designed for 12V, you need to convert 24V to 12V and it will end up loosing power due to converter efficiency and you are likely to get 80% max. Thus you may continue search for a power supply that would deliver 2 amp or above at 12V, so that 1.6 amps is at 80% and that would be a safer limit.

PS: I meant 24V 0.4a and not 9.4A-- sorry for the error.

 

[KMoffett]

no! No! NO! You can't use it at all for a 12v 20w lamp. A "24v 0.4A" supply is capable of only 24V x 0.4A = 9.6W. There is no magic way to reduce the voltage (V) and increase the power (W) output of your 24V supply. You need a "12V" power supply that in capable of "at least" 1.6A.

"I can nah change the laws of physics captain!"

Ken

 

[X3msnake]

Hehe

Thanks guys


So resuming to see if I get this right

I Could power up a 12V bulb with a 4.5v 5A?
By just pluging it to the output line?


Best regards.

 

[X3msnake]

BTW as for the resistance value, It is the dissipated power right?

Thanks

 

[silvarblade]

i think people did say that you need a 12v supply ot power your light bulb with the needed ampere rating of course,so your 5v supply wont do unless you use a converter like sarma said but that would make it too complicated. since you are in the way of buying power supplies why dont you buy one of 12v?? i think they are easily available!

 

[X3msnake]

i think people did say that you need a 12v supply ot power your light bulb with the needed ampere rating of course,so your 5v supply wont do unless you use a converter like sarma said but that would make it too complicated. since you are in the way of buying power supplies why dont you buy one of 12v?? i think they are easily available!

Thanks

Just tring to understand a bit of how the volt/amp/watt relation works.
I was trying to see if I could use the Power supplies I have @ home, didn't wanted to buy a PS before I had exausted the possibilities.

Best regards

 

[KMoffett]

Thanks

Just tring to understand a bit of how the volt/amp/watt relation works.
I was trying to see if I could use the Power supplies I have @ home, didn't wanted to buy a PS before I had exausted the possibilities.

Best regards

Yes, you could power it up, but it may not even light up. Because of the resistance of the lamp, it requires 12v to push the needed current through. The resistance of the lamp is the constant here, about 7.2Ω (Ohms). The power is what determines how hot (bright) the lamp filament gets. This lamp requires 20W to operate at the correct brightness. Halogens need high heat (brightness) or they don't last long.

So: Voltage =E, Current=I and Resistance=R, Power=P

This is Ohm's Law in several arrangements: E=IR, I=E/R, R=E/I, , I=√(P/R), E=√(PR)

Because of the bulb's design, the Power to work correctly is constant at 20W and resistance of the filament is constant at 7.2Ω.

E=√(PR)=√(20x7.2)=√144=12V

I=√(P/R)= √(20/7.2)= √2.77=1.66A

Higher voltage will push too much current through the filament and burn out the bulb. Lower voltage will not push enough current and the bulb will not light properly.

With your power supplies you want a "12v output". The current rating is the "maximum current" it can provide. So, a 12V supply with an any output current rating of greater than 1.6A.

Ken

 

[X3msnake]

Thank you v. much Kmoffett
That really helps a lot.

Btw I found miself a great PS I had laying around. A PC Supply, the 12v is able to wistand 9A, wich makes possible to connect more than one lamp at a time better than all it is air cooled

Thank you all for helping me understand a bit more about the basics and put an end to the supply killing spree hehe .

 

[Hero999]

You might need to provide a load on the 5V rail for the power supply to work properly.

 

[Willbe]

You can parallel several 12v supplies (wall adaptors from Hosfelt.com) so the current totals to the required amps, but you may need low-ohm current sharing resistors.

 

[X3msnake]

You can parallel several 12v supplies (wall adaptors from Hosfelt.com) so the current totals to the required amps, but you may need low-ohm current sharing resistors.

Hum... that sounds interesting

How would the calculations go?
For example if I had a 12v 1A and a 12v 0.5A and a 12V 1.6A, in parallel it would make the equivalent of a 12v 3.1A?

And what do you mean by using a sharing resistor, why?
Would 100Ω resistors do or would it be too much? It would bridge trough the supply's negative poles right?

Thanks

 

[KMoffett]

Hum... that sounds interesting

How would the calculations go?
For example if I had a 12v 1A and a 12v 0.5A and a 12V 1.6A, in parallel it would make the equivalent of a 12v 3.1A?

And what do you mean by using a sharing resistor, why?
Would 100Ω resistors do or would it be too much? It would bridge trough the supply's negative poles right?

Thanks

It is possible...for an emergency...but I can't see any reason to ever buy small DC power supplies (wall warts) and parallel them for more current. Definitely not if they're "regulated". They do made transformers that have two secondaries that can be put in series for more voltage, or paralleled for more current. But, their windings are pretty well matched for this purpose.

If you don't need to, don't go there.

Ken

 

[X3msnake]

It is possible...for an emergency...but I can't see any reason to ever buy small DC power supplies (wall warts) and parallel them for more current. Definitely not if they're "regulated". They do made transformers that have two secondaries that can be put in series for more voltage, or paralleled for more current. But, their windings are pretty well matched for this purpose.

If you don't need to, don't go there.

Ken

Like you said Ken, just packing up info for an emergency and trying to learn a bit from you guys . Like I said before I found the perfect solution for this particular case in an old 350w PC Power Supply

Thanks again for all the tips

 

[Willbe]

How would the calculations go?
For example if I had a 12v 1A [#1] and a 12v 0.5A [#2] and a 12V 1.6A, in parallel it would make the equivalent of a 12v 3.1A?
Yes.
Would 100Ω resistors do or would it be too much?

For identical output voltages at rated current, no current sharing resistors are necessary.
12v/1A = a 12Ω load, 12v/0.5A = 24 Ω, 12v/1.6A = 7.5Ω.
12||24||7.5 = a 3.9 Ω load. 12v/3.9Ω = 3.1A.
I don't believe open-circuiting this arrangement will harm any transformers.

If the voltages and current ratings are different for each xformer then you need current sharing resistors, on the order of a few ohms each.

 

[X3msnake]

You might need to provide a load on the 5V rail for the power supply to work properly.

Interesting, i think missed this before.

So far I have used the PS and it worked properly I guess, altho i'm just powering one lamp.

Can you explain a bit more?

best regards

 

[mneary]

Switching power supplies usually require a load on at least one of the outputs to keep the regulator happy. Often this is the 5V rail. Others want a load on the 12V, but some power supplies don't require a load.

Yours appears to be working properly, so no action is required. If adding more lamps to the 12V causes problems, you know what to try.