whats a triac and how is it different from a FET or solid state relay?

[ETmonster]

hi i think this is my first post. i am an electric bike entusiast that has built battery chargers and am in need of electronics knowledge.

whats a triac and how is it different from a FET or solid state relay? and could we use it to control battery charging or in diy motor controllers?

i read in wiki that it can be used in light dimmers to limit power? is this more efficient than charging a battery using a light bulb to limit current or will the triac just overheat & melt ? my bike battery is 180v 7ah.

 

[ETmonster]

could i charge my battery with something like this?

**broken link removed**

 

[ericgibbs]

could i charge my battery with something like this?

**broken link removed**

hi,
No, it would be a bad idea and dangerous to connect a dimmer to a battery.

Most SSR's solid state relays are made using Triacs.

The FET, there are quite a number of different types, it depends upon the application which one you choose.
They are VERY different from Triacs.

 

[crutschow]

A triac provides AC voltage or current control.

A FET is a type of transistor which can be used to amplify ac signals or control dc currents.

A solid state relay is a circuit generally used to turn ac or dc (depending upon their design) power on and off. The usually have an optical isolator between the input control and the output connections, and use a triac or FET to provide the switching control.

I would suggest you read some tutorials about electronics and battery chargers before you try to build one (try Google). You appear to require more explanation than can be reasonably provided in these forums.

 

[ETmonster]

i already built one battery charger but now i'm trying to make it more efficient. here is my circuit. its only 65% efficient and it wasts a lot in the bulb. i use it every day and its really fast charger.

i was hoping i could replace the bulb with a triac and that would still limit the current but not waste energy as heat. sorry my electronics is really basic. i may buy that art of electronics book.

 

[crutschow]

i already built one battery charger but now i'm trying to make it more efficient. here is my circuit. its only 65% efficient and it wasts a lot in the bulb. i use it every day and its really fast charger.

i was hoping i could replace the bulb with a triac and that would still limit the current but not waste energy as heat. sorry my electronics is really basic. i may buy that art of electronics book.

A triac dimmer requires a resistive or possibly inductive load to work properly. The battery is a very low impedance load which would create very high current spikes in the triac so you still need some resistance in series with the battery. If you reduce the voltage with the triac then adding more bulbs in parallel would reduce the loss for the same 7A current. Since light bulbs change resistance significantly with load, you'd have to experiment with the number of extra bulbs need for optimum operation. (You do have a meter to measure battery current, I assume?)

CAUTION: Your schematic shows no isolation from the mains. I trust you know that's very dangerous since there are lethal voltages everywhere in your circuit. Touching any part of the operating circuit while grounded will likely kill you.

 

[ETmonster]

bulbs in parallel with the triac? could you write a circuit diagram for me? cheers matthew

 

[crutschow]

bulbs in parallel with the triac? could you write a circuit diagram for me?

They are just in parallel with the bulbs you are already using. The triac would work off the lines and drive your bridge rectifier.

 

[ETmonster]

of course i do have a multimeter. i would have to use it in AC amp-meter mode to measure the current before the rectifier, right? because after the rectifier there would be pulsed DC which might not read well on a multi meter?

ill post a circuit diagram of what i am thinking of when i get home. thanks for your help

 

[crutschow]

of course i do have a multimeter. i would have to use it in AC amp-meter mode to measure the current before the rectifier, right? because after the rectifier there would be pulsed DC which might not read well on a multi meter?

The current before the rectifier is also pulsed since the rectifiers do not conduct for the whole sine-wave but only when the sine-wave voltage is greater than the battery voltage. And the current out of a triac is a truncated sine-wave. So a multimeter will not measure the correct ac current in either case, unless it is a true rms type.

Measuring the battery current with the meter in the dc mode should be the most accurate since the meter will tend to read the average of the current pulses, which is the dc current of interest.

 

[ETmonster]

would this work? what are sugestions for component ratings for 240v mains 144v battery ?

Cheers, Matthew

 

[Nigel Goodwin]

No, you can't charge a battery with AC.

 

[ericgibbs]

hi Nigel,

My concern is the total lack of isolation from the mains supply when using this circuit and what happens if the triac fails s/c.

IMHO this circuit is potentially lethal, I have cautioned the OP, perhaps he may listen to you.

EDIT:
I Googled Darwin Awards....

 

[Nigel Goodwin]

hi Nigel,

My concern is the total lack of isolation from the mains supply when using this circuit and what happens if the triac fails s/c.

IMHO this circuit is potentially lethal, I have cautioned the OP, perhaps he may listen to you.

Perhaps he's going for the Darwin Award?

 

[ETmonster]

ops, i missed out the bridge rectifier! assuming this is present will it work? it is going to be a light weight emergency charger so that i can carry it on the bike all the time for if i run out of charge unexpectedly. it needs to be light weight so no transformers.

but honestly would a transformer make it any safer anyway? -its 144v at 5 amps! i use one of those fast trip plugs for lawn mowing just incase, but i don't know if that is adequate either.

 

[ETmonster]

hi Nigel,

My concern is the total lack of isolation from the mains supply when using this circuit and what happens if the triac fails s/c.

IMHO this circuit is potentially lethal, I have cautioned the OP, perhaps he may listen to you.

EDIT:
I Googled Darwin Awards....

all mains devices are potentially lethal

 

[ericgibbs]

all mains devices are potentially lethal

Normally commercial manufactured equipment is designed to minimise or prevent any electrical hazard
and is supposed to meet the statutory regulations.

That charger circuit you are proposing does not.

If I did not warn you of this hazard, It would be a failure on my part.

I consider the safety of any forum member paramount and I will always err on the side of caution,
even though it may irritate the member.

 

[Nigel Goodwin]

I agree Eric, it's potentially VERY dangerous, and you wouldn't be allowed to sell such a device commercially.

 

[crutschow]

but honestly would a transformer make it any safer anyway? -its 144v at 5 amps! i use one of those fast trip plugs for lawn mowing just incase, but i don't know if that is adequate either.

The transformer makes it safer since it isolates the circuit from ground.

But one of the fast trip plugs (which I assume is a ground fault interrupter or GFI as it's called in the U.S.) should definitely be used if you insist on connecting the charger directly to the mains. They're designed to open the circuit before a lethal current (which can be as little as 15mA) can flow through your body to ground.

NOTE: A GGI provides no protection if you touch the two mains at the same time, they only protect against mains to ground current (by checking for an inbalance between the two mains currents).