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SCR vs Triac Heat dissaption?

A

Adam Aglionby

Jan 1, 1970
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In a 10A lighting dimmer, will a triac generate less heat than an inverse
parallel pair of SCRs if all other things are equal?

Have in my mind that SCRs run hotter than triacs but this may be just be
nonsense, would appreciate clarification.

Thanks

Adam
 
S

Spehro Pefhany

Jan 1, 1970
0
In a 10A lighting dimmer, will a triac generate less heat than an inverse
parallel pair of SCRs if all other things are equal?

Have in my mind that SCRs run hotter than triacs but this may be just be
nonsense, would appreciate clarification.

Thanks
Adam

Similarly rated (eg. triacs rated at double the SCR rating) will have
fairly similar power dissipations (around 1W/A), maybe a bit higher
for the triacs than for the SCRs, but the SCR dissipation is split
between two devices so they may be easier to cool.

This info is in the data sheets, BTW.

Best regards,
Spehro Pefhany
 
T

Terry Given

Jan 1, 1970
0
Spehro Pefhany said:
Similarly rated (eg. triacs rated at double the SCR rating) will have
fairly similar power dissipations (around 1W/A), maybe a bit higher
for the triacs than for the SCRs, but the SCR dissipation is split
between two devices so they may be easier to cool.

This info is in the data sheets, BTW.

Best regards,
Spehro Pefhany

also it depends on how you drive the SCRs - applying gate pulses to a
reverse-biased SCR greatly increases the leakage current, and as the SCR is
reverse-biased this can be significant. I watched a guy do this with a
3-phase half-controlled bridge rectifier for a 600kW drive - the SCRs got
HOT, so much so that our first thermal test showed the problem quite
clearly. The fix is simple - if its reverse-biased by more than a few volts,
block the gate pulses.

Cheers
Terry
 
S

Spehro Pefhany

Jan 1, 1970
0
also it depends on how you drive the SCRs - applying gate pulses to a
reverse-biased SCR greatly increases the leakage current, and as the SCR is
reverse-biased this can be significant. I watched a guy do this with a
3-phase half-controlled bridge rectifier for a 600kW drive - the SCRs got
HOT, so much so that our first thermal test showed the problem quite
clearly. The fix is simple - if its reverse-biased by more than a few volts,
block the gate pulses.

Cheers
Terry

I didn't know that. I suppose it's only a problem with 3-phase AC
circuits because in single phase designs the other SCR will turn on
and remove the reverse bias. Thanks.

Best regards,
Spehro Pefhany
 
T

Terry Given

Jan 1, 1970
0
Hi Spehro,

Spehro Pefhany said:
I didn't know that. I suppose it's only a problem with 3-phase AC
circuits because in single phase designs the other SCR will turn on
and remove the reverse bias. Thanks.

Best regards,
Spehro Pefhany

neither did I until I saw them break. IIRC the reverse leakage increased
about 1000x, but I imagine that is a strong function of the SCR. Mistakes
are a great way of learning things, especially if you get to observe others
making the mistakes :)

I hadnt thought about the single-phase arrangement but yeah, you're right.

cheers
Terry
 
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