Propeller control of household type freezer appliance

ElectricAye

Hi all,

I'm sure this has been discussed before but I can't find it on the search.

I would like to use the Propeller to control a household type freezer. It has a standard 3 pin plug, operates on 120 VAC. Not sure about its exact amps yet but its owner's manual says an extension cord must be rated for at least 15 amps. I do NOT need wireless control but an off-the-shelf gizmo would be ideal.

thanks for your time,
Mark

Cole Logan

So are you just wanting to turn it on and off. Or what exact control do you want over the freezer
·

ElectricAye

Cole Logan said...
So are you just wanting to turn it on and off. Or what exact control do you want over the freezer

I'm monitoring its internal temperature using DS18B20's. I want the ability to turn its main power on and off based on various criteria. Such control doesn't give me a lot of accuracy insofar as controlling its temperature, but at least I can cycle it for experimental purposes.

mctrivia

use a 15 amp relay. you will need a step up transistor to run coil with reverse diode across coil to protect

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

mctrivia

for reference sake there is always a panel somewhere saying how many amps it draws but you can make a safe reference by the plug type. according to canadian electrical code diagram 1 the standard plug we all know is good to a max of 15 amps at 120 v. 20 amp plugs have the neutral pin sideways

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

ElectricAye

mctrivia said...
use a 15 amp relay. you will need a step up transistor to run coil with reverse diode across coil to protect

Is there anything off-the-shelf UL certified that I can just hook up to for this sort of thing? I'm concerned that if I have to kluge it myself, it will raise eyebrows about fire hazards and electrical codes and all that sort of thing.

SRLM

How about this spark fun tutorial:

http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=119

Also, you could probably hack one of those $10 appliance/light timers.

mctrivia

yes I use commercial relay panels all the time but they are not cheap.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

mctrivia

as a commercial electrician I build my own control boxes all the time. to be legal you need proper metal box ul rated. din rail and ul rated relay. put step up transistor outside of box. low voltage is considered safe.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Jonathan

Why not use a SSR (solid state relay)? You can get ones that will trigger from 3V. They are optoisolated, detect zero crossing (most of them) have no back EMF and are very reliable. I have a mushroom culture incubator I made from a box freezer, maintains a constant temp with a heater and the cooling unit of the freezer. Has worked great for years.

Jonathan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

TreeLab

I am with Jonathan : use an SSR that can handle the AC motor. They are esentially bullet proof, and the worst that can happen is that they lock-up in the On position. No moving parts, no noise spikes ...

Think about the delay between on cycles; I am told that most compressors require a minimum time for the pressures to equilibrate prior to restarting the motor.

Cheers!
Paul Rowntree

mctrivia

SSR are better though also more expensive. Relay or SSR the only real difference is SSR does not need amplifying transistor or protective diode.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Jonathan

Well, SSR's are also opto isolated from the mains and have no moving parts and hence are better in harsh enviorments, which is nice. Also, if you get a zero crossing type, it always switches on and of at the zero crossing point. Not too important for most things, but good for some, like certain types of heating elements.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

ElectricAye

TreeLab said...
I am with Jonathan : use an SSR that can handle the AC motor. They are esentially bullet proof....

Okay, bulletproof is my kind of thing. I'll try the SSR and dress it up to look like it's off the shelf. Then maybe nobody will notice it's another kluge by me.

thanks, everyone!

Beau Schwabe

ElectricAye,

I have had great luck with these SSR's. They are rated for 25Amps. I use 4 of them in a DPDT arrangement to switch between the AC mains and my Solar equipment when I have enough "Solar reserve".

cgi.ebay.com/New-in-Box-Solid-State-Relay-SSR-24-480V-AC-25A_W0QQitemZ120387412336QQihZ002QQcategoryZ36328QQssPageNameZWDVWQQrdZ1QQcmdZViewItem



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

ElectricAye

Beau Schwabe (Parallax) said...
ElectricAye,

I have had great luck with these SSR's. They are rated for 25Amps. I use 4 of them in a DPDT arrangement to switch between the AC mains and my Solar equipment when I have enough "Solar reserve".

cgi.ebay.com/New-in-Box-Solid-State-Relay-SSR-24-480V-AC-25A_W0QQitemZ120387412336QQihZ002QQcategoryZ36328QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Beau,

awesome info. Being a newbie, I'm helped a lot when I see a specific component like this. You just saved me hours of clueless searching.

thanks so much!
Mark

BradC

TreeLab make comment about compressor off times, and he's very right. Look at specifying minimum on and off times to prevent trying to start the compressor against a high pressure load (off time to short) or "short-cycling" (on time to short). When I did my AC thermostat I had a minimum off time of 5 mins and a minimum on-time of 10 mins. They were recommended to me as safe minimums by a fridge-mechanic mate of mine..

I used a 3A SSR from futurlec. It was cheap and can be driven directly from a Propeller port pin (triggers reliably at 3v). Having said that, I'm on 240V so my current is not as high as you guys have to put up with.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Cardinal Fang! Fetch the comfy chair.

ElectricAye

BradC said...
TreeLab make comment about compressor off times, and he's very right. Look at specifying minimum on and off times to prevent trying to start the compressor against a high pressure load (off time to short) or "short-cycling" (on time to short).....

Yes, thank you, Brad, Tree's is a very good point to bring up. Luckily, my cycling time will be about 10 hours or more, so this is not an issue. However, if someone else were trying to control a cheap freezer to a tighter temperature range, then this cycling time issue would be very important.

much appreciated,

Mark

PJAllen

Ah, yes, SSRs and inductive loads.· If the SSR isn't "impedance protected" then results will prove unsatisfactory.··A motor/compressor is an inductive load.·

Here is some text from the spec provided in the link (emphases mine) --

Load could be resistance type, and maximum current is less than 25A RMS. Load could not be DC type. Load could not be inductive type.

Jonathan

@Brad - Thanks for that info. I don't think my setup ever cycles faster than 5 mins, but it might turn on for less than 10 mins. I'm going to add that to the program right now, although it has been fine for years now, but why take chances. I don't need super tight temp. control, so if I have to run it a little longer, that;s ok.

@ PJ - I took a look at the spec sheet for my SSR and didn't see anything about inductive or resitive loads. I have one running a compressor, another a small motor and one a heater element for a radiant floor. Are most SSRs not for inductive loads?

Jonathan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

PJAllen

Though it's often portrayed as such, an SSR isn't a panac

Jonathan

Which leaves me wondering, as my spec sheet says nothing about load side impedence protection or inductive VS resistive loads. I'll see if I can dig up another spec sheet on it. Thanks for the heads up, I like to at least *know* that I am driving 70 mph on a space saver spare. [noparse]:)[/noparse]

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

ElectricAye

PJ Allen said...
Though it's often portrayed as such, an SSR isn't a panac

TreeLab

I should have been more explicit when I said to use an SSR 'that can handle the AC motor'; I apologize!

That being said, we use SSRs for big motors on vacuum pumps and it is easy to find models that handle inductance-related phase shifts correctly.

Another approach that may work, depending on what exactly you want to do, is to run the freezer unit pushed to its lower-temperature limits, then use a heater (easily controlled by any version of SSR, since the load is resistive) to raise the temperature within a smaller volume or for the experiment itself. If the test volume/thermal mass is relatively small, you will get much faster and precise control this way.
Cheers!
Paul Rowntree

ElectricAye

Grasshopper said...
I would take apart the freezer and find the temp sensor bypassing all whats discussed above.

To Grasshopper,
That makes sense, provided the temp sensor switch is lower power. Why didn't I think of that in the first place? Thanks for the idea!

Paul Rowntree said...
Another approach that may work... is to run the freezer unit pushed to its lower-temperature limits, then use a heater (easily controlled by any version of SSR, since the load is resistive) to raise the temperature within a smaller volume or for the experiment itself.

To Paul,
yes, this is a fantastic idea. In fact, to get the kind of accuracy I'll need later on, this might be the only way to go for doing it cheaply. The actual sample space isn't very big and, without thermal mass in the freezer, the temperature of this home freezer isn't very stable anyway. What a cool concept!

Thanks, you guys, for shining a little of your brilliance my way!