Dear all,

I am a complete novice with circuits (but enjoying the steep learning curve - chemist by training, so can handle the physics, but the practical aspects are new to me), so please be gentle and use short words

I'm trying to make a device to heat a sample using ceramic resistors (a trick borrowed from the 3D printing fraternity). My setup is incredibly simple - 2x 10Ω, 5W resistors in series powered by a 12V, 1.5A 'switchmode plug pack' power supply. The idea being, when the power is on, the resistors will dissipate around 7.2W of heat (between them). They will only be used in short bursts, so they should be relatively happy?

The mystery is thus: when i turn the power on, nothing happens. I measure that there is 12V throughout the circuit, but no heating. However, if I use my multimeter as an ammeter (in series), the circuit functions as planned, and the resistors heat up.

Am I running into some problem with the power supply interpreting the low resistance as a short circuit and refusing to supply current? If so, why does the multimeter measuring amps correct for this?

Any and all advice would be very gratefully received!

many thanks,

Rico

 

What do you mean by "I measure that there is 12V throughout the circuit"?

Do you have different resistors that you can test with the power supply?
Something like a 100Ω resistor?

Maybe you have a faulty connection?

Can you post a photo of your setup?

 

Thanks for the fast response

Oh I just meant that using the multimeter in voltage mode, i measure 12V with one probe on the negative and one probe anywhere else in the circuit.

The picture is not great, but might help? The box is just to house the power jack and an on/off switch.

 

With two resistors in series, there are only three nodes in the circuit.
The first node is where the 12V supply connects to the first resistor.
The second node is where the first resistor is connected to the second resistor.
The third node is where the second resistor connects to to supply return.
(I have ignored the ON/OFF switch for the time being.)

If you connect the voltmeter with the negative lead to the third node (supply return)
then there are only two nodes where you can measure voltage with the positive lead, i.e. the first and second nodes.

At the first node you should measure 12V.
At the second node you should measure half the voltage, i.e. 6V.

If you measure 12V at both nodes then you have a bad connection at the third node.

(Sorry, I know a circuit diagram is appropriate but I'm on my iPad at the moment.)

 

Dreadful job of twisting the wires together. Try some wire nuts or solder, or use pliers to make several turns twist tightly together.
Most circuits have dozens to hundreds of connections, so we who do this all the time become quite good at ensuring proper connections. It gets insanely difficult if you don't.

 

+1
Sometimes, in a pinch, I will use insulated ring or spade crimp connectors to connect wires and the cut off the spade or ring itself and leave just the insulated crimped portion. Maybe with a bit of electrical tape around it for good measure.

 

Thanks for the trouble-shooting tips guys - found it!

Now don't laugh... I went through and checked at each point in the circuit and was reading 12V (fishy), so followed your suggestion that I had a dodgy connection.

It turns out that I have a plug jack with a bum solder tag. When i connect the negative wire to another part of the jack (where i was touching the multimeter probe) all is well.

I will add a photo and some temperature info on the final product once I've gotten to work tomorrow and soldered it together, in case anyone may see this post and find it useful.

Thanks again for helping me out (and starting basic!)

 

If your power jack has three solder terminals, you've inadvertently discovered the switching function of the jack. Those jacks are frequently used in battery powered devices and the switch disconnects the batteries when an external power source is plugged in.

Here's a data sheet for a typical jack with an internal switch:

http://www.cui.com/product/resource/pdf/pj-008a.pdf

 

If your power jack has three solder terminals, you've inadvertently discovered the switching function of the jack.

You were of course correct - and that's actually a really clever feature! The solder tag that I wanted was hiding underneath (at least, it was the way I'd screwed the jack on!)

Assembled, soldered, tested and working today. Will be back shortly with some usage info.

Thanks again guys for the advice.

 

As promised, the final item. I guess this is more an 'alpha' than a beta (hence the masking tape holding things in place...)

The sample is sandwiched between two cover slips (faster heat transfer than microscope slides), and placed onto the resistors. The copper wire is a corral to hold the resistors in place and keep them nice and flat (didn't want to use epoxy due to temp increases).

Once switched on, the resistors take a couple of minutes to heat the sample to ca. 80 degs Celsius, which is about right for observing the liquid crystal phase transitions of our samples. You can get over 100 degs by leaving it on longer, and I assume you could get higher final temperatures and heating rates by using differently rated resistors. However, this works perfectly for our current samples.

Temperature is measured by a calibrated thermocouple connected to a standard multimeter.

The next stage is to implement a temperature control circuit, so that the sample can be held at a fixed temperature (presumably using a calibrated thermocouple). I'll think about that more in the coming weeks. I think I'm going to need your advice again...