Hi from a Brit in Sunny Venezuela. A colleague from the HVAC-Talk forum kindly referred me to you guys. I hope you don't mind an outsider dropping by to ask for some advice.
Can anyone help me out with this one?
I rebuild quite a lot of icemakers, all kinds, especially old ones. Parts are hard to come by down here, and if the icemaker used an electronic control board that I can't find a replacement for, I generally just rip it out and control the machine electro-mechanically. (Did I hear a scream?)
I've got an old, but mint-condition SERVEND icemaker in the shop right now, with a good control board and a defective evaporator (ice sensor) thermistor. I'd like to leave the control board in the machine, but Manitowoc/Servend in the U.S. tell me I probably won't find an appropriate replacement thermistor in Venezuela, and they've stopped making SERVEND ice machines. They commented that it's possible to solder a resistor across the control board's thermistor terminals, and then connect a regular electro-mechanical pressure switch or thermostat to the board instead of the thermistor, but my contact wasn't sure exactly how it would be done, or what value the resistor should be.
Any suggestions? Thanks in advance.
<small>[ November 10, 2005, 11:37 AM: Message edited by: Mister Frio ]</small>
Help with electronic to electro-mechanical conversion.
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Mister Frio
- Posts: 3
- Joined: Thu Nov 10, 2005 1:01 am
- Location: Venezuela
- Contact:
Re: Help with electronic to electro-mechanical conversion.
There's nothing special about a thermistor,
they are only a resistor that changes resistance
with the temperature changes.
Has your thermistor got any markings ?
They could be coded e.g 103, 104 etc or banded
like a resistor e.g. orange body, brown band,
black band, gold band = 10k.
They are rated by their resistance @ 25°C e.g. -
1k = 1000 Ohms
5K = 5000 Ohms
10K = 10,000 ohms ( will all measure close
47k = 47,000 Ohms to this @ 25 Deg °C )
100k =100,000 Ohms
500k = 1/2 1Meg Ohms
They (NTC most common types) decrease in
resistance as the Temp rises.
If you know what value the original was you could
then measure a new device (@ or just above
freezing piont) to select a resistor needed.
BUT YOU WOULD JUST USE THE REPLACEMENT THERMISTOR
ANYWAY
Any Thermistor with the same resistance rating
should work as a replacement.
Unless it was a special magical device that was
custom designed for this device(highly doubt it).
they are only a resistor that changes resistance
with the temperature changes.
Has your thermistor got any markings ?
They could be coded e.g 103, 104 etc or banded
like a resistor e.g. orange body, brown band,
black band, gold band = 10k.
They are rated by their resistance @ 25°C e.g. -
1k = 1000 Ohms
5K = 5000 Ohms
10K = 10,000 ohms ( will all measure close
47k = 47,000 Ohms to this @ 25 Deg °C )
100k =100,000 Ohms
500k = 1/2 1Meg Ohms
They (NTC most common types) decrease in
resistance as the Temp rises.
If you know what value the original was you could
then measure a new device (@ or just above
freezing piont) to select a resistor needed.
BUT YOU WOULD JUST USE THE REPLACEMENT THERMISTOR
ANYWAY
Any Thermistor with the same resistance rating
should work as a replacement.
Unless it was a special magical device that was
custom designed for this device(highly doubt it).
Re: Help with electronic to electro-mechanical conversion.
By jove, a blooming Brit, and to boot…in Venzuela….and with a name like frio….do they call you Mr “Cold”. What a coincidence for a refrigeration oriented person.
They certainly used mechanical control before electronics was introduced, so retro fit is for sure a possibility. I just went the opposite on an old Frigidaire of the ‘70s that I still have out in the garage in which I replaced my failed thermostat with a thermistor (BTW....Neg Temp Coeff) and a home designed control box electronics for it on the outside…still operating after 1984-2005 times passage.
How about the exact physical placement of the thermistor?and within or upon what sensed source?
Is connector/water/ or bare wire erosion a possibility for its failure?
Also relevant to its evaporator related function, is it a threshold type of sensing where it is responsible a for shutdown of the system ( or even its initial activation and start up ) in case off an ice up condition ?. Ergo, is its open circuit condition keeping your unit from even running at all.
The other question is if the units placement is anyway associated to monitoring the created internal ice slabs temperature.
As far as the value I would guess anywhere of 1-10 K extremes in value. The big kicker here is you not KNOWING or having data on the original unit.The factory’s suggestion was apparently the use of a mechanical sensing with its closed circuit conditon being used in series with a fixed resistor to simulate the thermistors value at the default temperature. So with the system as it is , I would expect one of two things,
system non operation is one possibility or failure to go into the default function if the thermistor is used in the other type of sensing function.
Pls come back with the further info above for further in depth analysis.
73's de Edd
[email protected] .........(Interstellar~~~~Warp~~~Speed)
[email protected].........(Firewalled*Spam*Cookies*Crumbs)
They certainly used mechanical control before electronics was introduced, so retro fit is for sure a possibility. I just went the opposite on an old Frigidaire of the ‘70s that I still have out in the garage in which I replaced my failed thermostat with a thermistor (BTW....Neg Temp Coeff) and a home designed control box electronics for it on the outside…still operating after 1984-2005 times passage.
How about the exact physical placement of the thermistor?and within or upon what sensed source?
Is connector/water/ or bare wire erosion a possibility for its failure?
Also relevant to its evaporator related function, is it a threshold type of sensing where it is responsible a for shutdown of the system ( or even its initial activation and start up ) in case off an ice up condition ?. Ergo, is its open circuit condition keeping your unit from even running at all.
The other question is if the units placement is anyway associated to monitoring the created internal ice slabs temperature.
As far as the value I would guess anywhere of 1-10 K extremes in value. The big kicker here is you not KNOWING or having data on the original unit.The factory’s suggestion was apparently the use of a mechanical sensing with its closed circuit conditon being used in series with a fixed resistor to simulate the thermistors value at the default temperature. So with the system as it is , I would expect one of two things,
system non operation is one possibility or failure to go into the default function if the thermistor is used in the other type of sensing function.
Pls come back with the further info above for further in depth analysis.
73's de Edd
[email protected] .........(Interstellar~~~~Warp~~~Speed)
[email protected].........(Firewalled*Spam*Cookies*Crumbs)
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Engineer1138
- Posts: 458
- Joined: Thu Feb 05, 2004 1:01 am
- Location: Minneapolis, MN
- Contact:
Re: Help with electronic to electro-mechanical conversion.
Venezuela? I still have relatives across the border in Guyana. Small world!
When you say you rip it out and control the machine electro mechanically, do you mean you use a relay/switch to directly control the compressor?
The reason I ask is because it should be simple to design a general purpose control board that you can use whenever you come across this.
For the immediate problem, is the function of the thermistor to detect that there is enough ice so it can stop icemaking? Is this the function you want to replace with a pressure switch?
If that's the case, you could probably connect a large value potentiometer or trimpot: 100k or so, and turn it until you get to the threshold where the icemaking function toggles on/off. That's the setpoint you want. Now you can connect your switch to the pot so that no pressure = make more ice; pressure = stop making ice.
Does this make sense? Send me a private message and I can explain in greater detail. Sometimes I forget steps when I'm posting to forums
When you say you rip it out and control the machine electro mechanically, do you mean you use a relay/switch to directly control the compressor?
The reason I ask is because it should be simple to design a general purpose control board that you can use whenever you come across this.
For the immediate problem, is the function of the thermistor to detect that there is enough ice so it can stop icemaking? Is this the function you want to replace with a pressure switch?
If that's the case, you could probably connect a large value potentiometer or trimpot: 100k or so, and turn it until you get to the threshold where the icemaking function toggles on/off. That's the setpoint you want. Now you can connect your switch to the pot so that no pressure = make more ice; pressure = stop making ice.
Does this make sense? Send me a private message and I can explain in greater detail. Sometimes I forget steps when I'm posting to forums
-
Mister Frio
- Posts: 3
- Joined: Thu Nov 10, 2005 1:01 am
- Location: Venezuela
- Contact:
Re: Help with electronic to electro-mechanical conversion.
My thanks to Michael, Edd and Engineer for their replies. I appreciate your interest.
Michael, I think I've found some information about the required resistance of the thermistor. Does 10 to 15K @ 25ºC, and 32K or greater when it's covered with ice sound about right to you? I don't know if I can actually purchase a thermistor of any kind here in Venezuela, but it's a start.
Edd, the factory rep was who suggested fitting a thermostat with a resistor in-line. Why he didn't just tell me to find a certain type of thermistor and fit it as a replacement, I'm not sure. The thermistor in this machine is responsible for deciding when the ice has formed to sufficient thickness, in order to set in motion the defrost mechanism which frees the ice from its mold and allows it to gravity-drop into the bin under the icemaker.
Engineer, when I say I "rip out" the electronics, I mean I duplicate their function with electro-mechanical bits and pieces like relays, timers, thermostat, etc. The compressor in an ice machine never stops running, so I don't need to control it other than an on/off switch. The trick is to trickle water over the cooled molds, and wait until cubes form to a correct size. At that point, (which is what the thermistor does)the machine is basically put into "reverse" (a bit like a heat pump in air conditioning)and heats the molds instead of cooling them, until the ice frees itself. This part is done by opening a valve that detours hot refrigerant across the molds. The next trick is to sense that the ice has dropped into the container, and put the machine back into cooling mode again. Depending on the model of ice machine, I generally sense the formation of ice with a thermostat, and then set a time limit on the hot gas bypass before reverting to cooling mode.
In the case of this particular machine, I could do all that, but I rather wanted to save the control board, given that the machine is in such good condition - hence the query about how to subsitite only the thermistor.
Once again, I appreciate all your inputs, and your patience with someone who understands very little about electronics.
Michael, I think I've found some information about the required resistance of the thermistor. Does 10 to 15K @ 25ºC, and 32K or greater when it's covered with ice sound about right to you? I don't know if I can actually purchase a thermistor of any kind here in Venezuela, but it's a start.
Edd, the factory rep was who suggested fitting a thermostat with a resistor in-line. Why he didn't just tell me to find a certain type of thermistor and fit it as a replacement, I'm not sure. The thermistor in this machine is responsible for deciding when the ice has formed to sufficient thickness, in order to set in motion the defrost mechanism which frees the ice from its mold and allows it to gravity-drop into the bin under the icemaker.
Engineer, when I say I "rip out" the electronics, I mean I duplicate their function with electro-mechanical bits and pieces like relays, timers, thermostat, etc. The compressor in an ice machine never stops running, so I don't need to control it other than an on/off switch. The trick is to trickle water over the cooled molds, and wait until cubes form to a correct size. At that point, (which is what the thermistor does)the machine is basically put into "reverse" (a bit like a heat pump in air conditioning)and heats the molds instead of cooling them, until the ice frees itself. This part is done by opening a valve that detours hot refrigerant across the molds. The next trick is to sense that the ice has dropped into the container, and put the machine back into cooling mode again. Depending on the model of ice machine, I generally sense the formation of ice with a thermostat, and then set a time limit on the hot gas bypass before reverting to cooling mode.
In the case of this particular machine, I could do all that, but I rather wanted to save the control board, given that the machine is in such good condition - hence the query about how to subsitite only the thermistor.
Once again, I appreciate all your inputs, and your patience with someone who understands very little about electronics.
Re: Help with electronic to electro-mechanical conversion.
Hi Mister Frio,
I've found a page with several manufacturers and distributors. Scroll to the Non linear resistor list. http://www.interfacebus.com/resistors.html#c
I think your NTC is 10k, thats a standard value. How is the NTC constructed, axial or disc style?
TOK
I've found a page with several manufacturers and distributors. Scroll to the Non linear resistor list. http://www.interfacebus.com/resistors.html#c
I think your NTC is 10k, thats a standard value. How is the NTC constructed, axial or disc style?
TOK
Gorgon the Caretaker - Character in a childrens TV-show from 1968.
Re: Help with electronic to electro-mechanical conversion.
A 10K NTC seems likely what it was,
and 32k whith ice covering sounds about right.
The values could vary from manufacturer to
manufacturer slightly though.
A 10K Disk NTC is a fairly common item, should
be able to obtain from a good electronics supplier.
If you did manage to get one that was quite a bit different e.g 26k, with ice covering it
you could cheat and ad a resistor to make up the
difference.
It all depends on what temperature and at what resistance the original triggered what.
You could probably test the unit using a potentiometer of 50k to see exactly where it activates.
Just be carefull though as the circuit may be
at full supply voltage potential unless it runs
from a stepdown transformer supply.
and 32k whith ice covering sounds about right.
The values could vary from manufacturer to
manufacturer slightly though.
A 10K Disk NTC is a fairly common item, should
be able to obtain from a good electronics supplier.
If you did manage to get one that was quite a bit different e.g 26k, with ice covering it
you could cheat and ad a resistor to make up the
difference.
It all depends on what temperature and at what resistance the original triggered what.
You could probably test the unit using a potentiometer of 50k to see exactly where it activates.
Just be carefull though as the circuit may be
at full supply voltage potential unless it runs
from a stepdown transformer supply.
-
Mister Frio
- Posts: 3
- Joined: Thu Nov 10, 2005 1:01 am
- Location: Venezuela
- Contact:
Re: Help with electronic to electro-mechanical conversion.
Hi everyone.
Solved the problem, and I thought you might like to know how is was done.
It was Michael J who got me thinking when he said "There's nothing special about a thermistor,
they are only a resistor that changes resistance
with the temperature changes." He also said "If you did manage to get one that was quite a bit different e.g 26k, with ice covering it
you could cheat and ad a resistor to make up the
difference."
I searched for a thermistor with the right characteristics without success, then it dawned on me that if I could "con" the circuit board into thinking there was a thermistor connected, it might work - and it did!
I'm not sure how to post a diagram, so I'll try to explain.
An icemaker thermostat is a simple pressure-activated switch with a Common pole, a Normally Open pole and a Normally Closed pole. The switch changes state at a set point temperature.
I soldered a 15K resistor to the NC pole (high temperature = warm water) and a 45K resistor to the NO pole (low temperature = ice), and connected the two resistors in parallel to one terminal on the circuit board.
I connected the Common pole to the other circuit board terminal.
It didn't work! But as we say in England "In for a penny, in for a pound", so I doubled the resistence to 90K on the NO pole of the thermostat - and it worked perfectly.
I'm not sure why I had to go to 90K to get it to work, when the original thermistor was supposed to release the ice when the resistence got to 32K, but so far the machine has been running perfectly for 2 days non-stop.
This is my first attempt to do anything even remotely "electronic", so now I feel like the proverbial "rocket scientist".
Hopefully no-one is going to tell me that I'm about to blow up the circuit board with my uneducated tinkering.
Anyway, thanks again to everyone concerned - your helpful hints were what solved the problem.
Best wishes from Venezuela.
Solved the problem, and I thought you might like to know how is was done.
It was Michael J who got me thinking when he said "There's nothing special about a thermistor,
they are only a resistor that changes resistance
with the temperature changes." He also said "If you did manage to get one that was quite a bit different e.g 26k, with ice covering it
you could cheat and ad a resistor to make up the
difference."
I searched for a thermistor with the right characteristics without success, then it dawned on me that if I could "con" the circuit board into thinking there was a thermistor connected, it might work - and it did!
I'm not sure how to post a diagram, so I'll try to explain.
An icemaker thermostat is a simple pressure-activated switch with a Common pole, a Normally Open pole and a Normally Closed pole. The switch changes state at a set point temperature.
I soldered a 15K resistor to the NC pole (high temperature = warm water) and a 45K resistor to the NO pole (low temperature = ice), and connected the two resistors in parallel to one terminal on the circuit board.
I connected the Common pole to the other circuit board terminal.
It didn't work! But as we say in England "In for a penny, in for a pound", so I doubled the resistence to 90K on the NO pole of the thermostat - and it worked perfectly.
I'm not sure why I had to go to 90K to get it to work, when the original thermistor was supposed to release the ice when the resistence got to 32K, but so far the machine has been running perfectly for 2 days non-stop.
This is my first attempt to do anything even remotely "electronic", so now I feel like the proverbial "rocket scientist".
Hopefully no-one is going to tell me that I'm about to blow up the circuit board with my uneducated tinkering.
Anyway, thanks again to everyone concerned - your helpful hints were what solved the problem.
Best wishes from Venezuela.
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