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Topic: Minimum load current for SSR (Read 4338 times) previous topic - next topic

broadband155

I am using a number of SSR-25da's to switch various lights from a Duemilanove and and Uno. All was working very well with the 50W led floodlights I was using but then I tried a 4W led 'bulb' that goes into the GU10 holder. I switches on ok when the Arduino triggers the SSR. When the Arduino output goes low, the bulb goes out but it then starts to pulse on and off, even though the indicator light on the SSR stays off. In other words, it isn't getting this pulse from the Arduino.

I assume it is a problem with a minimum load current for the SSR but I can't find that figure anywhere. I have swapped it for a 20W halogen GU10 and that solves the problem but i don't want a halogen as the final solution. It won't last given that I am switching the light on and off very often.

If I knew there was a minimum load current and that is the issue, I could perhaps put in a dummy load of some sort. All suggestions appreciated.

Thanks, Neil.

retrolefty

Perhaps a simple resistor properly sized (ohms and wattage rating) could provide the minimum load current you seem to require. Also the problem might be caused by inductive load that the led load represents (vs what the halgen lamps represents) and a 'snubber circuit' circuit is needed to insure proper turn off control?

Lefty

jackrae

Rather than a resistor, which creates heat, you could try a mains rated capacitor.  This will draw current but, because it is out of phase with the voltage, produces "no" heat.  Old style fluorescent lights used capacitors to control power factor and that might be a suitable source.

retrolefty


Rather than a resistor, which creates heat, you could try a mains rated capacitor.  This will draw current but, because it is out of phase with the voltage, produces "no" heat.  Old style fluorescent lights used capacitors to control power factor and that might be a suitable source.


The problem with capacitance and inductive loads used with SSR (thyristor based switching) is that if forces the current and voltage to go out of phase with each other and can upset the zero crossing turn off and turn on properties. That is one function of a 'snubber circuit' to add capacitance to offset existing inductive values to keep I and E closer in phase to allow for auto zero crossing turn-off characteristics.

Lefty

dc42

Yes, the problem is that SSRs pass a small amount of current when they are turned off (5mA for the SSR25DA according to http://www.fotek.com.hk/solid/SSR-1.htm). Mains LEDs have a mains rectifier, capacitor and inverter to feed the LED. The SSR leakage current will charge the capacitor until it reaches a point where the inverter starts, then the inverter will run until it discharges the capacitor.

The load needs to be sufficient to prevent the voltage reaching the point where the inverter can power the LED. If the mains supply is 240V and you aim for a load that passes 5mA at one third of mains voltage, then that load will consume 15mA at 240V, which is 3.6W if it is resistive - hardly ideal.

If your final solution will use 4W LED bulbs then I suggest you use a device designed for much lower currents that the SSR25DA, for example http://uk.farnell.com/vishay/vo2223-x001/phototriac-power-dip-8-0-9a/dp/1870800.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

broadband155

Hi All,

Thanks very much for the replies. I think that I will go with a resistive load. I have found some 25W resistors on Farnell which are next day delivery for me and they will draw about 10W each. I know it isn't the absolutely ideal scenario but the lights aren't on for that long each time, perhaps a few minutes out of each hour in total and for ten hours a day.


broadband155

Just a quick update to say that I tried the resistive load and it was terrible. I realised how it was a lazy approach and very inelegant. I bought some AO2223's and they worked very well.

Thanks for the advice.

dc42

I'm glad you got it working, You did use a resistor between the Arduino and the VO2223 input, didn't you? Your SSR will have had one built in, but the VO2223 doesn't.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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