Triac questions

Thread Starter

l0vot

Joined Apr 29, 2013
104
I'm trying to switch a low KHz square wave (4-6KHz 70-90V nominal) using a DC trigger signal, 12V would be convenient, but I can step up the voltage if necessary, a triac seemed like the correct device to accomplish this, however I seem to have a knack for using transistors incorrectly, and destroying them in the process, so I figured I would ask how I need to wire up this triac in order for it to function like a relay.

https://www.digikey.com/en/products/detail/stmicroelectronics/T2535T-8T/13559154

I don't know if it can control high frequency AC or not, the datasheet doesn't really say, if it can, it's massive overkill because it should only be switching a couple amps, and if it has a heatsink it's max voltage is around 10X what it should be switching.
 

crutschow

Joined Mar 14, 2008
27,184
A TRIAC also has a significant voltage drop which a MOSFET does not.

Does the AC voltage have only positive polarity?
 

Thread Starter

l0vot

Joined Apr 29, 2013
104
It's proper AC, not pulsed DC, a FET can only switch DC because body diode. The triac isn't generating the AC, it's blocking the AC from getting to the load until a signal is provided, it needs to close the circuit and allow AC to pass for the duration of the signal, then turn off when the signal shuts off, the signal is DC. I am pretty sure a triac is the only transistor that can switch AC.
 

BobaMosfet

Joined Jul 1, 2009
1,768
I'm trying to switch a low KHz square wave (4-6KHz 70-90V nominal) using a DC trigger signal, 12V would be convenient, but I can step up the voltage if necessary, a triac seemed like the correct device to accomplish this, however I seem to have a knack for using transistors incorrectly, and destroying them in the process, so I figured I would ask how I need to wire up this triac in order for it to function like a relay.

https://www.digikey.com/en/products/detail/stmicroelectronics/T2535T-8T/13559154

I don't know if it can control high frequency AC or not, the datasheet doesn't really say, if it can, it's massive overkill because it should only be switching a couple amps, and if it has a heatsink it's max voltage is around 10X what it should be switching.
@IOvot Just throwing this out there, just in case. Do you understand what a triac does? A triac, or SCR passes signal when the gate voltage is positive and when that occurs, it is locked in the state until all 3 terminals are grounded, irregardless of what the gate does by itself. SCRs are frequently used in AC voltage (dimming circuits) because all 3 terminals are disconnected every time 0V is crossed. Normally you can use it in conjunction with a ZCD circuit.

You need to describe better what exactly it is you are trying to accomplish, please.
 

MisterBill2

Joined Jan 23, 2018
8,736
I'm trying to switch a low KHz square wave (4-6KHz 70-90V nominal) using a DC trigger signal, 12V would be convenient, but I can step up the voltage if necessary, a triac seemed like the correct device to accomplish this, however I seem to have a knack for using transistors incorrectly, and destroying them in the process, so I figured I would ask how I need to wire up this triac in order for it to function like a relay.

https://www.digikey.com/en/products/detail/stmicroelectronics/T2535T-8T/13559154

I don't know if it can control high frequency AC or not, the datasheet doesn't really say, if it can, it's massive overkill because it should only be switching a couple amps, and if it has a heatsink it's max voltage is around 10X what it should be switching.
There is no mention of a power level, but the triac listed is a very high power device, 25 amps and 800 volts working. The voltage is not that unreasonable but with no hint as to the current suggesting a suitable switching scheme is not likely to be correct.
So let us know the power level or current.
 

Ian0

Joined Aug 7, 2020
2,189
I think that the dI/dt might be a problem for the triac, if it goes abruptly from a positive voltage to a negative voltage a triac won't have zero current for long enough for it to switch off. Two back-to-back thyristors might do it, though.
 

MisterBill2

Joined Jan 23, 2018
8,736
WE stilll have no hint about how much current is involved, nor about the required switching rate. So there is a whole lot of guessing being done here. The simple and cheap way to do it is with a transistor and a diode bridge, except that it adds two diode drops in series.
 

Thread Starter

l0vot

Joined Apr 29, 2013
104
It SHOULD only be passing 7A max, probably more like 1-3A since it was barely getting warm when driven continuously the way i'm driving it, load has a primary resistance of ~1.5 ohms and it gets really hot when passing 6-7A, the AC source is a secondary winding on what is basically royer oscillator, it doesn't spend very much time at 0V at all. A 1V drop would result in a max heat dissipation of around 7W, a nominal heat dissipation of 3W or less, during normal operation it should have less than a 20% duty cycle, the triac can also run pretty hot continuously, it might not technically need a heatsink during normal operation, but I'm going to add one anyway, if it works. If 2 triacs could handle the square waves, the additional voltage drop is acceptable

It needs to switch up to 60 times per second with a duty cycle less than 20%, a relay really isn't the right choice, and it wouldn't be reliable if it did work.
 

Danko

Joined Nov 22, 2017
1,081
I'm trying to switch a low KHz square wave (4-6KHz 70-90V nominal) using a DC trigger signal, 12V would be convenient
load has a primary resistance of ~1.5 ohms
Use FETs, they can easily switch 60A through 1,5Ω load with AC 90V 5kHz.
1623125035415.png
This does require the 0V of the control signal to be floating with respect to the 90V signal. If the two V- connections are tied together it doesn't work.
How about some imagining?
1623134403218.png
 
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Thread Starter

l0vot

Joined Apr 29, 2013
104
Ian0: basically a flyback transformer

Danko: f I used FETs in that configuration, the body diodes will end up passing the full current with a 50% duty cycle, the body diodes can only do a fraction of what the FET is rated for, the shottkeys I have are rated for 20A, they can do more current than the body diodes, but 20A is the cap, this is sufficient, but the FETs I have are only rated for 100V anyway, so they would probably end up fried. A pair of SCRs is typically used when a triac can't do the job, if i can't get the triacs to unlatch i'll probably go the SCR route.
 

Danko

Joined Nov 22, 2017
1,081
the body diodes can only do a fraction of what the FET is rated for
They are rated equally.
I drain @25°C:
1623684144109.png
---------------------------------------------------------------------------------------
I body diode @25°C:
1623683950651.png
Danko: f I used FETs in that configuration, the body diodes will end up passing the full current with a 50% duty cycle
Body diodes do not work in this circuit at all,
because Drain-Source is simple resistor and conducts current in both directions, when transistor is ON.
You can see on diagram that D-S voltage of transistors is not more than ±75 mV @20A current.
So body diode can not be open by such small voltage.
1623683753785.png
 

cmartinez

Joined Jan 17, 2007
7,332
I'm trying to switch a low KHz square wave (4-6KHz 70-90V nominal) using a DC trigger signal, 12V would be convenient, but I can step up the voltage if necessary, a triac seemed like the correct device to accomplish this, however I seem to have a knack for using transistors incorrectly, and destroying them in the process, so I figured I would ask how I need to wire up this triac in order for it to function like a relay.

https://www.digikey.com/en/products/detail/stmicroelectronics/T2535T-8T/13559154

I don't know if it can control high frequency AC or not, the datasheet doesn't really say, if it can, it's massive overkill because it should only be switching a couple amps, and if it has a heatsink it's max voltage is around 10X what it should be switching.
Take a look at this thread, you might find it interesting.
 
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