Need to heat an IC...will my solution work?

I had built this remote temperature sensor circuit and it works just fine...

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Source – Maxim’s MAX6648/MAX6692 Datasheet

According to the datasheet I can increase remote accuracy if the IC temperature is kept at 85C, so I’m trying to add some sort of IC heater mechanism to this circuit. It just so happens that the OVERT pin is an open-collector that goes low when the IC is at 85C. So I thought I could use this signal to toggle a heater circuit. That way, the circuit would maintain the IC at 85C.

Here’s the solution I came up with...

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I added a JFET to the circuit. This JFET will be bonded to the MAX6692 IC. The idea is that when OVERT is high (actually, just disconnected) there's 0V at the gate and current flows, heating the IC. When OVERT goes low at 85C (actually, just gets grounded,) 3.3V will turn the JFET off. When OVERT goes high, the 10K resistor, which used to be a pull-up, now has a new job of shunting the JFET and then current flows again.

The JFET I have in mind is a J113. Cutoff is 0.5V - 3V. The resistance across DS is 100 ohms. I figured I'll just connect a 9V battery right to the JFET and let it heat up. The JFET is rated at 400mW, but I'll be pumping 720mW into it. Hopefully, the fact that I turn it off at 85C will keep it from being destroyed, but I guess I just have to try it and see.

Does this make sense? Any and all thoughts are greatly appreciated. Thanks!

Maxim MAX6692 datasheet
https://www.electro-tech-online.com/custompdfs/2004/08/MAX6648-MAX6692.pdf

J113 JFET datasheet
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Where exactly in the datasheet did you read remote accuracy increases if the chip is 85°C ?...

Doesn't make much sense to me, and I can't find it in the datasheet.
Remote accuracy may increase if the remote sensing element's (the transistor itself) temperature gets higher...

And even so, it'll probably be 0.5°C of precision that's at stake, what's your application that requires such precision?

Also, the FET suggestion you made is doubtfull. You're shorting your supply (a feeble 9V battery) with the FET's on resistance of max 100 ohm (so it may be lower). Thats 90mA!, even a rechargable battery would be drained in no time...
The FET won't like it eighter...

if you really want to heat the ic (but I really don't see a reason). you should use a fet or transistor to turn on a heating element (resistor) that does the actual heating.

Hello Exo! Thanks for taking the time to respond. I appreciate it.

Exo said:

Where exactly in the datasheet did you read remote accuracy increases if the chip is 85°C ?...

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Under the Electrical Characteristics there is a parameter called “Remote Temperature Error.” This parameter lists the accuracy rates under various conditions. The first condition is where Tambient = 85C. In fact, at the top of the chart you’ll see that operation at 85C is considered typical.

The application is to read the thermal diode that is built into AMD Athlon processors. The purpose is to evaluate and collect performance information on different waterblocks, which are part of a water-cooling system for the computer.

Exo said:

Also, the FET suggestion you made is doubtfull. You're shorting your supply (a feeble 9V battery) with the FET's on resistance of max 100 ohm (so it may be lower). Thats 90mA!, even a rechargable battery would be drained in no time...
The FET won't like it eighter...

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Yes, you’re absolutely right about draining the battery. I figured that it will only last a few hours, but that’s okay. That’s more than enough time to do what I have to do, and at this point electrical isolation is more important than a steady power supply.

However, could you please elaborate on your “FET won’t like it” comment? Are you saying that, even though I’m shutting the FET off at 85C that it will still suffer damage?

Exo said:

if you really want to heat the ic (but I really don't see a reason). you should use a fet or transistor to turn on a heating element (resistor) that does the actual heating.

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Yeah, that’s option number two! I was hoping to be slick and get away with adding only a single device. But if the FET is going to be damaged then I just might need to do that.

Thanks!

One option for heat generation would be to use a LCD display heater. Many are the thickness of flexible circuit boards, self regulate their temperature, and are simple to drive. Well over 100 degrees is obtainable, since they are designed to keep an outdoor LCD warmed to around 25 degrees in winter.

Just a thought.

zevon8 said:

One option for heat generation would be to use a LCD display heater. Many are the thickness of flexible circuit boards, self regulate their temperature, and are simple to drive. Well over 100 degrees is obtainable, since they are designed to keep an outdoor LCD warmed to around 25 degrees in winter.

Just a thought.

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Sounds good if they're small. I did a little searching and didn't come across any small ones yet. I'm just trying to heat up an 8 pin SOIC. These things are about 4mm X 5mm. That's why I like the JFET idea. The TO-92 package is the same size as the SOIC.

I wonder if a miniature peltier will work...gotta look into that.

Thanks!

That FET will never heat your chip to 85C. The Rds=100 ohms spec is for Vds=100mv. Idss at 15 volts could be as low as 2ma (see spec). At 9v, it could be even lower. As a side note, you drew the 9v battery upside down. The drain on the JFET has to be positive relative to the source.

You might want to try a PN2222A. Connect the emitter to GND and the base to *OVERT (this assumes you don't need a logic level output from *OVERT). Change the *OVERT pullup resistor to 1.8k. Connect the collector to 3.3v. Bond the transistor to the MAX6692. You should get between 150ma and 500ma of collector current, depending on the beta of the transistor. This will give you 0.5 to 1.5 watts dissipation, which should be enough to heat the combination to 85C. I wouldn't recommend this for production, but it should work for a one-off experiment.

I'm having a look through my "archives" ( LOL ) to see if I can find the ones I used years back. They were trimable with sharp scissors, and we used thermal epoxy to attach them to a LCD. I like suggetion by Ron H, similar to what we did for air flow sensor, using a dual transistor pakage, one "heated" the package, the other was used to measure the cooling effect of the airflow, and the changes in current flow during the heating - cooling. Another possibilty is a small ceramic heater found in curling irons. Some are plates, others are rods.

Ron H said:

That FET will never heat your chip to 85C.

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Thanks for that info. I guess I was reading my JFET ratings incorrectly.

I didn’t pay much attention to the battery orientation because the datasheet says that the drain and source are interchangeable.

I’ll take a look at the PN2222A. My initial problem with using a BJT was that the OVERT is an open-collector input. Basically it’s an open switch normally, and closes when the temp hits 85C. That’s the opposite of what I need to operate a BJT. So that means additional devices need to be added to the circuit. I was excited about the JFET because it looked like I could do it with one device.

I guess I should just control the PN2222A with the JFET. That looks like it should work. I’ll try to work that out in the circuit.

I didn’t pay much attention to the battery orientation because the datasheet says that the drain and source are interchangeable.

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The source and drain are interchangeable, but the battery polarity is definitely not.

I’ll take a look at the PN2222A. My initial problem with using a BJT was that the OVERT is an open-collector input. Basically it’s an open switch normally, and closes when the temp hits 85C. That’s the opposite of what I need to operate a BJT. So that means additional devices need to be added to the circuit. I was excited about the JFET because it looked like I could do it with one device.

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This is not true. When OVERT is low, the NPN turns off, as did your JFET. When OVERT turns off (goes open), the pullup resistor supplies base current to the NPN, turning it on. You don't need to power the NPN through the JFET or any other sort of inverter.

Ron H said:

The source and drain are interchangeable, but the battery polarity is definitely not.

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Oh, okay. The datasheet listed drain-source voltage as +/-Vds 40V so I thought I could connect the battery either way.

This is not true. When OVERT is low, the NPN turns off, as did your JFET. When OVERT turns off (goes open), the pullup resistor supplies base current to the NPN, turning it on. You don't need to power the NPN through the JFET or any other sort of inverter.

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AH! Okay. So I have to adjust the pull up resistor to give the correct base current for operating the transitor, BUT it must not allow more than the 50mA that OVERT can sink when it goes low. That shouldn't be a problem cause the controlling current should be lower than that. I'll start working on that.

Thanks much!

Graystar said:

Ron H said:

The source and drain are interchangeable, but the battery polarity is definitely not.

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Oh, okay. The datasheet listed drain-source voltage as +/-Vds 40V so I thought I could connect the battery either way.

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The gate pinchoff voltage is relative to the more negative of the other two pins, so that more negative pin becomes the source.

This is not true. When OVERT is low, the NPN turns off, as did your JFET. When OVERT turns off (goes open), the pullup resistor supplies base current to the NPN, turning it on. You don't need to power the NPN through the JFET or any other sort of inverter.

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AH! Okay. So I have to adjust the pull up resistor to give the correct base current for operating the transitor, BUT it must not allow more than the 50mA that OVERT can sink when it goes low. That shouldn't be a problem cause the controlling current should be lower than that. I'll start working on that.

Thanks much!

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The absolute maximum current rating on *OVERT is 50ma. If you put a pullup on it that required it to sink that much current, the voltage would never go low enouth to turn off a transistor. You don't want to drive the transistor that hard anyway. In fact, I think you could get by with a pullup larger than 1.8k, but I concluded it was a safe value.

Hey, thanks for all the help so far. Here is the latest schematic:

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I hope this is it. I replaced the JFET with an NPN BJT. I tied the emitter to ground to complete the base current path. I replaced the 10K pull-up for OVERT with a 200 Ohm resistor to get the proper base current.

So here’s how it works, when OVERT is high (open switch) then a current runs from 3.3V to GND through the 200 Ohm resistor and through the base. This base current turns on the transistor and the collector current flows and heats the transistor.

When the IC hits 85C, OVERT goes low and creates a direct connection to GND. This ties the base to ground. With the base and the emitter both tied to ground, the current takes the path of least resistance through OVERT and no current flows through the base and the transistor turns off.

A 16.5mA current is still flowing through the 200 Ohm resistor and into OVERT. That’s fine since OVERT can sink 50mA max.

So NOW does it look like it has chance of working?

EDIT: Sorry Ron...I posted before reading your note on the OVERT current. Will review.

I know you said you're going to review, but I'm going to give you this caveat anyway - I'm trying to help you, but I feel like you're not paying attention. I'm not just shooting from the hip here. I actually know what I'm doing (most of the time).
Just like medicine - because some base current is good, it doesn't mean more is better. I will guarantee you will burn up that transistor if you use 200 ohms. Stick with the 1.8k if you connect the collector to +3.3 volts. And if you insist on using the 9 volt battery, raise the value of the resistor to 4.7k. Otherwise, you stand a very good chance of letting the smoke out of the transistor. As we all know, it's the smoke inside that makes it work. If you let it out, it quits working.

I apologize if I’ve come across as not paying attention. Let me say now that I greatly appreciate the time you’ve taken to help me with my project.

Please understand, if I don’t seem to be picking up on something important it’s not because I’m dismissing it, but because I simply don’t realize the importance. The fact is that I don’t know about circuit design. I can follow a simple schematic and build a circuit, but I can’t modify it. I’ve spent the last couple of days reading up on semi-conductors and transistor operation just to get to this point. Obviously, I’m still very, very shaky on the subject and I’ll try to pay closer attention.

I think I have to buy a minimum of 10 of these transistors, so I might just experiment with a range of resistors to see what works (if it works at all!) Then I wouldn’t mind letting the smoke out of a few of them!

Sorry I was cranky - I'm an old fart. I guess what annoyed me was the fact that the schematic you posted was not what I had recommended. And now you say that

I can follow a simple schematic and build a circuit, but I can’t modify it.

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Well, you did modify it. Don't get me wrong - I believe experimentation is good. You can learn a lot from it. And it's not that I'm so proud of my little circuit. The problem is, if you don't start with the guidelines you were given, and it doesn't work, you may give up before you give the idea a chance.

I wish you luck. Let us know what works and what doesn't.

Ron...I must apologize again. After carefully reading through the posts again I realize that you were right...I hadn’t paid close attention. Let me explain a couple more things about the circuit.

The MAX6692 is to be powered by another 9V battery, regulated by a voltage regular with a max output of 250mA. That’s why I couldn’t power the heater circuit with the 3.3V source, so I was sort of blinding following my 9V battery setup.

My first sensor seemed to have some unresolvable grounding issues, so I decided that this one must be electrically isolated. That's why I'm powering the system with a battery.

I’m going to order a regulator that can supply 1000mA. With that I should be able to build the circuit as you described it.

I might be back to the two-battery system. A suitable regulator will only supply 350mA.

There's another design-driving consideration for me. I was hoping to write an article describing exactly how to build this temperature sensor. I already wrote an article describing how to chop up a CPU socket in order to access the thermal diode. Here it is:

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Now I would like to follow that up with a how-to on building the temperature sensor. I could do a very simple one, like the one pictured, but I wanted to provide a more accurate design with only a minimum of additional parts. The target audience knows less about electronics than I do (if that’s possible.)

So I need things like electrical isolation with batteries...a voltage regulating IC that needs nothing more than a cap added to it...an IC heater that’s just one device and a battery...and so on. If I can do this and be very thorough in my description, then my target audience has a shot at building this thing.

Here’s the complete circuit as it stands now:

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I think this is the circuit you originally described, which I’m keeping for when I happen upon the right voltage regulator:

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All that's left now is to build it!

Thanks so much!

If you want to make a how-to for other people to make this, the i would drop the heating element. It's not good design practice to use a transistor as a heater. As Ron said, "it should work for a one-off experiment".

Ever considered that before the wafer in your chip gets to a 85°C the transistor is already boiling at over a 100? It won't last forever...

You won't see any commercial product do this. The chip is accurate enough on it's own...
It'll solve your power issues as well, because the chip probably uses only a little current, you could make a voltage regulator with a zener and a little buffer transistor.

Exo, you’re right about the misuse of the transistor, and I am wondering myself how long it will last. The datasheet lists an operating temperature of 150C. So I believe I should be okay even if the transistor temp reaches 100C.

With the transistor being thermal-glued right to the IC, I can’t think of a better way to watch the temp of the transistor. I mean, in essence, the IC is monitoring the temp of the transistor.

I’ve been searching all the tech docs at Maxim to see what they say on this issue. They obviously intended the IC to be operated at 85C but they never say how that’s to be achieved. I haven’t found anything yet.

I’ll look into that regulator design. There should be a page somewhere that explains how it works.