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| Comparator with dual supply and excessive current draw [solved] |
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| geo_leeman:
I'm working with a comparator circuit that looks at a signal (range -15 to 15 VDC) and a reference voltage (range -15 to 15 VDC) and gives me a logic out for when the signal meets or exceeds the reference voltage. The output goes into a latching circuit, so no need to worry about hysteresis, etc. I've implemented this with the LM339, but after a bit of operation suddenly the negative supply draw increases and the chip gets pretty hot. Looking back at the datasheet, I see the inputs are -0.3-36V (DOH!). Hmmm, so there's the issue I think. The question is, what is a comparator you'd recommend for this? Looking through digikey I'm not finding a suitable part and would like to keep my footprint (14-SOIC 3.9 mm) so I don't have to re-spin a board here. My first thought was to move to a simple quad op-amp, but those pinouts are different. |
| wraper:
--- Quote from: geo_leeman on April 23, 2019, 09:31:44 pm ---I see the inputs are -0.3-36V (DOH!) --- End quote --- Doh what? There is no such thing as GND pin on LM339 in addition to negative power pin. -0.3V is in regards to it's negative power pin. You have negative and positive power supply pins. And you are allowed to push inputs only a little bit beyond rage of supply voltage (0.3V). It's true for basically any other IC, be it comparator, opamp or even digital IC. If you need to put -15V to it's input, it's negative power pin should be powered by at least -15V or more negative. "Ground" is completely arbitrary for it, nothing to do with it's power supply. |
| wraper:
If LM339 is powered properly, I'd be more concerned about what is connected to it's output. As it will pull that towards it's negative pin which likely is at -15V. So if that something has 0V/GND as it's negative power supply, you have a problem. In any case you should provide your circuit. |
| geo_leeman:
Ok, so my initial thought was indeed that then since I'm having issues I decided it must be -0.3 ground relative, but yes, with no ground, no problem... Hmmm. So I must be dumping current somewhere else there. I've attached a schematic (this section is duplicated again so I use two of the total 4 comparators). It's output is going right into a 4072 to get or'ed together with more error conditions. As you can see the two unused comparators have their inputs tied to ground via 10k. |
| wraper:
Where logic out is connected? And you probably shouldn't connect a cap to the output of LM339. There is no issue with input signal voltage range of LM339. |
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