Hello, I am very new to circuit design and assembly, and an important piece of lab equipment died suddenly and needs to be fixed. However, some of the components are no longer available (the equipment is VERY old) and in lieu of building a whole new instrument again, I am trying to assemble a circuit to supply 4V at 2 Amps to a "low-resistance" coil heater that will be heating up a gas sensor to its working temperature (~700C) - I am unsure of what low-resistance means with this device. Also, the voltage supply needs to be adjustable to switch between standby and on states (I assume this will involve digital control of the high/low states?), as the heater will not be required to work continuously. The circuit also needs to be as low noise as it can be to minimize measurement errors.

What would be the best way to approach this project? I am sure it is probably relatively simple, but I am having a hard time getting started!

Thanks for any help!

 

What is the make and model of the piece of equipment?

Do you have the schematic for it?

If so, it would be very helpful if you posted it.

If not, can you create a schematic of how it currently exists, and post it?

It would be best to make as few changes as possible where lab equipment is concerned.

 

I don't have the make or model of the current sensor. It has been here for at least 20 years and I suspect was old even then. The sensor is part of a large custom made system. The only information that is available was the voltage/current requirements that I listed earlier. I THINK it might be similar to this Honeywell O2 sensor which apparently shares the voltage/current requirements, but I am not entirely sure as there is little information available and I don't want to take it apart to look at the construction. I am not currently able to look at the boards in the instrument now, as I am away from the lab, but I was hoping to get a start on the mental design of this supply while I was away. That was really enlightening wasn't it! Sorry about the lack of information -it is part of what has frustrated me. The system has plenty of room for expansion so adding some additional power supplys/transformers, circuits won't be a problem.

Thanks!

 

Well, you'll need to figure out HOW it is currently maintaining the temperature. It likely has a thermocouple connected to an instrumentation amplifier that was controlling the current regulator circuit, which would give a pretty stable closed loop.

If the exact temperature wasn't critical, it might've used a simple thermal switch to turn current on and off.

 

If your assessment is that you just need 4 V at 2 A that needs to be switched on and off by a signal, a straightforward solution would be to use a lab DC power supply for the power (or a well-regulated wall wart) and switch it in and out of the circuit with a relay. I'd assume that the "logic" signal to turn the power on from standby probably won't be able to switch a relay directly, so the relay's coil current will probably need to be switched by an active device like a FET or bipolar transistor. This could require another small wall wart for powering the relay. Or the whole thing could be driven by a 5 V wall wart and a regulator would derive the needed 4 volts. The folks on this board can help with the design and it would probably be less than 10 components total.

 

The sensor heater is controlled by an RTD (pt100). I am pretty sure that the sensor needs a fairly consistent operating temperature to minimize any measurement artifacts that might pop up during operation. The temperature of the sensor determines that rate of movement of oxygen ions through the ZrO2 matrix.