24v power supply for 25watt resistors.

[cluelesshillbilly]

I am undertaking my first project since 2004 and even then my skill was minuscule. I have what "I think" will work and I would appreciate your guidance in arranging the components correctly.

I will mount 4 24VAC/36VAC Compact Power Transformers to an aluminum enclosure. I will provide the power for each transformer through a lug strip and ground to the aluminum chassis and ground that to the 120v ac plug. I will then use 4 insulated fuse holders with 1.25 amp fuses to power each resistor (that I have yet to figure out how to suspend). I will then use a common ground for the opposing resistor leads. Please don't bash me for my ignorance I know for most it is a very simple project.

The resistors will be mounted in a small metal box inside a 4ft x 4ft x 18in three quarter inch plywood box with two 12v fans (I already have the power source for them) constantly recirculating the air over the resistors to heat the enclosure. I am slowly working on a sketch I will post as soon as I finish.

Thank you very much.

 

[MikeMl]

Why does each resistor need its own fuse? I would put just a single fuse in the transformer primary. I see that you are still bound and determined to start a fire...

 

[cluelesshillbilly]

I tend to over fuse to avoid fires ironically! LOL I will be able to put an exaust fan and introduce more fresh air with the more powerful heating elements. Could you please explain what you mean by putting a single fuse in the transformer primary?

 

[Ubergeek63]

The resistors will be mounted in a small metal box inside a 4ft x 4ft x 18in three quarter inch plywood box with two 12v fans (I already have the power source for them) constantly recirculating the air over the resistors to heat the enclosure. I am slowly working on a sketch I will post as soon as I finish.

Thank you very much.

if you are not concerned with safety or can screen it... just put all the resistors in series to the power line and control the power with a lamp dimmer

dan

 

[MikeMl]

I suggest that you get a normally-closed thermostat set to ~100degF and put that in series with your resistor(s).

The goal of fusing is to prevent a fire anywhere, including in the transformer. Wire a slo-blow fuse in series with the transformer primary. Start out with a 15A fuse. Get your project running, and measure the AC current flowing through the fuse into the transformer primary. Now replace the 15A with a fuse that is about 25% bigger than what you measured. If a short occurs on the transformer secondary side, the increase in primary current will blow the fuse. If the transformer develops an internal short, the increase in primary current will blow the fuse...

 

[Ubergeek63]

you could even use a PTC thermister to control a triac for temp control

 

[cluelesshillbilly]

**broken link removed**

OK I added fuses. I will also put a thermostat in the box before the 120 ac as I have one of these laying around.

Again please don't bash my ignorance if I got it completely wrong.

 

[MikeMl]

The whole reason for using a transformer in a project like this is to provide isolation between the AC line and the heater side and to reduce the voltage on the load side to minimize shock hazard. As you have drawn it, you have the load side connected to the line side, so why bother with the transformer(s)?

If you dont care about shock hazard, then do what the previous poster suggested; namely eliminate the transformers, and connect the resistors in series, and power them directly from the 120VAC line.

Why don't you just discard the resistors, and wire up a single 120V lamp socket. Then everything is insulated, and you can pick an appropriate Wattage to heat your box by just screwing in a different lamp; anywhere from 10W to 150W.

 

[Ubergeek63]

The whole reason for using a transformer in a project like this is to provide isolation between the AC line and the heater side and to reduce the voltage on the load side to minimize shock hazard. As you have drawn it, you have the load side connected to the line side, so why bother with the transformer(s)?

If you dont care about shock hazard, then do what the previous poster suggested; namely eliminate the transformers, and connect the resistors in series, and power them directly from the 120VAC line.

Why don't you just discard the resistors, and wire up a single 120V lamp socket. Then everything is insulated, and you can pick an appropriate Wattage to heat your box by just screwing in a different lamp; anywhere from 10W to 150W.

lol i believe i did state that if you could shield it so as to keep your fingers from the "elements" ... even toasters have 110VAC exposed, after all.

light bulbs are not as efficient at making heat, and they would tend to expose you to burns as well...

dan

 

[MikeMl]

...

light bulbs are not as efficient at making heat, and they would tend to expose you to burns as well...

Last I looked, a standard 100W incandescent puts out 97% heat, and only 3% light. In a closed box, the 3% light becomes heat, too

The lamp will burn you about as well as his resistors will burn you.

 

[cluelesshillbilly]

The whole reason for using a transformer in a project like this is to provide isolation between the AC line and the heater side and to reduce the voltage on the load side to minimize shock hazard. As you have drawn it, you have the load side connected to the line side, so why bother with the transformer(s)?

If you dont care about shock hazard, then do what the previous poster suggested; namely eliminate the transformers, and connect the resistors in series, and power them directly from the 120VAC line.

Why don't you just discard the resistors, and wire up a single 120V lamp socket. Then everything is insulated, and you can pick an appropriate Wattage to heat your box by just screwing in a different lamp; anywhere from 10W to 150W.

Ok I thought I needed the trans formers to lower the voltage. but if I can run the resistors straight off the 120v I will. I like the resistors because its new to me and I like the idea of never changing a bulb and reliable heat. Oh well I thought I had figured something out on my own. Ego check #3 today.

 

[tcmtech]

OK I thought I needed the trans formers to lower the voltage. but if I can run the resistors straight off the 120v I will. I like the resistors because its new to me and I like the idea of never changing a bulb and reliable heat. Oh well I thought I had figured something out on my own. Ego check #3 today.

They make long life bulbs that have 20K hour ratings. Put one of those in with a diode in line with it so it only runs are half power and you will probably die of old age about the same time it does.

A lamp socket and an optional dimmer working with a long life bulb just seems to look a lot simpler and cheaper from my point of view at this time.

You can even add a thermistor into the lamp dimmers control circuit in series with the potentiometer and have a self regulating adjustable controller too all for under $20 - $25.

 

[MikeMl]

Ok I thought I needed the trans formers to lower the voltage. but if I can run the resistors straight off the 120v I will. I like the resistors because its new to me and I like the idea of never changing a bulb and reliable heat. Oh well I thought I had figured something out on my own. Ego check #3 today.

If you want to use the transformer for safety, then here is my recommendation:

 

[cluelesshillbilly]

If you want to use the transformer for safety, then here is my recommendation:

Thank you. I Like! I agree the lightbulb is cheaper but using resistors for a heat source is new to me and gives me something to do while bored at work. Where do you go to build the circuts like that?

Mike can you please show me how to make it work with these trans formers I can't beat the price. https://www.goldmine-elec-products.com/prodinfo.asp?number=G12951

 

[MikeMl]

Use two of them. Dont hook up the 36V winding. Use two resistors in parallel per transformer. I still think that you do not need all four resistors, so start with one transformer, and one or two resistors.