LM2907-N: 5V pulse train input

Hi Jeffrey,

My suggestion is that you would need to measure temperature at the laser's heatsink interface and the measure the the flow rate of the coolant system, where the coolant is circulated through the cooling system. The temperature at the laser heat sink block should take the higher priority for the safety interlock system.

 The F-to-V simulation example is shown below. I would need additional requirements to complete the simulation.

LM2917-14 flow rate F-V converter 06282024.TSC

LM29x7 Comparison & FAQ Revised 04192024.pptx

If you have other questions, please let us know. 

Best,

Raymond

Hi Raymond,

So I don't know that I can directly measure the temperature of the laser (the coolant flows through a tube that surrounds the actual laser) but I do have a thermistor measuring the temperature of the coolant. I took a stab at the circuit based on my understanding of the datasheet. You can ignore K2 and the other named nets, plus J1 except for pin 6 which is where the Hall effect output comes in from. My thinking is that R4 and R5 will form a voltage divider (values to be determined through experimentation) that sets the minimum flow rate threshold, and by connecting the charge pump output to the negative input of the high-gain amplifier the relay (40mA coil current) will trip if the rate drops too low. Does this make sense or am I missing something?

Hi Jeffery,

Since you are using low voltage in the application, you are limited to 14 pin package. Use the package without the internal 7.5Vdc Zener clamp. The input frequency signal in 8 pin LM2917 packages need to be above and below GND. 

BTW, your Vs is on the load side, and the output voltage swing may not be able to reach 5Vout in full scale. 

What is the max. input frequency? 10V to 12V may be better option for 5V F-V conversion (unless you are going to do less than 5V conversion).  

Regarding to the coolant temperature. It will be good to measure the bulk temperature near the laser emitter (measure the metal tubing temperature), especially this is the high power laser application. The coolant temperature is important, but the trigger point should be the metal casing at the laser emitter, which should be the hottest part of coolant module.

If you have other questions, please let me know. 

Best,

Raymond

Hi Raymond,

My best estimate is that the highest frequency is only about 250Hz. And I'm fine with using a narrower output voltage range than 0-5V; I'm only trying to confirm if the frequency is above a specified threshold and throw an error if it isn't.

Hi Raymond,

That second part is the heart of my question. Since I'm using the 14-pin package, the positive input of the built-in comparator is not tied to the charge pump output. Can I connect the charge pump output to the negative comparator input and use a voltage divider to put a constant threshold voltage on the positive input, so that current flows through the output transistor (tripping the relay and opening the interlock) when the F-to-V voltage is below the threshold? Or do I need to add an external comparator?

Perfect. Thank you!