Thanks spinnaker! That makes sense to me.

Kiss, these are 24VDC LEDs.

I guess you're calling my nichrome wire a "heater"? The nichrome wire is bare and 8mm long. It's a consumable fuse, that will heat up and ignite a sample in a high pressure environment.

The 24 V LEDs I understood once you clarified.

Other's (alec_t) though the nichrome was a heater too because heating is the usual application.

Not sure why you need the "controlled burn" then? More of a safety issue or don't want a DC field messing up the experiment?

 

Thanks!

I need a controlled burn time because I'm measuring the specific heat given off in the high pressure environment. The temp, and therefore the timing of the current, of the nichrome wire will be a standard for each burn.

The ultimate goal is to create a bomb calorimeter to measure the caloric content, through temperature emitted, of organic samples.

I'm a complete noob when it comes to electricity. Could you expand on what you mean when you say wire across the load and the power source when it comes to wiring the LEDs?

 

Load, your wire. Load is "somthing" that consumes power.

Across the load = parallel.

We have series elements and parallel elements. If two lamps were connected in series and one lamp burnt out, that would be a series circuit. The newfangled xmas lights.

Parallel - If one bulb burns out, the other stays lit. The old style xmas lights

Now we usually find three wires in a light set. One wire is power, the other is neutral. The LED's might tbe borken down as 50 light in series and three sets of 50 in parallel.

 

8mm of nichrome wire will look like a short-circuit to the power supply. The spec for that supply doesn't hint at any form of current limiting, so without that the supply will die as soon as you switch it on, unless you have a high-power (50W) 27 Ohm resistor in series with the nichrome wire to limit the current to about 1 Amp! A better supply (for the wire, not the timer) would be a switch-mode controllable 'constant-current' supply (as is used with high-power LEDs, for example), but you would need to check that it can drive your load at a very low voltage.

 

Thanks!

I need the nichrome wire to get as hot as possible. Can I place the resistor after the nichrome wire?

 

Thanks!

I need the nichrome wire to get as hot as possible. Can I place the resistor after the nichrome wire?

But it won't get hot with the resistor. Read up on Ohm's law.

 

The reason you are putting that resistor in series with the LED is to dissipate current. What do you think will happen to the wire if you place a resistor in series?

 

So, if I have a resistor rated at 24 ohms, and my voltage is 24 volts, then my amperage is limited to 1 amp. Correct?

I could run my LEDs in parallel with separate resistors to limit their supplied amperage?

 

So, if I have a resistor rated at 24 ohms, and my voltage is 24 volts, then my amperage is limited to 1 amp. Correct?

That is what Ohm said.

I could run my LEDs in parallel with separate resistors to limit their supplied amperage?

The nic wire will be a short across the LED and the LED will not light.

The right way to do this would be to have some sort of sensor that is placed near the nic wire. Perhaps a thermistor wired to an OP Amp and your LED. That way if the nic wire burns out you can use the LED as an indicator that the nic wire is not doing it's job.

Also, assuming you have multiple contacts on your relay, you could use one for the LED and the other for the nic wire. But not the ideal indicator since one of the contacts could fail or the nic wire could be burned out as mentioned.

 

Thanks!

Could I run wire for the circuit like the one I have attached here? Or am I shorting out the relay?

I could indicate that power is being sent to the NC circuit (post 9) and that power is being sent to the NO circuit (post 8) that has my nichrome wire. I would have a 24 ohm resistor at the beginning from the power source to limit amperage to 1amp. If the LEDs do not have built in resistors, I can place appropriate sized resistors before them. More then likely the nichrome wire will be consumed (i.e. broken) during normal operation, it's a fuse that will light a string and then combust my sample.

 

That won't work. What you have is a double throw relay. The LED will be on when the load is off. You could add a general purpose transistor to the LED and make an inverter. Look up inverter switch on the internet.

Or get a double pole single throw rely.


But like I said, the right way to do this is to build a sensor for the thermistor. You could even get fancy and have a bar of LEDs to indicate temperature. Add an MCU and you could have a digital display!

 

Thanks!

How would I add a transistor to this circuit to make it work? Would this work with the transistors? I have a basic ideas of how they work

I like the LED light temperature, but I have a thermocouple and logger in this system that measures temperature every 1/64 second.

 

You are going going to have the same issues with that LED on the load as you did wit the LED alone.

You need a base and possibly collector resistor on the other transistor. Look up transistor invertor.


If you already have a logger, why bother wiith the LED?

 

Thanks spinnaker!

I like the idea of having an LED signifying I have power to the switching device. The logger and thermocouple are separate and plugged into a computer.

I think a SPDT relay or switch like this could work, plus resistors in front of the LEDs:

 

What gauge wire do you intend to use? 30AWG, for example, has a resistance of about 6.5 Ohms per foot, = 0.17 Ohms for an 8mm length, so the 'fuse' would be dissipating only about 170mW with 1A through it: hardly enough to get even warm.
Connecting your load in series with the transistor base won't work.

 

Alec_t,

I've used that 8mm nichrome wire connected directly to a 12VDC, 500mA power source and it glows red hot after a few seconds... Why would this be the case with half the voltage and amperage?

 

I bet the wire was drawing a lot more than 500mA! Either that, or it was a lot thinner than 30 gauge.

 

Hey guys!

I got it all wired up and it is running great! Went with two power sources. For the nichrome wire fuse, I repurposed a microscope light (the light was burnt out) with a rheostat that will allow me to draw 6 amps. Used the original DC power source to operate the time delay relay. Got one LED to light when power is supplied to the time delay relay and the other LED to light when the time delay is switched. If I could upload a video of it working, I would...

I know how frustrating it can working with someone who has very little knowledge in a particular subject. So, thanks for helping me out along the way, greatly appreciated!

 

Good to hear it's working!

 

Hey guys!

I got it all wired up and it is running great! Went with two power sources.

Thank God, this thread was a painful read. It was pretty obvious that your nichrome wire was way too low resistance and shorting out your power supply every time the relay switched on.