Transformer Powered Heated Bed

Happy New Year! I've had two silicone heated beds fail so I'm switching over to a 24 volt Kapton heating element I have laying around. I don't have a 24v PSU but I do have a 24v step down transformer (110 vac to 24 vac) from a foam cutter project. The primary and secondary are isolated and it currently has a full wave rectifier on the 24 volt side to convert to DC. I use a SSR and plan to use it with the transformer. The relay is controlled by the RAMPS. My questions are, since I am plugging in to the mains, are there any safety concerns I can address or prepare for? Will the constant switching on and off from the SSR be hard on the transformer? Thanks for your help!

Beware if you are going to be switching a transformer primary with a SSR.

Depending on the phase at the moment it's switched there can be a huge inrush current (hundreds of amps). What you are planning would do that randomly.

See this

Regards

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Thanks for the insight! Can you suggest any options to an SSR? Does it make a difference if I switch the secondary, not the primary?

Edited 1 time(s). Last edit at 01/04/2015 01:29AM by airkuld.

Quote
airkuld
Thanks for the insight! Can you suggest any options to an SSR? Does it make a difference if I switch the secondary, not the primary?

Firstly will your Transformer supply enough current ? will need to be around 10 amps I would guess but not knowing the kapton heater you are using it is hard to tell.

Also if you measure the output from your fullwave rectifier I would expect to see around 34V not 24V so you may have to regulate it in some way.

HTH

Doug

I agree with Doug, the rectified voltage will be higher than the secondary voltage.

If you have measured the output voltage, and the heater's resistance then you can use Ohms Law to determine what the current will be. The voltage will drop a bit under load, but you need safety margins anyway.

Make sure the transformer's secondary and the rectifier are rated for more than that calculated current.

Make sure the heater is rated for the wattage.

Switching the secondary does not have the same problems as switching the primary would.

You could use an n-channel mosfet (or more than one) to switch the DC. I just built a power supply that does exactly that.

There are further complications - you need to be able to turn on the mosfet with the proper voltage and current, or it will only turn partly on and overheat, or too much voltage and it will be damaged.

Also you must determine what signal you are going to use to turn the mosfet on. Most boards use N-channel mosfets to switch the DC to the heated bed and hot end. That means the positive pin is always active so you can't use that. You could use a digital pin and a transistor to do it, or make a driver using a PNP transistor using the heated bed output (which I did)...

You can also get a DC SSR and switch the secondary, using a digital pin. I suppose you could also switch the secondary (before the rectifier) with an AC SSR. Someone tell me if I'm wrong.

So now I caution you to make sure you know exactly what you are doing before attempting this. I'm not an electrical engineer so I am not comfortable sharing actual circuits. Be safe.

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I was attempting to make do with what I have on hand. After the comments from you both, I looked more closely at my components and noted both transformers I have are only 5 amps at 24 volts. That sort of kicks the whole project into the water as that will not be enough to drive a 300 x 300 bed. Looks like I'm buying a PSU. Thanks very much for your advice and your consideration for my safety!