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Little electronics project/problem
TheFish
Registered User regular
Registered User regular
I'm trying to put something together to control a car heated seat to stop it overheating. This is cobbled together from a couple of circuits I found online, and I don't really know what I'm doing, so there's probably something obviously wrong with it:
It uses an 741 op-amp as a comparator, and I want the output to switch off when the seat goes above a pre-set temperature.
R1 & R2 provide a reference voltage.
R3 is a NTC thermistor attached to the seat (reads approx 4-6k ohms in the temp range I want)
R4 sets the temperature cut-off point
LOAD is the heater element itself - approx 5 amps, 2.5 ohms.
The circuit I based it on used a transistor to control a relay, but I wanted to avoid this as the relay clicking on/off every few seconds would be annoying. So I replaced it with a MOSFET (Q1). Have I done this right? What do I need to look for when picking a MOSFET, just the current and wattage rating? I'm also not sure about the value of R6.
It uses an 741 op-amp as a comparator, and I want the output to switch off when the seat goes above a pre-set temperature.
R1 & R2 provide a reference voltage.
R3 is a NTC thermistor attached to the seat (reads approx 4-6k ohms in the temp range I want)
R4 sets the temperature cut-off point
LOAD is the heater element itself - approx 5 amps, 2.5 ohms.
The circuit I based it on used a transistor to control a relay, but I wanted to avoid this as the relay clicking on/off every few seconds would be annoying. So I replaced it with a MOSFET (Q1). Have I done this right? What do I need to look for when picking a MOSFET, just the current and wattage rating? I'm also not sure about the value of R6.
TheFish on
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I'm a noob electronics engineering student, I'm half way through my first op-amp course. So if something I say sounds retarded, it likely is.
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To my knowledge, you can't power an op-amp with a non-symmetric input. It needs +12V on its positive side, and -12V on its negative side (Or +/-13, or 14, etc). (This is assuming the particular op-amp you're using doesn't make mention of it accepting +12 and GND.)
Also, the way you have that feedback loop, you've configured it as a Schmitt Trigger. This will introduce a memory effect and may not allow it to turn off when you want it to.
I would personally use a BJT instead of a MOSFET, but thats because I love using BJT's as switches for some reason.
I'm hesitant to give specific part numbers and calculations, as I'm not 100% confident in my abilities. I hope this helps somewhat though.
I'm an EE, but it's been a while since I've done circuits. But I was usually pretty good at them.
Non-symmetric rails on an op-amp are fine. Not sure about the Schmitt trigger, I'll have to look at the circuit better.
Stick with the MOSFET, it's better for power dissipation.
EDIT: Schmitt trigger is fine, it's exactly what you want. Haven't verified the values of your resistors, I'd have to do calculations. But they look good.
You're right to be concerned about the MOSFET, that's where things get tricky. I'm not sure if you NEED R6, but it's fine if you have it. I'm not sure if you know how MOSFETs work, but I'll tell you anyway.
Where R6 is connected to is the gate. The source is connected to GND, and drain is connected to the load. Your gate-to-source voltage (Vgs) needs to be higher than the MOSFET's threshold voltage, in order for it to turn on and allow current to flow between drain and source, thus turning on your load. A voltage applied to the gate induces a channel between drain and source to allow electrons to flow. If it's less than threshold, the induction isn't strong enough to form a uniform channel. Why's this important? Well one, you need to make sure you're circuit's applying enough voltage to turn it on when appropriate. You'll have to calculate this out, but looking at it briefly, it'll probably be fine.
The second, more concerning part, is that you have 5 A flowing from source to drain to drive the load. That's a LOT of current, and it's going to require a large power MOSFET, most likely with a heat sink. You want to get a good quality MOSFET with low drain-to-source resistance values, otherwise you'll be dissipating away a lot of current. It could get very hot, very quickly, and it could become dangerous. And this becomes more of a concern at lower Vgs voltages, because as Vgs goes down, that resistance goes up, causing more heat to dissipated.
That's why a relay is technically better. It's either on, or off; when off, the connection is physically broken. That's not to say you should be using a relay, I still would prefer a MOSFET here. Just, calculate your values, be careful, and choose your components wisely. Hope this helps.
Thanks. I'll get the calculator out. At least now I know I'm thinking along the right lines and hadn't made some obvious mistake.
I've been looking at MOSFETs in the 10A/125W range to provide plenty of safety overhead. Cooling is something I was prepared for, so that's not a problem.