# Mysterious schematic symbol? TL494

#### MacIntoshCZ

##### Member
Hello,
I am wondering whats this symbol really means?
Its connected to oscillator and to reference pin in Op amp.
BTW why is there current source connected to ground?
Its like there is by default 0,12/ 0,7 V or higher due to pin 5 and 4?
Thanks

#### Nigel Goodwin

##### Super Moderator
I would presume it's a pair of internal voltage references?, used to create the 'dead band' along with the comparators.

But why worry about what's inside the IC?, it's only a crude representation of what's there - and probably not at all accurate (there's no reason for it to be).

#### MacIntoshCZ

##### Member
I would presume it's a pair of internal voltage references?, used to create the 'dead band' along with the comparators.

But why worry about what's inside the IC?, it's only a crude representation of what's there - and probably not at all accurate (there's no reason for it to be).
I always trying to figure out how it works from schematic. Thanks

#### Nigel Goodwin

##### Super Moderator
I always trying to figure out how it works from schematic. Thanks
I take you are aware that this is a prehistroic chip?.

#### MacIntoshCZ

##### Member
I take you are aware that this is a prehistroic chip?.
=D heh. I need chip for half bridge. This works. Which chip i shoud use instead? I needed variable duty cycle and frequency.

#### Nigel Goodwin

##### Super Moderator
=D heh. I need chip for half bridge. This works. Which chip i shoud use instead? I needed variable duty cycle and frequency.
How about using a chip designed for that?, the TL494 is a 1980's switch-mode PSU chip.

#### MacIntoshCZ

##### Member
How about using a chip designed for that?, the TL494 is a 1980's switch-mode PSU chip.
Can you please give me a example? I searched alot but i not found anything. Also i make a thread about it. No one reccomended me something. Thanks

#### ronsimpson

##### Well-Known Member
Can you please give me a example?
What kind of a power supply are you building? input voltage, output voltage, power, isolated?

The LTC3721 is also old but at least it is designed to drive power MOSFETs not transistors.

#### MacIntoshCZ

##### Member
What kind of a power supply are you building? input voltage, output voltage, power, isolated?

The LTC3721 is also old but at least it is designed to drive power MOSFETs not transistors.
View attachment 126358
Halfbridge for Tesla coil. .tl494 drives irf21864pbf which control igbt transistors

#### alec_t

##### Well-Known Member
The recommended output transistor current of the TL494 is only 200mA. That may not be enough to drive sufficient gate current for rapid switching of big IGBTs to avoid significant heating.

#### MacIntoshCZ

##### Member
The recommended output transistor current of the TL494 is only 200mA. That may not be enough to drive sufficient gate current for rapid switching of big IGBTs to avoid significant heating.
I though that output from tl494 is only logic level for gate driver irf21864

#### alec_t

##### Well-Known Member
The TL494 outputs are open collector and open emitter, so can be configured with external resistors to give whatever level you want (within reason!).

#### ronsimpson

##### Well-Known Member
Halfbridge for Tesla coil.
There are a number of ICs that have the PWM and High/Low gate driver in one package. UCC256302,

#### MacIntoshCZ

##### Member
There are a number of ICs that have the PWM and High/Low gate driver in one package. UCC256302,

View attachment 126360
Can i ask where do you searched for components? When i want to find something i am using tme.eu. I could use also mouser or farrnel. But thats all whats avaible in CZ. I am prefering tme.

#### ronsimpson

##### Well-Known Member
I use digikey.com but mouser and farrnel are good. This is a good choice because they work with almost all products.
In this case I know TI.com makes many power supply parts. Analog.com is a good place.

If you want half bridge, FairChild now OnSemi has FSFR parts.

#### ronsimpson

##### Well-Known Member
I forgot about these parts. Actually almost any of the ICs for compact florescent bulbs will work.

#### unclejed613

##### Well-Known Member
the TL494 is a 1980's switch-mode PSU chip.
yet i still see them from time to time in new equipment... not much as i used to, but they're still using them.

#### Nigel Goodwin

##### Super Moderator
yet i still see them from time to time in new equipment... not much as i used to, but they're still using them.
Certainly in domestic electronics they disappeared decades ago, and were replaced by more modern, more efficient, and (most of all) cheaper products. It's an antique, and requires a LOT of external circuitry to work.

I can't help thinking that anyone using them in new equipment has a seriously poor designer?.

#### MacIntoshCZ

##### Member
Certainly in domestic electronics they disappeared decades ago, and were replaced by more modern, more efficient, and (most of all) cheaper products. It's an antique, and requires a LOT of external circuitry to work.

I can't help thinking that anyone using them in new equipment has a seriously poor designer?.
Thanks anyone for help. NCP1396A is in our country quite costly, about 10$. Tl494 i bought cost me about 0.5$ but additional driver chip another 2.5\$.
But i realized that danyk solution ( diodegonewild etc. He has a lot channels on YT) could work with adjusted capacitence. One pair of Caps ( 2x 1u) limits power to 317W (230V AC / 3khz).
*UPDATE
Actually when one transistor is open second capacitor is charging through load / second cap. Power should be higher than.

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