Drive a 555 with an op amp

Jun 14, 2011 4 Replies
M

Hi every one I am a new bie. I am a bit rusty on my electronics as I trained 30 yrs ago. I am making a small project for home that uses the basis of this circuit

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It will drive my imersion heater with spare electric off the roof pv system I have. What I want to achieve from this group is some help on the design of the stage between the op amp which compares the outputs from my current transformers to the control of the triac by the 555. I have the circuit on the bench working perfectly via the pot on the 555 this is 0 to 37 k ohm. Does any body have any ideas as to the best way to link an output from an op amp into that 555 so that I can obatain a smooth progressive output? Thanks ever so much for replying.


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R

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If the opamp is an analog summing amplifier, just scale its output and send it to the control voltage of the 555, which I think is pin 5. If it's a comparator with a bang-bang output (one rail or the other), then you shouldn't need the 555 at all - just figure out how to use the comparator output to directly trigger the triac.

Good Luck! Rich

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W

First problem: 'pv system' usually means DC power, why do you want to use a triac? They cannot be turned OFF while DC power is present.

Second problem: you link to an optocoupler data sheet. Why? What is the purpose of using an optical coupler? It oughtn't be for AC isolation, you mention 'current transformers' which are intrinsically isolated anyhow.

Third problem: what function does the '555 serve here? There's lots of reasons to combine an op amp and an NE555, and none of them usefully relate in my mind to triacs or photovoltaics.

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J

Your description is ambiguous enough that, right now, the best thing you could do would be to post a schematic of what you've actually built instead of referring to something that we can only guess about how you hacked.
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G

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I thought he might have an inverter for his PV system.

Yup it was confusing. A schematic of what he's got on the bench would help.

George H.

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