EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Neukyhm on March 02, 2018, 10:07:45 am
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Hi there, I have a positive voltage that can be from 0v to +150v. I would like to measure when it's 0v, so the attached circuit is what I was thinking about.
The IC is a TC4468. The idea is that when the voltage is >0v, the diode blocks and the 12v goes to the TC4468.
When the voltage is 0v, the diode pulls down the TC4468 input. However, the UF4007 has a voltage drop of 0.8v, just the upper limit for a logic
0 input. The question is, is this a good idea? Is there a better method to do this?
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That's a pretty usual way to do this - should work
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The circuit looks more sensible when you draw so that the resistor is a pullup to the +12V supply to the TC4468 (I assume).
Yes, it should just about work but the guaranteed logic low input to the TC4468 is 0.8V, so you might be a bit close depending on how close to 0V your 0-150V rail gets (ie. it actually needs to get within 100mV or so of 0V). A suitably voltage rated schottky would give you just a little more leeway but not much.
You need to consider what happens when your 0-150V supply is in the 0.5-3V range.
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Thank you for the replies. Can you recommend me an IC with higher threshold? For example, I liked that the IR2110 had a very high threshold and proportional to Vdd.
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Without knowing more about your application and the varying 150V rail, I think you would be far better to use a proper analogue comparator, with hysteresis, to ensure a clean and predictable transition. Without that, the logic level that the TC4468 'sees' is going to transition several times due to noise as the voltage passes through its undefined logic transition region. Probably not what you want in a power mosfet driver!
http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf (http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf)