Using a solid state relay to switch a heating element

[iep]

Hi,
I have a heating element (750w) that forms part of the pre-heating stage of a house ventilation system (takes the edge off incoming cold air).

The problem is that the switched supply (S1) that enables it (and the fan that forces the air) cannot handle the ~3 amps required to run the heater.

So, first thought was to use a relay or contactor to switch a fused 'live' to the heater when S1 is on. Problem is that S1 drives the fan motor directly and also controls fan speed by varying its voltage from 100V to 230V. This varied voltage would also be driving the input coil of the relay and I have not been able to find a standard relay or contactor that will switch reliably over such a wide voltage range.

This leads me to title of the post. Can I use a solid state relay for this application? There are some fairly beefy 25A ones available with input signal range from 90V to 280V AC. Seems like an ideal solution but I've never used one before so I'm not sure.

I gather they can get a bit warm but there is loads of space in the housing to add a heatsink so no problem there. Plus the required heater current is only 3A vs the 25A rating so may not even be needed.

Any pitfalls anyone can think of to this approach?

Cheers,

iep

 

[bernardgreen]

How does S1 control the voltage to the fan ? Does it have a transformer with 230 and 110 volt tappings. Or does it insert a resistor as a voltage dropper, or a capacitor as a current limiter ?

 

[iep]

It's a transformer with multiple taps and relays to switch them to S1.

Cheers,

iep

 

[bernardgreen]

Feeding the "coil" of a solid state relay with an AC voltage will cause the relay to switch the output ON and OFF at 50 or 100Hz You would need to rectify the AC and smooth the DC into the "coil". There are solid state relays that have this rectification built in but they may require and externally fitted smoothing capacitor

If S1 always supplies 230 when turned from OFF to ON (fast) and only give the lower voltage when turned from ON (fast) to ON (slow) (i.e fan always starts on fast) then it may be possible to use an AC relay that pulls IN at 230 volts and and remains held IN on a lower voltage. It would not pull IN at the lower voltage so S1 would have to start the fan at full voltage before changing to SLOW.

Problem is data sheets do not always provide the hold IN voltage for AC relays as they do for DC relays

 

[iep]

Hi, there are AC input solid state relays with built in rectification. Things like this:

https://www.ebay.co.uk/p/577043315?iid=253061249422&trdt=0&rdtsrc=vi

Obviously I'd be buying a new one. Just found this as a handy link.

Fan does not start 230 and drop back. Comes up in 80v and stays there most of the time. That's why the SSR appeals. Nice wide input range.

Cheers,

Iep

 

[Echo the husky]

What ventilation system is it?

 

[iep]

It's an old husquvarna system in Sweden that I'm fixing up for a friend.

Cheers,

Iep

 

[SUNRAY]

Hi,
I have a heating element (750w) that forms part of the pre-heating stage of a house ventilation system (takes the edge off incoming cold air).

The problem is that the switched supply (S1) that enables it (and the fan that forces the air) cannot handle the ~3 amps required to run the heater.

So, first thought was to use a relay or contactor to switch a fused 'live' to the heater when S1 is on. Problem is that S1 drives the fan motor directly and also controls fan speed by varying its voltage from 100V to 230V. This varied voltage would also be driving the input coil of the relay and I have not been able to find a standard relay or contactor that will switch reliably over such a wide voltage range.

This leads me to title of the post. Can I use a solid state relay for this application? There are some fairly beefy 25A ones available with input signal range from 90V to 280V AC. Seems like an ideal solution but I've never used one before so I'm not sure.

I gather they can get a bit warm but there is loads of space in the housing to add a heatsink so no problem there. Plus the required heater current is only 3A vs the 25A rating so may not even be needed.

Any pitfalls anyone can think of to this approach?

Cheers,

iep

Are you saying the heater has been running on this varying voltage? and is the transformer always in circuit? what controls the relays which vary the voltage and do they have spare contacts or can they be changed?
We need more info, I suspect you don't need to add the complication of SSR's

 

[iep]

Hi,
I'll try to add a bit more info on the overall problem.

1. The ventilation system controller is built into the kitchen cooker hood extractor fan. This might seem a bit odd but is standard practice for this type of system (the cooker hood connects to the ventilation system outlet so needs to increase the central ventilation fan speed to maintain negative system pressure when the hood is running). The issue here is that the original 1970s cooker hood has recently been replaced with a modern one.
2. The original controller (in the 1970s hood) had two features no longer supported by modern hoods:
   • It could source large currents on it's S1 connection (so it could drive the heater directly)
   • S1 was either 0V or 230V (no 100V option). Instead, fan speed control was achieved by having a dedicated 'Neutral' for the fan motor which included the speed controller circuitry. So, for low speed fan operation the motor would be driven between 230V from S1 and 130V on it's 'Neutral' line.
3. As I say, these old features are not supported on new cooker hoods. Instead they use the new standard which is a single 'S1' with 4 output voltages (0, 100, 130 and 230V) which can drive ~250W (enough to support fan operation only). Not all systems include the additional electric preheater element and those that do have additional circuitry within the heater control box to translate the incoming variable 'S1' voltage to a fixed, high current, 230V signal. It is this last function I am trying to emulate by adding the SSR.

Sunray, in direct answer to your questions, the heater has never been run on the varying 'S1' voltage at the output of the new cooker odd. The heater connection was disconnected when the new cooker hood was fitted as the installer recognised it would draw more current than the new hood could provide. This is okay for now but, come winter, will lead to a build up of condensation in the passive heat exchanger. All relays, transformer that control the system are mounted inside the new cooker hood and cannot be modified. Basically, I have to work with the 'S1' output as it is provided from the new hood.

Hopefully this explains why I am looking at the SSR solution. Basically I need to translate a 100-230V, low current signal into a 230V 3A supply for a heater element. I'm happy to look at other solutions if anyone can suggest one. If not though then I guess I just need to know if an SSR can be used to drive a 750W heater element and if any additional components would be required.

Thanks,

iep

 

[ericmark]

I seem to remember when living in a house with hot air heating, there was a little flap and micro switch, so heating could not switch on without air flow, it seems there was a worry about heater switching on with no air flow and so over heating, may be this is the way forward?

 

[SUNRAY]

Thanks for the fantastic reply, as far as I'm concerned you have covered everything.
I think you probably have the solution as long as you can find a SSR which will latch correctly at all voltages.

Hi,
I'll try to add a bit more info on the overall problem.

1. The ventilation system controller is built into the kitchen cooker hood extractor fan. This might seem a bit odd but is standard practice for this type of system (the cooker hood connects to the ventilation system outlet so needs to increase the central ventilation fan speed to maintain negative system pressure when the hood is running). The issue here is that the original 1970s cooker hood has recently been replaced with a modern one.
2. The original controller (in the 1970s hood) had two features no longer supported by modern hoods:
   • It could source large currents on it's S1 connection (so it could drive the heater directly)
   • S1 was either 0V or 230V (no 100V option). Instead, fan speed control was achieved by having a dedicated 'Neutral' for the fan motor which included the speed controller circuitry. So, for low speed fan operation the motor would be driven between 230V from S1 and 130V on it's 'Neutral' line.
3. As I say, these old features are not supported on new cooker hoods. Instead they use the new standard which is a single 'S1' with 4 output voltages (0, 100, 130 and 230V) which can drive ~250W (enough to support fan operation only). Not all systems include the additional electric preheater element and those that do have additional circuitry within the heater control box to translate the incoming variable 'S1' voltage to a fixed, high current, 230V signal. It is this last function I am trying to emulate by adding the SSR.

Sunray, in direct answer to your questions, the heater has never been run on the varying 'S1' voltage at the output of the new cooker odd. The heater connection was disconnected when the new cooker hood was fitted as the installer recognised it would draw more current than the new hood could provide. This is okay for now but, come winter, will lead to a build up of condensation in the passive heat exchanger. All relays, transformer that control the system are mounted inside the new cooker hood and cannot be modified. Basically, I have to work with the 'S1' output as it is provided from the new hood.

Hopefully this explains why I am looking at the SSR solution. Basically I need to translate a 100-230V, low current signal into a 230V 3A supply for a heater element. I'm happy to look at other solutions if anyone can suggest one. If not though then I guess I just need to know if an SSR can be used to drive a 750W heater element and if any additional components would be required.

Thanks,

iep

 

[iep]

Hi Erikmark. That's a nice idea and there is a similar control flap on this system. Unfortunately it is spring return and so heavily 'damped' that it takes about 5 mins to close after the fan motor is deactivated. This would mean the heater running with no airflow for several minutes.

Hi Sunray,
Glad my post made sense. It's quite a complex system and difficult to describe. I'll be back in Sweden this Autumn so will try to source an appropriate SSR in the meantime.

Cheers,

Iep

 

[Lectrician]

Fit a pressure switch in the duct.

 

[Adam_151]

I'd go with the pressure switch idea, unless there is a reason not to do this.

If however you were looking for an answer on your original lines, you can get switch mode 24v dc power supplies with a wide input voltage range, the one I am looking at on RS having quickly googled it, is good for 85v to 264 v.ac . You could then use a 24v dc contactor to switch the heater. Its a faff, but uses off the shelf parts.

 

[iep]

Hi, ta for responses. Pressure switch sounds interesting. Can you provide a link to an example of the type of switch you mean? I've not been able to find anything that seems appropriate.

Thanks,

iep