Ultra simple oven controlled resistor for analog temperature control

Question

Background:

We're having a problem with a sensitive analog circuit. It has an analog supply voltage from a LT3045 Linear Regulator, whose output voltage is set by an external resistor. We're using 10ppm temperature coefficient resistors, but even these don't seem to be stable enough, and in practice, we're seeing too much sensitivity to temperature.

One solution to this would be to put these resistors into an oven, i.e. heat them up to a controlled 70ºC, so that they do not experience temperature fluctuations.

Question:

Is there is a very simple way to create a temperature stable oven using a only a couple of components, for example a thermistor and a transistor? Since we would be retro-fitting this to an existing board, with very little available space, it would need to be pretty simple and compact.

Added:

I have confirmed that the resistor is the culprit. I carefully touched a hot soldering iron onto each component in turn to see if it affected the output. The SET resistor produced by far the most significant change in output when heated.

Answer 1

Is there is a very simple way to create a temperature stable oven using a only a couple of components, for example a thermistor and a transistor?

One way to do this with a double-sided PCB is to put one or more large surface mount power resistors on the bottom side, and put the sensitive circuitry directly opposite them on the top side with a surface mount temperature sensor. Then find a small plastic cover for the top circuitry to prevent air draughts. If you can't find a cover of the right size, do a quick CAD design and get it 3D printed.

A simple potential divider using the temperature sensor via a comparator (with a bit of hysteresis) to a FET switch will turn the power resistor on and off to maintain the target temperature of the components on the top side of the PCB. You would ideally set the target temperature to be nicely above ambient, but not too hot, so perhaps 50C depending on your requirements.

The components on the top side of the PCB are now in a nicely temperature controlled "oven" without any draughts, so will be stable.

We used this at a previous company to get an extremely stable clock source from an off the shelf crystal oscillator module. A proper OCXO was way beyond budget. This approach cost almost nothing (less than $1) but worked extremely well.

If you have a microcontroller in the mix, you can read the temperature via an ADC with the FET switch on a GPIO, and create whatever optimal PID you want. That will give you better temperature accuracy and save you the cost of the comparator.

Answer 2

The rule of thumb is: the absolute value of a single resistor, no matter how good its tempco, can be handily beaten by the ratio of resistances of two or more much cheaper resistors.

Precision applications where an absolute value of a resistor is of essence indeed require expensive parts and ovenizing.

But nobody forces you to do it this way. The current source in LT3045 is a convenience for less critical applications. It is designed to be better than reasonably priced resistors, and nothing more than that. It basically lets you assume that if you select a typical metal film resistor, the I_SET tempco can be ignored. But this chip is simply not designed to use ultra-precise external reference resistors: this architecture simply doesn't make much sense then.

You shouldn't be using it when temperature stability you require is of essence. The SET terminal can be driven from a voltage source. There are no guarantees that the LT3045's current reference is any more stable than the rather expensive resistor you chose.

The tempco of the current source isn't even specified (sic!), but it's somewhere in the 50ppm/K ballpark around 35C, looking at the typical I_SET vs. temperature graph.

Also remember that resistor tempco may be specified over a much narrower temperature range than you'll get by touching the resistor with a soldering iron tip. Never mind that such precision resistors are only meant to provide specified tempco when they themselves are isothermal with the substrate they are on. Touching them with a soldering iron after the board is assembled is a surefire way to shift their value due to thermomechanical hysteresis, and is a rather destructive thing to even contemplate. Do not test precision resistors' tempco that way! If you contact the resistor manufacturer's application engineering, they'll probably tell you as much. Getting the tempco they specify requires some care, and abrupt temperature changes is definitely not on the path to long-term stability.

It is way, way easier to make a voltage source to your specs using an external voltage reference and a resistor network, than using a single ultra-expensive resistor and an oven. Especially that the current source is not as good as many even semi-competent voltage references would be.

For the price of the resistor you've shown, you can get a voltage reference with a tempco an order of magnitude better than that resistor's in practice, and that's without trying super hard either.

You haven't mentioned what the output voltage you need is, so let's assume it's 3V. With a 10ppm/K resistor, the voltage will drift 30uV/K. That's not a particularly impressive performance from 30 quid worth of parts! And that's assuming no drift from LT3045. And that's a wrong assumption. LT3045 is not 10ppm/K good. Not unless you'd select them for that tempco within the operating temperature range in your application.

You could probably get similar ballpark performance if you selected from a small batch of LT6650 parts for low drift around 35C, and used that to generate the reference voltage. LT6650 can be configured as a shunt regulator working from the 100uA current source, so the whole thing could be connected directly between SET and GND. Similar performance for 1/10th the cost.

With some selection, perhaps TL4050C could offer similar temperature stability.

If the voltage you need is covered by standard voltage references, then for a couple quid you can get references that go down to a couple ppm/K guaranteed, and will handily outperform the thermal errors within LT3045. At that point, you'll need an external DC servo that generates a voltage setpoint for the SET terminal, since the thermal drift of the internal error amplifier within LT3045 won't be a match for voltage references in this class.

You'll also find out that very likely the voltage distribution on the PCB, and the assorted thermal voltages in other circuitry, will be large contributors to the effective voltage tempco, even though the regulator won't be of fault.

I wonder why you need such a precise absolute voltage anyway? Most measurements can be made ratiometric versus a reference voltage, and then the absolute value of the reference voltage doesn't matter. If your application uses a sensing bridge, like a strain gage bridge, thermistor bridge, etc. - trying to keep the voltage reference to a low tempco is entirely pointless, since ultimately the A/D converter will be just dividing this voltage reference down and using it for reference. The value anywhere within 1% of the desired one will be plenty good.

Focusing on just this single resistor is missing the forest for the trees. In order to make any guesses as to how salvageable the whole design is, it'd help to have an overview of the system design for the whole thing. It may even be that the voltage doesn't matter, e.g. if the customer doesn't understand the ratiometric nature of certain common measurements. I have no idea how sophisticated the customer is, but such snafus aren't unheard of. Sometimes the customer just cares that the output from the ADC is stable, and they don't understand that this may or may not require absolute-value stable system voltages... Never mind that around 10ppm/K things kinda get hard, and tempcos stack up. Everything in the system that deals with absolute voltages would have to be much better than 10ppm/K in order for the overall system performance to be 10ppm/K.

Answer 3

Is there is a very simple way to create a temperature stable oven using a only a couple of components, for example a thermistor and a transistor?

I've made temperature controlled zones as small as 1" square before. This was done using an aluminum block with a thermistor and 100Ohm resistor glued into the block connected to the pcb with 0.1" headers. But you could take this a step further, and simply put the resistor and thermistor on the pcb and use either aluminum or some other thermally conductive covering that covers all the resistors, and while you are at it, you could cover the whole LT3045 if it doesn't dissipate a lot of heat.

There are a few gotchas if you do a temperature controlled zone:

The first is the zone will need to run higher than any ambient temperature (both air and PCB). The reason for this is you are using the ambient to provide the negative portion of the temperature control (the resistor provides the positive temperature control). A good rule of thumb is 5C above max ambient. If the max ambient on the PCB and air is 35C, then run the temperature control at 40C.

The second thing is making sure you have enough power through the resistor to keep the zone at that temperature. If you have 1W of heat leaking into ambient, you'll need more than that (like 1.5W), or the control will saturate (rail out) and it will lose control. A PWM will be acceptable if it does not cause noise on some of the other circuits.

The third thing is there needs to be separation between the thermistor and heater, otherwise there will not be a delay between the control output and sensor and the control loop will become unstable. I only needed about 0.1C of control for most of my circuits, so make sure you have an adequate circuit that can detect better resolution than that on the thermistor.

I used a simple digital PID control. One could also use an analog circuit although it would be harder to tune.