Can we use the coil circuitry of a SPST relay to power a 12v fan at about 300ma.?
Craig
Mike Harding said
04:58 PM Mar 7, 2018
Your question makes no sense. Tell us what you want to do and why and we'll try to help.
Kellaig said
05:28 PM Mar 7, 2018
I want to place a spst relay into my existing 12 v fan sitting on my fridges cooling fins. I only have 0.5 amp to play with. By placing the coil pins 85 and 86 into this circuit when the fan comes on it will energise the coil and allow me to place more fans on a separate circuit to help vent out hot air from the fridge cavity. When the fridge turns off its fan it will also open the other circuit and all fans stop saving power. Does this make sense.
herks said
06:56 PM Mar 7, 2018
SPST refers to switches not relays
I used a thermal switch (75deg c, normally open N/O) attached to the condenser to achieve what you are trying to do. Available at JayCar etc.
oldtrack123 said
07:13 PM Mar 7, 2018
Kellaig wrote:
I want to place a spst relay into my existing 12 v fan sitting on my fridges cooling fins. I only have 0.5 amp to play with. By placing the coil pins 85 and 86 into this circuit when the fan comes on it will energise the coil and allow me to place more fans on a separate circuit to help vent out hot air from the fridge cavity. When the fridge turns off its fan it will also open the other circuit and all fans stop saving power. Does this make sense.
YES , you should have no problem using a SP/ST NO 12V relay in that situation. And your terminology is correct
The coil current should be well under 500mAs
Just use the smallest you can get with a DC contact current rating a bit above the current of the extra fans
Or you could go the other way get one with sufficient current rating to handle all the fans & only connect it's coil to the sensor
just make sure that the relay contacts have sufficient DC amp rating.& the coil is continuous rated
Besides Contact area, that is affected by how fast it opens & the open gap between contacts
A Jaycar SY4076 or SY 4077 should do the job
--
-- Edited by oldtrack123 on Wednesday 7th of March 2018 07:30:16 PM
Mike Harding said
05:35 PM Mar 8, 2018
OK, that's a little clearer.
My understanding is that you wish the relay to energise when the fan switches on.
Also the fan draws about 300mA and the power supply is 500mA maximum.
Is that correct?
If so: Wire the relay in parallel with the fan ie. the positive connection of the fan is connected directly to one side of the relay coil and the negative of the fan to the other side of the relay coil.
This relay will do the job: https://www.jaycar.com.au/12v-dpdt-power-relay-10a-240vac-24vdc-contacts/p/SY4065
And you can buy a base in which to mount it and wire to - easy :)
Although this is a DPDT relay you simply need to use just one pole and wire between the normally open and the common contacts.
T1 Terry said
04:00 PM Mar 9, 2018
No one has mentioned the need for a diode across the relay coil to protect the electronics that power the existing fan. Does that mean they don't see a problem or that the need for the diode has missed? I only use solid state relays where electronic low current circuits are involved, had a few small transistors fail trying to power up mechanical relay coils. Something like this www.ebay.com.au/itm/Heat-S-Input-3-32V-DC-Output-5A-200V-DC-SSR-Solid-State-Relay-N1N5-F3E5/282694982241~P-
there are other even smaller is size relays they we use quite bit but I can't find a listing any more, we buy them in bulk direct.
Please understand the way solid state relays mark their connections, the control side is + and - as marked, but the load side refers to the more positive and more negative in regards to current flow. For example, to power the fan the battery positive would go to the load "+ " and the fan positive would go to the load "- " because the current is flowing from the battery to the fan. the fan negative would go to the battery via the shunt if one is used in your system.
T1 Terry
oldtrack123 said
04:52 PM Mar 9, 2018
T1 Terry wrote:
No one has mentioned the need for a diode across the relay coil to protect the electronics that power the existing fan. Does that mean they don't see a problem or that the need for the diode has missed? I only use solid state relays where electronic low current circuits are involved, had a few small transistors fail trying to power up mechanical relay coils. Something like this www.ebay.com.au/itm/Heat-S-Input-3-32V-DC-Output-5A-200V-DC-SSR-Solid-State-Relay-N1N5-F3E5/282694982241~P- there are other even smaller is size relays they we use quite bit but I can't find a listing any more, we buy them in bulk direct.
Please understand the way solid state relays mark their connections, the control side is + and - as marked, but the load side refers to the more positive and more negative in regards to current flow. For example, to power the fan the battery positive would go to the load "+ " and the fan positive would go to the load "- " because the current is flowing from the battery to the fan. the fan negative would go to the battery via the shunt if one is used in your system.
T1 Terry
The 12V power source is already feeding the existing fan!
The back kick from the coil would be no different to that of the fan ,that the relay coil is connected in parallel with
. That fan itself would absorb the back EMF
But if the OP is concerned ,a diode, reverse connected across the relay coil is a very small additional expense, but it needs to be selected to handle the max back emf
A diode & series resister takes some of the load off the diode as the energy is dissipated in the resistor
Yes ,lots of experience with BIG DC solenoids & contactor coils , they can generate a very nasty back EMF & give the actual opening contacts hell., unless arc suppression is used
Before the days of diodes ,capacitors were used for that purpose.
-- Edited by oldtrack123 on Friday 9th of March 2018 04:55:10 PM
Mike Harding said
05:18 PM Mar 9, 2018
It is unlikely a back EMF diode is required in this case.
Terry: you are correct in your assertion the relay coil will produce a high voltage as its magnetic field abruptly collapses at switch off but it will be of low energy and will easily be absorbed by the fan motor or, with a 0W9 relay coil, by the PSU I am sure.
Peter: the fan motor will not behave in the same manner as the relay because once the motive force is removed from the motor coils it will being to act as a generator as it slows to zero, there will be no sudden change to the current flow through the fan motor it will be a gradual one and the di/dt rate will be slow so no spike of significance will occur.
oldtrack123 said
05:45 PM Mar 9, 2018
Mike Harding wrote:
It is unlikely a back EMF diode is required in this case.
[a[Terry: you are correct in your assertion the relay coil will produce a high voltage as its magnetic field abruptly collapses at switch off but it will be of low energy and will easily be absorbed by the fan motor or, with a 0W9 relay coil, by the PSU I am sure.
Peter: the fan motor will not behave in the same manner as the relay because once the motive force is removed from the motor coils it will being to act as a generator as it slows to zero, there will be no sudden change to the current flow through the fan motor it will be a gradual one and the di/dt rate will be slow so no spike of significance will occur.
Hi Mike
[a]Totally agree
Yes, I stand corrected on that part ,on it's significance , I was referring to that generated voltage
-- Edited by oldtrack123 on Friday 9th of March 2018 05:48:14 PM
T1 Terry said
06:36 PM Mar 9, 2018
Ah, you plan to use the fan motor as the drain, do you think the in rush current for the fan motor plus the current to pull the relay coil will total less than 0.5 amp instantaneous load? The cell monitors we use have a relay contact supposedly rated at 500mA and they have been reported to have a nasty habit of failing instantly when attempting drive a relay, but there could have been other forces involved such as rapid switching etc. I have always used the mini SSR to switch the loads to avoid destroying a rather expensive circuit board and I would wire all the fans to the relay output if I was doing the job, no point in some being relay powered and others being circuit board powered and makes fault finding just that bit harder.
T1 Terry
oldtrack123 said
12:02 AM Mar 10, 2018
T1 Terry wrote:
Ah, you plan to use the fan motor as the drain, do you think the in rush current for the fan motor plus the current to pull the relay coil will total less than 0.5 amp instantaneous load? The cell monitors we use have a relay contact supposedly rated at 500mA and they have been reported to have a nasty habit of failing instantly when attempting drive a relay, but there could have been other forces involved such as rapid switching etc. I have always used the mini SSR to switch the loads to avoid destroying a rather expensive circuit board and I would wire all the fans to the relay output if I was doing the job, no point in some being relay powered and others being circuit board powered and makes fault finding just that bit harder.
T1 Terry
Perhaps I have misread the op, but it seems to me that he is saying he will run the additional fans of 12V , so we are only talking about the relay coil current from the existing fan source
I also suggested he could run all the fans by the relay leaving the existing current limited source to just supply the relay coil
Perhaps you were so eager to have another go at me ,that you missed that
That should make fault ?? finding even easier'.
-- Edited by oldtrack123 on Saturday 10th of March 2018 12:05:35 AM
T1 Terry said
11:22 AM Mar 10, 2018
Sorry if you fell I'm having a shot at you Peter, that was not the intent and I did not follow that you were suggesting to remove the fan from the electronics supply and only use that for switching the relay. I was only putting forward an alternative method that would reduce any risk of damage to the fridge electronics because I'd imagine they would not be cheap to replace.
T1 Terry
oldtrack123 said
12:10 PM Mar 10, 2018
T1 Terry wrote:
Sorry if you fell I'm having a shot at you Peter, that was not the intent and I did not follow that you were suggesting to remove the fan from the electronics supply and only use that for switching the relay. I was only putting forward an alternative method that would reduce any risk of damage to the fridge electronics because I'd imagine they would not be cheap to replace.
T1 Terry
Accepted
Kellaig said
07:18 PM Mar 12, 2018
I am the op. I connected a Relay( with a resistor) coil circuit to the original fridge fan circuitry. It draws 0.15 amps. both the original fan and new fan get their power directly from the battery when the fridge powers the coil. Is this safe or do I need a diode or something similar to protect fridge electronic control unit
Hi all,
Can we use the coil circuitry of a SPST relay to power a 12v fan at about 300ma.?
Craig
Your question makes no sense. Tell us what you want to do and why and we'll try to help.
I want to place a spst relay into my existing 12 v fan sitting on my fridges cooling fins. I only have 0.5 amp to play with. By placing the coil pins 85 and 86 into this circuit when the fan comes on it will energise the coil and allow me to place more fans on a separate circuit to help vent out hot air from the fridge cavity. When the fridge turns off its fan it will also open the other circuit and all fans stop saving power. Does this make sense.
SPST refers to switches not relays
I used a thermal switch (75deg c, normally open N/O) attached to the condenser to achieve what you are trying to do. Available at JayCar etc.
YES , you should have no problem using a SP/ST NO 12V relay in that situation. And your terminology
is correct
The coil current should be well under 500mAs
Just use the smallest you can get with a DC contact current rating a bit above the current of the extra fans
Or you could go the other way get one with sufficient current rating to handle all the fans & only connect it's coil to the sensor
just make sure that the relay contacts have sufficient DC amp rating.& the coil is continuous rated
Besides Contact area, that is affected by how fast it opens & the open gap between contacts
A Jaycar SY4076 or SY 4077 should do the job
--
-- Edited by oldtrack123 on Wednesday 7th of March 2018 07:30:16 PM
OK, that's a little clearer.
My understanding is that you wish the relay to energise when the fan
switches on.
Also the fan draws about 300mA and the power supply is 500mA maximum.
Is that correct?
If so:
Wire the relay in parallel with the fan ie. the positive connection
of the fan is connected directly to one side of the relay coil and
the negative of the fan to the other side of the relay coil.
This relay will do the job:
https://www.jaycar.com.au/12v-dpdt-power-relay-10a-240vac-24vdc-contacts/p/SY4065
And you can buy a base in which to mount it and wire to - easy :)
Although this is a DPDT relay you simply need to use just one pole
and wire between the normally open and the common contacts.
there are other even smaller is size relays they we use quite bit but I can't find a listing any more, we buy them in bulk direct.
Please understand the way solid state relays mark their connections, the control side is + and - as marked, but the load side refers to the more positive and more negative in regards to current flow. For example, to power the fan the battery positive would go to the load "+ " and the fan positive would go to the load "- " because the current is flowing from the battery to the fan. the fan negative would go to the battery via the shunt if one is used in your system.
T1 Terry
The 12V power source is already feeding the existing fan!
The back kick from the coil would be no different to that of the fan ,that the relay coil is connected in parallel with
But if the OP is concerned ,a diode, reverse connected across the relay coil is a very small additional expense, but it needs to be selected to handle the max back emf
A diode & series resister takes some of the load off the diode as the energy is dissipated in the resistor
Yes ,lots of experience with BIG DC solenoids & contactor coils , they can generate a very nasty back EMF & give the actual opening contacts hell., unless arc suppression is used
Before the days of diodes ,capacitors were used for that purpose.
-- Edited by oldtrack123 on Friday 9th of March 2018 04:55:10 PM
It is unlikely a back EMF diode is required in this case.
Terry: you are correct in your assertion the relay coil will produce a high voltage as its magnetic field abruptly collapses at switch off but it will be of low energy and will easily be absorbed by the fan motor or, with a 0W9 relay coil, by the PSU I am sure.
Peter: the fan motor will not behave in the same manner as the relay because once the motive force is removed from the motor coils it will being to act as a generator as it slows to zero, there will be no sudden change to the current flow through the fan motor it will be a gradual one and the di/dt rate will be slow so no spike of significance will occur.
Hi Mike
[a]Totally agree
Yes, I stand corrected on that part ,on it's significance , I was referring to that generated voltage
-- Edited by oldtrack123 on Friday 9th of March 2018 05:48:14 PM
T1 Terry
Perhaps I have misread the op, but it seems to me that he is saying he will run the additional fans of 12V , so we are only talking about the relay coil current from the existing fan source
I also suggested he could run all the fans by the relay leaving the existing current limited source to just supply the relay coil
Perhaps you were so eager to have another go at me ,that you missed that
That should make fault ?? finding even easier'.
-- Edited by oldtrack123 on Saturday 10th of March 2018 12:05:35 AM
T1 Terry
Accepted
I am the op. I connected a Relay( with a resistor) coil circuit to the original fridge fan circuitry. It draws 0.15 amps. both the original fan and new fan get their power directly from the battery when the fridge powers the coil. Is this safe or do I need a diode or something similar to protect fridge electronic control unit
No, you don't.