I've been working on a project that involves switching some relays with a Basic Stamp microcontroller, with the idea that the relays would be wired into my house's doorbell circuits. For the microcontroller to close relay1 would be the same as pushing the front doorbell, to close relay2 would be the same as pushing the back.
Everything works fine. The microcontroller's output pins drive a couple of switching transistors, which sink the relay coils.
Except that when I actually connect the relays to the doorbell circuits, when the microcontroller closes the relay, it immediately resets.
This doesn't happen when the relay is connected to nothing, or when it's connected to LEDs or lightbulbs, so I'm pretty sure it's a matter of needing to add some filtering caps.
I've had similar problems, driving small electric motors, and I just stuck whatever capacitors I had lying around whereever they would fit.
I'm hoping that someone can give me some more precise ideas as to what type and size of capacitors to use, and where exactly they should be placed.
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Hi, Jeff. Sorry for your troubles. You're switching a high-current, low voltage inductive AC load, and you're getting sparking at the relay contacts. The EMI Graham mentioned is coming from the sparking at the relay contacts. That is very probably what's causing your uC to reset.
Here's the drill. First, remember Sesame Street -- "The Solution ... To Pollution ... Is Dilution!" Speaking seriously, though, the intensity of the electro-magnetic interference will decrease dramatically with distance. Ideally, you might want to have the relays located several feet or more away from your PIC. That can often be done by putting them on another board and connecting them with a ribbon cable. However, the arcing of the relay contacts will also lead to reduced relay life, with pitting on the contacts eventually causing them to stick. That leads to the second solution, which is snubbing the relay contact.
When you're switching line volltage, you want to use a resistor in series with a cap as a snubber across the contacts. A good non-technical method of intuitively finding appropriate values of R and C goes as follows:
1) Find the maximum switching current of the relay (often printed on the side of the relay). Choose a resistor such that the Current across the resistor would be about 1/2 of that maximum with the switching voltage applied.
2) Remove the cover from the relay. Place a snubber across the contacts with a given capacitance. Switch the relay, and see if there is a visible contact spark. If there is, increase the capacitance and try again. Find the capacitor value at which the relay contact just stops visibly sparking, and you're set.
This is actually somewhat easier to do with 120/240VAC switching than
10VAC switching, because your doorbell coil probably draws an amp or so, more current than many AC loads. Let's assume you have a relay that can switch three amps, you have a 1 amp load, and you're switching
10VAC. That would mean you'd start with a 6.8 ohm, 2 watt resistor (10V/1.5A), and find a fairly big AC rated cap that can handle at least twice the peak voltage (10 * 1.4 * 2 = 28V non-polarized). For a 1 amp inductive load, I'd assume your ideal would be somewhere between 1uF and 22uF.
A somewhat easier way to do this is to get a low-voltage MOV surge suppressor, which can clip the maximum voltage excursion until the relay contacts are far enough apart to stop the arc. This can be very effective, because most of the energy from the switching event is absorbed by the MOV. Littelfuse makes MOVs which will help you here -- I'd like the V24ZA50, which is rated for 14Vrms/18VDC, and isd rated to take a 100 joule hit. Possibly overkill, but bigger is better here.
Better still is to put the R-C suppressor in parallel with the MOV. That ensures the maximum voltage across the cap won't exceed the cap's rated voltage, and also will allow you to reduce cap size (do the above experiment with the MOV and see for yourself).
I hope this has been helpful. Dilution through distance, and snubbing are two good first choices. Feel free to post again if you have any questions.
The contact arching is producing lots of high frequency energy that is getting back into the microprocessor, especially into the master clear pin, evidently. You can attack this at both ends. You can try to make the microprocessor circuits more robust and you can add filters to the relay contacts, to reduce their high frequency generation. Probably the simplest in this case is a series RC connected directly across the contact. If the load is a low voltage door bell, try something like 22 ohms and a .47 uF 100 volt mylar capacitor.
As far as making the micro more robust, you have to look at what opportunities you have created for energy in the contact wiring to couple back to the relay coil wiring and back to other microprocessor lines and components.
Is the contact wiring well separated from the driver side wiring? Id the micro enclosed in a grounded metal box?
Is there some grounded capacitance on the reset pin?
Is the coil of the relay bypassed with a diode to cut the coil spike when the coil is de energized?
Is there a bypass capacitor across the coil and driver transistor?
As isolated as it can be, given the physical constraints.
The reset pin is pulled high, internally. I'm not using it, so I've not connected anything to it.
Yep.
No. Where should this be, and what would its value be?
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"Across the contacts" means across the switch terminals (i.e., not across the coil terminals)?
By "visible spark", you mean in a lighted room? Or should I turn off the lights?
My multi-meter says the doorbell is 20VAC. I've had other people tell me it's spec'ed at 16VAC - which would be consistent if my meter was returning peak-to-peak and the spec was RMS.
But there can't be many all that many amps through the thing - bell wire is 18 gauge.
The V24ZA50 is a non-stock item in Digikey's catalog, with a minimum order of 1500 units. But I'm sure they stock something equivalent.
I'll give it a try.
It's been a lot of help - not just in giving me approaches to solving the problem, but in explaining the issues well enough that I can do a web search and find more detail about what's going on.
(I'm a software type playing around with circuits as a hobby, and while the digital stuff is simple enough, when it comes to the analog issues, I often don't know what questions to ask.)
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The load is a standard household doorbell - 16VAC driving a solenoid.
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A .1 to .47 uF film or ceramic should connect the emitter (or source) or the driver transistor to the supply connection of the coil, acting as a local power source for the edges of the power pulse applied to the coil.
Are you absolutely sure that, in this mode, the pin is immune to noise? Is it worth doing the experiment of connecting the pin to ground with a .1 uF ceramic cap, to see if this alters the result?
Hi, Jeff. You've got a 16VAC system rather than a 10VAC system, but most things are kind of the same. Let's see...
Before anything else, I'm assuming you have a diode across your relay coil already. If not, that definitely could cause your processor to reset. Lenz' Law will rear its angry head, and put a voltage spile on the switching side of the coil that will feed into your processor output pin or power supply, if it doesn't kill the switching transistor. Put a 1N400X diode directly across the coil as shown _now_. That should have been first.
1) "Across the contacts" means directly next to, and in parallel with, your switch contacts. It should look like this (view in fixed font or M$ Notepad):
2) It helps to turn down the lights, but all you really need to do is keep it out of sunlight, and put a cardboard box cover over your stuff so there's no shop light shining directly on it. You don't have to be picky about this -- it's "rule of thumb" here. If you want technical rigour, look toward s.e.d. ;-)
You assume you've got a 1 amp load with doorbells and chimes -- it could be less, and could be a little more. Most bell transformers are rated for about an amp. We're trying to be non-technical with this, so I just made some assumptions.
By the way, since you've got a 16VAC system (you're reading 20VAC unloaded), the choice for MOV isn't correct. Try the Littelfuse V47Z20 (Mouser P/N 576-V47ZA20, in stock, $0.42 ea. in single qty). For caps, you might want to look first at what you have in your junkbox. This all presupposes you can substitute the caps in the snubber for optimum. But if not, you might want to look at
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and find a good cap. In order to complete things, if you don't happen to have a well-stocked junkbox, here's your shopping list from Mouser:
The PIC datasheets I've read generally want some kind of cap + resistor. Apparently, on some devices, the reset (MCLR) pin is prone to latchup if it sees spikes. The circuit I usually use is this, stolen from some PIC datasheet:
However, if he has MCLR disabled in the config word, he probably is not going to see any issues with it. He is probably seeing some kind of power droop. There is a brown-out detection in many PIC devices which he might be able to use to see if that is the problem. ISTR that it sets a bit someplace if the device reset due to brown-out. Or, he could just use a couple of 0.1uF caps between Vss and Vdd.
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I don't know why everybody keeps talking as if I was using a PIC. I stated in the original post that I was using a Basic Stamp 2.
Yes, the Basic Stamp includes a PIC, but it includes a great deal of supporting circuitry, as well. The reset pin is pulled high. It's not the reset pin that is causing the Stamp to reset, it's the brownout detector.
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Add a small resistor, 10 ohms, in series with the Vdd pin. Use a 10uF electrolytic and a 0.1uF ceramic cap between the Vdd and Vss pins, very close to the stamp. Add a 10nF cap from /RES to gnd.
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The BASIC Stamp usually uses a PIC. There is a configuration word bit that re-assigns the /MCLR (reset) pin to become a Schmitt trigger input pin. Hopefully the OP doesn't have it floating.
Well, if you knew that the brown out detect was causing the reset, why didn't you say so? That means that your power supply must be sagging. Are you using the same power supply in all of your testing, or are you making changes (including using longer wiring)? BTW, how do you "know" that the BOD is triggering the reset?
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