cDAQ module to power a small heater

[chrisff]

Hi forum friends

I have a probe that approximates a heat conduction line source. Basically a steel hypodermic needle which encloses a thermocouple and a length of resistance heater wire. Power is supplied to the heater wire which heats the needle. Heat is dissipated throughout the surrounding medium. During the heating stage the probe internal temperature is monitored. Presently I am logging the probe temperature with a cDAQ 9174 backplane and 9214 Temperature module. Very happy with the results. (Using DAQmx and text programming for the data acquisition and analysis).

However the probe heating side is currently an external homebrew prototype circuit. We want to replicate this system to a number of facilities so a modular solution supported by the cDAQ system is required. We don't want to do any more work than simply connecting devices or sensor wires together. After very much browsing of this forum I think I have found a solution. However I could use a sanity check because my electronics knowledge is very basic (V=RI right?!)

Heater wire resistance is 25-50 Ohm. Heater voltage is 1-5 V DC. So the current ranges from 20 mA (1/50) to 200 mA (5/25). Heater requires continuous power for 30-300 seconds. There don't seem to be a cDAQ analog output modules that will do this at the current levels required. However it looks like the digital output module 9478 will meet the requirements with ease, with the addition of an external power supply. There was a thread on this forum that suggested the 9478 for powering a solenoid with a low frequency pulsed output and similar voltqage and current requirements. Hence my belief in this as a viable solution.

Now since I basically want to use the 9478 as a programmable battery I have some questions to be sure my understanding is reasonable.

1. Can I generate a continuous voltage output from the 9478?

2. Can I generate an arbitrary value of  voltage between 1 and 5 V, e.g. 4.2V.

3. Can I set the voltage value programatically (DAQmx)?

4. Can anyone recommend a suitable external power supply for this setup?

5. Are there any other modules that might so the same job at a lower price? (For example, the 9478 has 16 outputs of which I require 1. The 9474 has 8 outputs and is half the price but I'm uncertain if it supports the voltage range I need).

Of course I am open to any other suggestions and I apprecaite any help you can provide.

regards

Chris

[Henrik_Volkers]

You can use a digital output as a PWM output to control a heater, IF your heater is slow enough. I would guess, that the heater in a needle is quite fast.

What are the requirements for the heater?

Do want to build a heater controller loop?

BTW are there any medical requirements? 

Another solution migth be a 9260 but the spec doesn't mention the output current/power rating. Good question for a NI application eng.

I would use 5V from a USB (powersupply)  a simple transistor as an emitter follower (buffer) and an analog voltage out..    

[Izzzzz]

Hello Chris, and Henrik_Volkers

The NI 9269 might be a better fit for powering a small heater than the NI 9260. The minimum load, 600 ohms, along with the output voltage specificaitons provides guidance on the current output capabilities of the NI 9260. 

Regards, 

Izzy O. 

Product Support Engineer

ni.com/support

[Henrik_Volkers]

---

@Izzzzz wrote:

Hello Chris, and Henrik_Volkers

The NI 9269 might be a better fit for powering a small heater than the NI 9260. The minimum load, 600 ohms, along with the output voltage specificaitons provides guidance on the current output capabilities of the NI 9260. 

Regards, 

Izzy O. 

Product Support Engineer

ni.com/support

---

As I read the spec that would fail to drive a 25 Ohm at 5 V . He needs to drive 1 W into his needle !   The 9269 can drive 20 mA all channels max !!

[chrisff]

Appreciate the feedback so far.

OK, after more reading, I've given up on the digital output approach. I think I had a basic misunderstanding of what they did.

So are there any cDAQ compatible analogue output modules that will provide up to 1 W of power (5 Volts at 200 mA)? All the modules I can find cover the required voltage range (1-5 V) but most are limited to 20 mA or less per channel.

Is the lack of more power in cDAQ modules a safety issue?

What other options do I have that don't require building external add on circuits to tweak NI module outputs?

Am I stuck having to create an external circuit to amplify an analogy output module current to higher levels?

Thanks

[Henrik_Volkers]

I found this one

http://wwwold.kgc.co.jp/hks-8306-en.html

@ NI:  the link on your site to this company is broken.

And you need to add a external powersupply.

Or you find someone who will put together a (USB) 5V powersupply, a transistor as a voltage follower (like a BC635) and two connectors.

[chrisff]

Henrik

you are a Legend!!! 🙂 🙂 🙂

I didn't know there were third party suppliers. Have you had any experience with this companies modules? They are designated as Compact Rio modules but I believe that also makes them compatible with Compact DAQ ... true?

I assume also that they can be driven programmantically via DAQmx?

btw - there is also the current output module http://wwwold.kgc.co.jp/hks-8307-en.html which outputs 4-500 mA at up to 14 V. Actually I think this one is the best option since the voltage module wont go below 3 V. 

Kind Regards (aus Australien)

Chris

[Henrik_Volkers]

With a current drive for a heater you have to keep in mind, that P=I²*R =U²/R . Usually the resistance of a heater increase with temperature. With a constant current drive you have to watch out for a heater 'run away'.

[chrisff]

Hi Henrik

Thanks for the advice. I'll keep that in mind.Temperature changes over the course of the heating part of a test are "small" 10-20 C but ambient test temperature is selectable at anywhere between room temperature and 400 C. Since I am able to check heater resitance with temperature so I will do that to assess the runaway likelihood.

btw - I think you asked whether this was a medial application ... no it's not. Its a thermal conductivity measurement by the transient line source method.

Kind Regards

Chris

[Henrik_Volkers]

Thank you for the information. If you use const. current, monitor the voltage over the heater (you want P= const.). Best done with Kelvin (4 wire) measurement, two extra sense wires as close to the heater as can be done. However, depends on the results you need 😉