# Diode calculations

Discussion in 'The Projects Forum' started by chardur, Apr 4, 2006.

1. ### chardur Thread Starter New Member

Apr 4, 2006
3
0
I am trying to build a projector and I need to wire a 115v outlet to run a 300watt 82 volt halogen light bulb.

I understand a diode would be a good way to drop the voltage, but Im not sure how to make the calculations to find out what diode I need at radio shack.

Thanks in advance for any help!

Charlie

2. ### paultwang Well-Known Member

Mar 8, 2006
80
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121 watts of heat from the diode has to go somewhere.

3. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
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I am not to sure how you would drop from 115V to 82V with "A" diode.

4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
You guys are sooo.... cold!

5. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
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Then warm us up.

6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
If the bulb can run on 82 Volts DC at 300 watts the the safe and practical way to do it is to use a transformer, and a rectifier diode, and a capacitor. Since these bulbs are probably expensive I would suggest a voltage regulator with a soft start as well. The very last thing you want to do is hit a cold filament with 3-4 amps of current.

If the bulb can run on AC then you might get away with just the transformer. You still have the problem of managing the thermal shock of hitting a cold filament with a blast of AC current. It is possible that some kind of thyristor dimmer circuit could be made to bring the lamp up slowly.

Can ya feel the warmth?

7. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
24
All toasty and warm thank you

8. ### chardur Thread Starter New Member

Apr 4, 2006
3
0
Thanks for all the input guys!

Im still confused on how to make the calculations for the diode. Is there a formula that I can use?

The guy at radio shack suggested a rectifier diode rated at 3amps and 400piv.

It doesnt have to be exactly 82v, 80v or as close to it as possible would be fine.

Thanks again!

9. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
24
Can you give us a bit more info on this bulb. Is it 82V DC or AC? What sort of projector is it you are trying to build?

I think you may need to build a power supply for it. At least a transformer and maybe some rectification. Any more info greatly accepted.

PS. I don't know what the staff in Radio Shack over there are like but I would trust the ones here to butter bread properly let alone know anything about electronics.

10. ### Papabravo Expert

Feb 24, 2006
10,340
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But you did not answer the question. Are you going to run the bulb on AC or DC?
We already pointed out that you cannot use just one diode to get line voltage to anywhere close to 80 Volts.

To answer your question which will do you no good at all if you insist on doing what I think you are going to do.

Line voltage is 115 Volts AC. Multiply by the square root of 2, or 1.414 to get a peak voltage of 162.6 Volts AC. Multiply this number by 2 to get 325 Volts Peak to Peak. Round up to 400 to get the PIV or peak inverse voltage. This number includes a generous safety factor.

At 82 volts and 300 watts the lamp will draw 300/82 = 3.66 Amps. I think the guy at Radio Shack gave you bad advice. He's a clerk at Radio Shack, not an engineer after all. You should never never never depend on salesmen or clerks for technical advice. A 3 Amp diode would be stressed in this application. I would go for a 5A or a 6A diode to have some margin.

Let me reemphasize this point. In order to limit the line voltage to 80 volts you would have to use approximately 115 diodes in series at 0.7V forward drop each.
Code ( (Unknown Language)):
1.
2. 162.6 VAC - (115 * 0.7) = 82.10 VAC
3.
4. You also need a capacitor to keep the voltage from decaying on the negative half cycle
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You need to use a transformer with about a 2:1 turns ratio to step the line voltage down. If you don't use some kind of circuit to limit the rush of current into a cold filament you will spend a large fortune on replacement bulbs.

There is also a personal safety issue, if you wire HOT AC into your project without a transformer. If the hot wire goes to your ground everything will work but you could get a fatal shock from touching any exposed metal part.

Long story short -- do it like a professional, don't mess around with shortcuts until you gain some experience and have a good idea what you are doing.

11. ### chardur Thread Starter New Member

Apr 4, 2006
3
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Yes AC current, directly to a wall outlet.

The projector assembly can be found here
(only 15 bucks!)

The main reason for the halogen assembly is that it does not require a ballast or ignition capacitor, reducing the costs. I have dissected a similar projector (82v 360watt) that used only a diode so I was thinking the same would be possible here.

If a diode is not reasonable do you think a standard light dimmer from home depot would be a better solution? I could connect a voltage meter until it hits 80-82v and just mark the switch location and block it from going higher.

12. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
I don't know what to tell you. Your link doesn't tell me anything. If you're convinced it can be done with just a diode then by all means go ahead and have a ball. Perish the thought that reason should rain on your parade.

13. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
24
Are you sure it is a diode??? Some items like this have a thermal fuse which can look like a diode.

Also the picture on that link doesn't show any other circuitry that could indicate a power supply. As such we can't offer much more advice as to how you shpuld go about it. The only thing I can suggest, and please don't take this the wrong way, but spend some money and build a decnt power supply and do it right. You can always earn more money but very hard to earn a new life.

14. ### Papabravo Expert

Feb 24, 2006
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Well spoken mate!

15. ### Spoggles Well-Known Member

Dec 2, 2005
67
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Hello:

Is there something that I do not get about this?

If you put a diode in series with a AC voltage source (120vRMS) and a load, the load will see 0.707 of 120Vrms or 85Vrms. (It is called a half wave rectifier).

All light bulbs draw about 10 times there rated power, initailly so nothing new here.

Spuggles

16. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
NO! NO! NO!

In a half wave rectifier, what you see is the positive half cycle with a peak at 1.414*120VRMS = 170Volts Peak. The diode itself will drop 0.7 Volts, for a peak voltage of 169.3 Volts Peak.

You should make sure of your facts before shooting your mouth off.

17. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6

No. Not directly to a wall outlet.

<!--QuoteBegin-chardur
@Apr 5 2006, 12:03 AM
The projector assembly can be found here