Electronics question -- remotely controlling an air cleaner

[Matt Meiser]

I've now got 2 air cleaners. The first is a very basic Delta (call it "B") with no remote control. I've been controlling it with an RF solution where there's a box that plugs into the outlet and the air cleaner plugs into that which has been fine. The new one is a newer Delta (call it "A") with a timer, 3 speeds and an IR remote. As a (maybe) bonus, the IR remote also works with my Grizzly cyclone and vise-versa. I'd like to use that remote to control both.

Since they will be located across the shop from each other, I want to run a low voltage wire between them and add some relays so that B turns on whenever A is on high. Poking around, I found that the speeds on A are switched by SCR's and a pretty low level signal, so I'm going to add a 120V coil relay on the blower wiring that will close when the blower is on high.

I see two possible options for the low level signal. 1 is my prefered option.

1: The control board in unit A has a small transformer and a bridge rectifier. The voltage on the DC side of the rectifier is about 6V. I'm considering switching the 6V signal with the relay I'm adding. Unit B would get 2 relays, a small 5VDC coil relay to switch a 120V relay that has an adequate rating to switch the blower. My biggest concern is damaging the control board by adding the load of the additional relay. I see this as a more efficient option though.

2: The relay in unit A would just be a set of dry contacts. In unit B I'd add a 24V transformer in B and 24VAC coil relay in the fan circuit. The 24VAC would be switched by the dry contacts from unit A. I don't see a 24VAC signal being a safety hazard since that's just like a thermostat and the back of my furnace out there even has exposed 24VAC terminals. The biggest drawback is adding the transformer which would always be powered--at least when the lights are on in the shop since this unit will end up getting plugged into one of my lighting circuit plugs.

[Jerome Hanby]

In solution 1, use a small signal transistor with enough current rating to pull in your small relay. Go to a book store and look in any of the volumes of the Electronic Circuit encyclopedias for the basic 1 transistor, 4 resistor layout. This will add almost no load to your control board. You could get fancy and add an opto-isolator in line with that 6 volt output and basically have practically total isolation between the two systems, but the circuitry gets a little more complex.

[Bob Winkler]

I might use this as an opportunity to be creative, inventive, or at least unconventional. I might consider an external air flow switch triggered by unit A. This could be as simple as a microswitch with a paddle on the actuator arm. The paddle size and position in the airstream could be calibrated to switch when you want it to. This gives you the iidea:
http://www.cherrycorp.com/english/AIRFLOW.HTM


The benefit here is that you don't "monkey" with any of the unit internals, which could hurt resale or worse cause unit failure. The microswitch switch can trigger any relay you choose. I could probably find a microswitch to send to you if you can't find one.

Just an alternative :)
Bob

[Dan Friedrichs]

I wouldn't worry about damaging the control board in "A" - really, you're just adding the load of a very small relay onto that power supply, which should be almost no additional load. I wouldn't worry about that, at all. Scheme 1 sounds doable. If you are really concerned about it, for some reason, you could also just hook a "wall wart" transformer up inside unit "A", to generate a +5 (or whatever) supply to send to the relay.

But, are you really set on using low voltage control? You could easily just add a single outlet on unit "A", tied to the blower "high" winding, and then plug the AC cord from unit "B" directly into it. Unless your shop is huge, the cost of an extension cord is probably less than the relays.

[Matt Meiser]

Jerome, I knew that could be done as well, but its been 18 :eek: years since I took EE201 (or whatever it was) and learned about transistor circuits. I've never done circuit design professionally. That knowledge is long gone!

Dan, I thought about that, but the combined ratings of the units is about 10A and the outlet A is plugged into happens to be on the same circuit as some other things--I think I'll have trouble overloading the circuit. Plus they'll be about 20' apart and I'm not crazy about draping the extension cord around the shop.

I didn't have the 24VAC relay I thought I did, but I did have some 24VDC relays that are rated for 1/2HP and a 24VDC wall wart salvadged from some piece of past electronic gear. I added the 120VAC relay to A with a set of dry contacts going out to a set of binding posts mounted below the control panel. I'll wire the power supply and relay into the motor wiring on B with a set of binding posts on the outside so I can run a piece of thermostat wire across the shop and back to trigger the relay. If I ever want B to work as originally designed I just have to short the binding posts, or it'll be easy enough to revert them to stock.