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Topic: Question: Driving a heating element (Read 1 time) previous topic - next topic


Jun 24, 2013, 10:39 am Last Edit: Jun 24, 2013, 10:42 am by Voidugu Reason: 1
I want to drive a 60 watt 24volt heating element of a 24volt 80 watt transformer. The trigger pin will be connected on a comparator that is capable of sinking up to 18 mA of current. I know that the ideal circuit for the job would be circuit 4 yet the MOC30xx family of triac drivers is incapable of handling 90 mA of current in order to latch the main triac. Moreover, i got 4 optocouplers and again none of them can handle 90mA of current.
> Which circuit do you suggest i use? (i was going to go with circuit 1 to be honest)
> Is there a better way of getting the job done?
> Do you know any other triac drivers capable of handling such currents?

Last question is a bit off topic:
Which would be the best way to switch ac current?
> Full wave rectifier and a MOSFET (Tried it and the power wasted in the diodes of the full wave rectifier  is huge)
> 2 MOSFETs connected source to source and the drain terminals connected to the AC(Tried it and destroyed 2 MOSFETs one after the other by unknown cause {note: the MOSFETs were at all times cool, no magic smoke was released in any case} Still haven't figured out why they got destroyed as the circuit was correct at all times. After disconnecting the MOSFETs and measuring them out, some of their pins would be shorted, in one case the gate with drain showing 0 resistance or the drain with source showing 0 resistance. Do you have any idea why this might have happened? {The circuit is shown in one of the pictures. The MOSFETs are IRF630MFP with a max gate voltage of +- 20V})

I am really sorry for the crappy pictures. I know i need to change my mobilephone.



That particular MOSFET is wrong for that circuit - you need logic-level MOSFET, you want one with appropriate on-resistance
(less than 0.1 ohm would be a good place to start, definitely not a 200V device, they have much larger on-resistances).

You also want resistors between gate and source to prevent switch on while the circuit powers up.  Try 10k.

Can't see that it would have failed without getting very hot though - I presume the 24V ac circuit is isolated other than at the MOSFETs?
If not then you blew the gate oxide.

You did isolate the drain tabs on the two MOSFETs?  they need insulating from any heatsink.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


The MOSFETs have a full plastic body so yeah they were isolated from eachother.


Then they almost certainly overheated - full plastic body doesn't conduct a lot of heat away in a hurry...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Is it okay and reliable to use a simple pnp transistor to trigger the power triac? or should i go for a triac driver?


I would not bother trying to switch between the transformer and the heating element.   I'd be switching on the primary side of the transformer,  which is presumably mains electricity ( although you didn't actually say so ).   Most straightforward method would be a regular relay.

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