Eliminate thermostat contact chatter

[Blippo]

I have a Grasslin Tower RS room thermostat controlling the 230V 50W fan of
a Smiths hydronic fan-assisted kickspace heater in my kitchen.

The thermostat contacts chatter for many seconds when opening causing
annoying RFI and I wondered how I could make them switch cleanly, and/or cut
out the RFI?

TIA

Blippo

 

[Jerry G.]

I would try another thermostat in case this one is defective. I am using
thermostats on a number of heating and cooling systems. When they give this
type of fault, I change them. If it is a design fault, see if you can use a
different model of thermostat.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
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I have a Grasslin Tower RS room thermostat controlling the 230V 50W fan of
a Smiths hydronic fan-assisted kickspace heater in my kitchen.

The thermostat contacts chatter for many seconds when opening causing
annoying RFI and I wondered how I could make them switch cleanly, and/or cut
out the RFI?

TIA

Blippo

 

[Asimov]

"Blippo" bravely wrote to "All" (02 Dec 03 10:47:36)
--- on the heady topic of "Eliminate thermostat contact chatter"

Try a snubber consisting of 120 ohms 1/4w and 0.1uF @ 400v (disk) across
the contacts. The contacts may simply need to be cleaned or replaced if
they are burned.


Bl> From: "Blippo" <[email protected]>

Bl> I have a Grasslin Tower RS room thermostat controlling the 230V 50W
Bl> fan of a Smiths hydronic fan-assisted kickspace heater in my kitchen.

Bl> The thermostat contacts chatter for many seconds when opening causing
Bl> annoying RFI and I wondered how I could make them switch cleanly,
Bl> and/or cut out the RFI?

Bl> TIA

Bl> Blippo

.... Reactance: your imaginary friend.

 

[Sam Goldwasser]

In general, thermostat contacts should open and close cleanly with some
hysteresis. Unless this is a design error, there might be a mechanical
problem with the unit.

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[Allodoxaphobia]

In general, thermostat contacts should open and close cleanly with some
hysteresis. Unless this is a design error, there might be a mechanical
problem with the unit.

Some I've seen use wee little permanent magnets to provide this
hysteresis. Maybe the OP's thermostat has had this magnet come
loose and it's fallen out or become dislocated.

Jonesy

 

[H. R. Bob Hofmann]

In general, thermostat contacts should open and close cleanly with some
hysteresis. Unless this is a design error, there might be a mechanical
problem with the unit.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.

There are two types of bimetallic thermostats. There are the kind
previously described here, that have or are supposed to have, a "snap
action" to the contacts.

The other type of thermostat is simply a bimetallic arm, without any
"snap". This is common in acquarium heaters and other devices using
very low-cost thermostats. Dirty contacts will make this type of
thermostat even more likely to arc, causing RFI. A nail file or emery
board will clean the contacts for a while, the snubbing circuit
described by a previous poster will help reduce the arcing that seems
to be the problem, but cannnot eliminate it entirely.

H. R. (Bob) Hofmann

 

[Asimov]

"H. R. Bob Hofmann" bravely wrote to "All" (03 Dec 03 17:59:38)
--- on the heady topic of "Re: Eliminate thermostat contact chatter"

HRBH> From: [email protected] (H. R. Bob Hofmann)

HRBH> There are two types of bimetallic thermostats. There are the kind
HRBH> previously described here, that have or are supposed to have, a "snap
HRBH> action" to the contacts.

HRBH> The other type of thermostat is simply a bimetallic arm, without any
HRBH> "snap". This is common in acquarium heaters and other devices using
HRBH> very low-cost thermostats. Dirty contacts will make this type of
HRBH> thermostat even more likely to arc, causing RFI. A nail file or emery
HRBH> board will clean the contacts for a while, the snubbing circuit
HRBH> described by a previous poster will help reduce the arcing that seems
HRBH> to be the problem, but cannnot eliminate it entirely.

Contacts will always burn somewhat but sometimes a little is desirable,
within reason, as this will automatically burn off condensed oils and
other types of oxidation films which prevent a good contact. A snubber
will help preserve a good set of contacts but it won't help much if they
are already badly burned. Back to back diodes in series will also work
as will a spark gap (glass enclosed or other), or a VDR etc.

.... The current limits placed are based on resistance

 

[Isaac Wingfield]

There are two types of bimetallic thermostats. There are the kind
previously described here, that have or are supposed to have, a "snap
action" to the contacts.

The other type of thermostat is simply a bimetallic arm, without any
"snap". This is common in acquarium heaters and other devices using
very low-cost thermostats. Dirty contacts will make this type of
thermostat even more likely to arc, causing RFI. A nail file or emery
board will clean the contacts for a while, the snubbing circuit
described by a previous poster will help reduce the arcing that seems
to be the problem, but cannnot eliminate it entirely.

Sometimes, the hysterisis is provided by a small permanent magnet
situated near the contacts -- might not be very obvious if you don't
know to look for it.

If you're up for it, rewire things so the thermostat just provides gate
drive for a triac. Then arcing is not a problem.

Isaac