Latching relay vs. Solid state relay

Hi all,

I am doing a project where one of its functions is to switch off the water heater when a pre-set temperature is exceeded. One of the targets for this project is to keep it low in power consumption as possible.
For this reason, i was considering using a latching relay so it wont be energized all the time, therefore consuming less power. Another option is to use a solid state relay (which as far as i know does not consume high power although it need continuous supply when contacts are closed).

Which type of relay do you suggest? a latching relay or a solid state relay?

Thanks in advance.

You do not mention the load current or voltage? However, using for example a SSR like this one the on current with 5 VDC applied for the On state the current draw is about 15 mA. Really depends on what is classified as a low on current? Then there is consideration for the other circuit components.

Ron

Hi Ron,

You do not mention the load current or voltage?

Click to expand...

The load voltage is 230VAC and the current is not known since it will be used on any water heater, but a 40A to 50A relay will be safe.

The relay will be controlled from a PIC micro controller.

Really depends on what is classified as a low on current?

Click to expand...

The less current consumed the better. What is the typical current consumption of a normal relay which can handle 230VAC 40-50A ? and will a lurching relay consume less then the SSR?

Thanks.

EDIT: will i need to use a heat sink if i used an SSR?

Hi adrianvon,

please let's be realistic.

Water heaters are normally designed for 1KW, providing hot water for a single shower.

There are 10KW water heaters on the market, but they are all three phase devices.

So a 1KW heater would require ~4.5A at 230VAC.

Solid state relays are designed for different AC voltages with and without zero crossing unit built in.

Here is an SSR I suggest for your application: SHARP S202S02, rated 230V/8A with zero crossing unit.

The LED forward voltage is 1.2V and the forward current is 8mA - no reason to use a mechanically latching relay since the heater will be switched on and off within short time intervals.

Using an SSR it's advisable to use a 0.6W VDR across the AC terminals to avoid back firing into the built in triac.

Have a look into the attached datasheet.

For higher currents look into the second datasheet of Carlo Gavazzi SSRs. They should do for three phase high current heaters. Carlo Gavazzi SSRs are internally fitted with a snubber network.

Boncuk

Hi,

Thanks for your reply. Can you please explain to me what are 'zero crossing unit' and 'VDR ' ?

I was considering the SSR in the link below. Is it good for my application?

**broken link removed**

and will i need a heat sink for it ?

Thanks.

adrianvon said:

Hi,

Thanks for your reply. Can you please explain to me what are 'zero crossing unit' and 'VDR ' ?

Click to expand...

A zero crossing unit fires the triac at zero AC voltage passing. A VDR is a "Voltage Dependent Resistor", taking care of eliminating sparks which "fly" between relay contacts when power is disengaged. There won't be sparks within a solid state relay, but using a properly rated VDR (e.g. 275V) it takes care of elimination of possible sparking.

I was considering the SSR in the link below. Is it good for my application?

Click to expand...

Nobody knows the power rating of your heater yet!

and will i need a heat sink for it ?

Click to expand...

That depends on the rated power of the heater and the SSR. The data sheet should have information about possible heatsinks for that.

Thanks.

Click to expand...

adrianvon said:

Hi,

Thanks for your reply. Can you please explain to me what are 'zero crossing unit' and 'VDR ' ?

Click to expand...

A zero crossing unit fires the triac at zero AC voltage passing. A VDR is a "Voltage Dependent Resistor", taking care of eliminating sparks which "fly" between relay contacts when power is disengaged. There won't be sparks within a solid state relay, but using a properly rated VDR (e.g. 275V) it takes care of elimination of possible sparking.

I was considering the SSR in the link below. Is it good for my application?

Click to expand...

Nobody knows the power rating of your heater yet!

and will i need a heat sink for it ?

Click to expand...

That depends on the rated power of the heater and the SSR. The data sheet should have information about possible heatsinks for that.

Thanks.

Click to expand...

Hi,

I was considering the SSR in the link below. Is it good for my application?
Nobody knows the power rating of your heater yet!

Click to expand...

It is a 1500 Watt element water heater.

Hi,

I was considering the SSR in the link below. Is it good for my application?

**broken link removed**

Nobody knows the power rating of your heater yet!

Click to expand...

It is a 1500 Watt element water heater.

Sorry,

the item has either been removed or is no longer available.

I suggest to use the SHARP S202S02 SSR. It can switch up to 8A loads.

At 6.525A (your heater) the power dissipation will be 8W. No info given about a heatsink in the datasheet. For safety reasons you might use a clip on heatsink.

Boncuk

The 10 amp unit you linked to should work just fine. I would mount it on a small piece of aluminum for a heat sink using some thermal compound and it should do just fine.

Ron

OK, thanks for your replies.