OSHA pain -step down 24v from 110


  #1  
Old 05-10-03, 02:38 PM
josh1
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OSHA pain -step down 24v from 110

"no 110v devices shall be less than 18" above the floor in regards to hazardous areas near flammables such as oil gas"grease etc.

OSHA is killing my invention skills.

I need to drop 110V down to 24V max, possibly by step down transformer.

The numbers

have a standard 120 volt device drawing 100 watts. =.83A

What does dropping the volts to 24 do?

by my math 100 watts @24v=5A.

Am I close? Can a simple AC resistance device such as embedded heating coils run on 24volts versus 120v or will there be something happening that I dont understand (most likely)

Thanks-Josh
 
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Old 05-11-03, 09:21 AM
J
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100/24 is much closer to 4 than 5.

Heating coils can run on 24 volts, but it'll take just short of forever to provide a significant amount of heat. If you take a heating element designed for 120 volts and run it on 24, it will produce one-twentyfifth the amount of heat it otherwise would. I'm not even sure that will feel warm to the touch. If it was designed to run on 240 volts, it will produce only one percent of the heat.

What the heck are you doing?
 
  #3  
Old 05-11-03, 11:41 PM
josh1
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well MY way just used straight 110volts for this heating appartus, but the OSHA/ansi/ul people arent keen on that.

the heating coils were designed for 110 so I gather that at 24v it will be pretty useless.

I have searched for 24v heating coils and all I can find is aquarium heat cables.

In essence, a heating cable is basically a wire and a resistor?

Is DC more effective or less effective at heat generation? Say I could use 24v DC for this heating, then my limits are at battery capacity. 4 amps per hour is going to take a hefty battery I would think. I could also rectify AC to DC? using a diode? I think, if that helps.

its like this invention is a very 'good idea' but the regs surrounding it may keep it grounded forever. I have working prototype and it works great! (even if osha wont let it pass)

Thanks for ideas-Josh
 
  #4  
Old 05-12-03, 02:54 PM
RickJ6956
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You can run a standard 120-volt 60 watt light bulb at 12 volts, but the current increase will cause it's tungsten element to fail prematurely.

DC is a better carrier of low-current devices over extremely short distances, but probably not suitable for your application. A 5-amp, low voltage diode (actually it should be a rectifier) may suffice, but it will need support circuitry and massive heat sinks to protect it.

Can you tell us more about the circuit or application without giving away the idea? Or ... what's the problem with the 18-inch proximity according to OSHA?

It is intriguing.
 
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Old 05-12-03, 04:13 PM
J
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There is no need ot convert to DC just for heater. You are just creating more failure points and heat loss area for no reason. I don't know what you are up to but what if it was in an explsion proof housing like used around gas pumps? I think it might be ok then.
 
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Old 05-12-03, 04:51 PM
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You can run a standard 120-volt 60 watt light bulb at 12 volts, but the current increase will cause it's tungsten element to fail prematurely.
Huh? A 120-volt 60-watt light bulb will run at 1/2 amp when presented with 120 volts, but only 1/20 amp when presented with 12 volts. I'd guess that a 120-volt 60-watt bulb would last darn near forever on 12 volts. Of course, only someone with very good eyes would even be able to tell it was on, since the bulb would be using only about one half of a watt!
 
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Old 05-12-03, 10:13 PM
RickJ6956
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If you divide the voltage by 10, the current increases by the same factor. That 60-watt lamp is now drawing 5 amps. Have you ever hooked up a 60-watt lamp to a car battery?

Er ... or maybe the 120-volt bulb I acidentally screwed into the 12-volt system of a boat was weak to begin with ...
 
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Old 05-13-03, 04:04 AM
josh1
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The problem is the "device" would be in voilation of OSHA and ansi /ul standards running on 110. I guess close proximity to the floor in hazardous areas can result in explosions. However I need to find a way to make 24V heating energy efficient using either step down transformer or fuel cell/battery type cordless design. Electrics are not my strong point. I used huge sheets of cardboard to map out the electrics of the current "prototype" and its really simple! The heating elements themselves should be spark/wet proof due to design, and all control boxes are epoxy filled. basically it "would" never casue a spark or problem, but tell that to OSHA!!! -Josh
 
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Old 05-13-03, 05:14 AM
josh1
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Spent some time at howstuff works.

If I want to drop voltage to 24, and yet still maintain the same POWER in watts, I need to also drop resistance of the heating coils. I=v/r The original R was 960 and 120 volts, so current was .125A. Reducing the device to 192 Ohms would bring the same Current as original (120 volts) using 24 volts.

If i want to increase power in watts, I need to increase current in Amps, If I want to increase current I may either Up the voltage ( cant do) or drop resistance (can do).

Basically Im just looking at stepping down the resistance?

Thanks-Josh
 
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Old 05-13-03, 06:30 AM
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Rick

Josh is correct on this one.
In fact the 120v bulb won't even light on 12v
 
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Old 05-13-03, 08:27 AM
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Putting smaller "heaters" in parallel will allow you to drop the equivalent series resiistance of your circuit. Basically, instead of using a 500W heating strip use 5 100W strips in parallel. They will each be applied the same voltage, and the current will be divided. Your total ESR of the parallel combination will be lower then 1 500W strip.
 
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Old 05-13-03, 11:54 AM
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Originally posted by RickJ6956
If you divide the voltage by 10, the current increases by the same factor. That 60-watt lamp is now drawing 5 amps. Have you ever hooked up a 60-watt lamp to a car battery?

Er ... or maybe the 120-volt bulb I acidentally screwed into the 12-volt system of a boat was weak to begin with ...
Problem with that logic is the resistance of bulb stays the same. It does not produce 60 watts on 12 volts. It only porduces 60watts on 120 volts.
 
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Old 05-13-03, 04:22 PM
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Josh:

In your original post you indicated that you want a 100W heater running off 24V. I will assume 100W, if you need a different value the math is the same.

You currently are using a 100W heat strip running on120V. A heat strip is simply a long, skinny resistor. Power dissipated in a resistor is proportional to the square of the voltage applied across it:

P = V^2 / R

Your 100W, 120V heat strip has a resistance of 144 ohms.

If you just connect that heat strip across your 24V, as John Nelson pointed out (and several posters seem to misunderstand) the power dissipated in the heat strip is only 4W - not very warm.

To get 100W with 24V you need a resistance of 5.76 ohms. You could get this in several ways:

- You could use a 2500W 120V heat strip

- You could use several lower-power heat strips in parallel, as long as their equivalent resistance is 5.76 ohms. Five 500W 120V heat strips in parallel would give you the right value.

- You could use power resistors in some series and/or parallel combination to give you 5.76 ohms. For example, six 1 ohm power resistors in series would get you pretty close at 6 ohms, but each resistor would need to be rated for at least 16W. Many other combinations are possible.
 
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Old 05-13-03, 07:30 PM
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Calculate the watts you needs (or estimate what you need). Then knowing your voltage you can calculate what kind of total ESR you need for your heating bank. There are several ways to calculate power, either using the voltage or the current. You can then also calculate the current you need (DC current) to run your bank. A resistor doesn't really care whether you apply DC or AC to it. The resistance will also stay constant with frequency. One you know what total ESR you need then you can measure the ESR for the strip you planned to use. You can then calculate how many strips you have to put in parallel to get your target ESR. Basically a lower voltage heating strip has a lower ESR.
 
 

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