Turning OFF the grill when the espresso maker is ON

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Postby vivo » Oct 26, 2016, 3:24 am

These are the variables to this problem: We have two 120 volt appliances:
- an espresso machine (draws 8 amps)
- and a grill (draws 12 amps).

The grill is ON all the time (it actually stops heating when it's at the right temp, but let's consider it ON all the time).

The espresso machine is ON at random times.

I need to turn OFF the grill when the espresso machine goes ON (basically when anything is drawing from the 'espresso' outlet) -> (Can't exceed 13 amps at any given time on this circuit).

The grill's heating element is turned ON by a relay until the temperature is reached.
The 12 volt relay coil is switched ON by a transistor that is itself switched ON by a 5 volt signal going into a 4.6K resistor then to the base (so 1mA at the base).

Current sensing of the espresso machine is accomplished by a split-core transformer that clamps around the live wire. This is actually a (cheap but fairly accurate) clamp meter. Now, inside this 9 volt clamp meter, I found the op-amp (lm358) that amplifies the tiny signal from the 'clamp'. If the meter reads 0 amps, the voltage at the 'output' pin of the op-amp is 6.30 volts. When the meter reads 8.5 amps (that's the espresso turned on), the voltage at that pin is 6.50 volts. If the meter reads 12 amps, that voltage climbs to 6.70 volts. So I can use this signal for something.

Now, this is the part where I get confused.
What's the best/easiest way to insert a 'switch', obviously/probably a transistor, somewhere in the relay's coil switching circuit?

Would it be better to pull the 5 volt 'on' signal to ground to turn OFF the transistor that drives the relay (maybe not a good idea, I don't know what actually produces the 5 volt signal, maybe doesn't like to be shorted to ground?)

Or would it be better to insert a transistor in the 12 volt line that would be 'ON' all the time, and then turned OFF when the signal from the clamp meter comes in?

Before all that though, I need to figure out even this simple thing: how can I have the transistor be off when the clamp meter's signal is 6.30 volts, and then have it on if the voltage is any higher than that? Making 6.30 volts the 'threshold' voltage so to speak?

I was convinced this whole thing was going to be trivial when I started thinking about it, but now I realize it is not. I am still convinced the answer is fairly simple!

Can anybody chime in with some great ideas? Maybe another approach altogether? Oh yeah, and the goal is to keep it simple and hopefully not have to order components. I have all the resistor values and a bunch of general purpose transistors, diodes and the like.

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Postby bostonbuzz » Oct 26, 2016, 10:04 am

Sorry I can't help. But, I can sympathize.

I have my Strega, a 1500 watt water kettle, and a microwave. All of which are used many times per day, and none of which can be on at the same time or it means a trip to the breaker.
LMWDP #353


Postby ChileBean » Oct 26, 2016, 9:49 pm

Okay. Let's see if I can help. First things first. You say the grill is turned on when 5v is present on the base of the transistor, and that the 5v on the base shows up via a resistor.

Turn the grill on. Short the base of the transistor to ground. (Should not damage anything if the only connection to the base is the resistor.) but be sure to short the side of the resistor connected to the base. Does the grill shut off?

Report back and we will go from there.


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Postby AssafL » replying to ChileBean » Oct 29, 2016, 5:35 pm

A few thoughts:
1. Isolation: You really want the two circuits galvanically isolated (otherwise you risk catastrophic failure if one becomes hot, or fails). So a relay in between them. (a relay is fully isolated between activation coil and contacts).
2. Preferences: Which is more important? Pulling an espresso or running the grill... Another option is just adding a hefty diode in series with the grill heater (so it won't turn off - just the RMS current will drop to about half).
3. Noise: 6.3V vs. 6.5V isn't a lot. Especially when 12A routed nearby is being cycled on-off... A better approach would probably be to connect in parallel to the espresso machine heater (usually crimp quick connect or screw lugs - so fairly easy to install and remove).

So an option is: if you were to take a large Rectifier, wire it in series with the Grill Heater, and use the contacts of a massive Normally Closed (NC) relay (or SSR) to short the Diode (so when the espresso machine is off the Grill is at full power) - and route the 110VAC coil (or SSR input) to the espresso machine's heater - that may do it.

You can of course build a comparator for 6.4V (easy with 1 operational amplifier) - and wire it to drop the voltage on the transistor - or add a second transistor. That is also fairly easy to do but make sure the grill uses an isolated supply, and that you have the circuit diagram handy... Or the comparator may be used on a safety switch (like a door switch) or to guilt the temperature controller to thing it is too hot. You need a circuit diagram for all of those options.

BTW - why is the clamp giving 6.3V with no current? Is it an AC clamp or a Hall-Effect (DC capable) clamp? If the former are you rectifying the voltage correctly (should show 0V@0A)? If the latter - it is difficult to extract RMS - did it for a motor chopper using an ATTiny - see Automating the Versalab M3 for details of a simple way to extract RMS current.

Edit: If you are measuring current using a clamp transformer remember you must load the output of the current transformer and measure the voltage on the load. This is especially true for a high winding count where the current would be low but the high winding ratio would mean that the open loop voltage can be very high. If this load is internal to the clamp figure out what it is. It is lower for a higher current probe (and the sensitivity will suck).
Caution! Water, heat, pressure and electricity don't mix! I want an espresso.


Postby jonr » Oct 29, 2016, 6:43 pm

I would probably wire a relay to the espresso maker heater. Ie, when the heater is on, the relay (controlling power to the grill) is off. No electronics needed.


Postby ChileBean » Oct 29, 2016, 7:54 pm

+1 to jonr's suggestion.