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LM317 digital control

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Maobuff:

--- Quote from: ZeTeX on May 22, 2016, 05:44:11 pm ---
--- Quote from: Maobuff on May 22, 2016, 03:52:58 pm ---
--- Quote from: ZeTeX on May 22, 2016, 03:41:31 pm ---no no no, its gonna drop the voltage to 0V because I'm using negative supply. Its a closed loop feedback, the op amp is going to do whatever it has to do to get 0V output.
relay is too slow for current limiting for sure.

--- End quote ---
Why relay is slow? In my design if load consume more current its disconnect load from PSU.To connect it back i need to reset flipflop manually (or with help of MCU).

--- End quote ---
so lets say your circuit tell the relay to switch off or switch on.
Relay takes some time to switch off or on because the mechanical pin has to move a couple of mm in the air, it might sound like really fast, but really if its over 1ms its too much. not to mention that a relay is kinda loud and you will probably hear it every time your PSU will go into cc mode and you will want to destroy it.

Also you circuit will oscillate badly, because when lets say your load want to draw 3A while your CC set to 1.5A so the relay is gonna disconnect the load, so the cc circuit will see 0A doe gonna switch the relay on, so the current is going to shoot up to 3A and then over and over again, with NPN or MOSFET his happens much faster that it is easy to stabilize, with a relay.. not really.

--- End quote ---
Sorry but you dont understand how flipflop works, if load draw more than I set its turn on relay and disconects output, and ONLY ONE WAY to connect it is to reset flipflop and turn relay off. Its acts like a RCD(Residual-current device).

ZeTeX:

--- Quote from: Maobuff on May 22, 2016, 06:13:45 pm ---
--- Quote from: ZeTeX on May 22, 2016, 05:44:11 pm ---
--- Quote from: Maobuff on May 22, 2016, 03:52:58 pm ---
--- Quote from: ZeTeX on May 22, 2016, 03:41:31 pm ---no no no, its gonna drop the voltage to 0V because I'm using negative supply. Its a closed loop feedback, the op amp is going to do whatever it has to do to get 0V output.
relay is too slow for current limiting for sure.

--- End quote ---
Why relay is slow? In my design if load consume more current its disconnect load from PSU.To connect it back i need to reset flipflop manually (or with help of MCU).

--- End quote ---
so lets say your circuit tell the relay to switch off or switch on.
Relay takes some time to switch off or on because the mechanical pin has to move a couple of mm in the air, it might sound like really fast, but really if its over 1ms its too much. not to mention that a relay is kinda loud and you will probably hear it every time your PSU will go into cc mode and you will want to destroy it.

Also you circuit will oscillate badly, because when lets say your load want to draw 3A while your CC set to 1.5A so the relay is gonna disconnect the load, so the cc circuit will see 0A doe gonna switch the relay on, so the current is going to shoot up to 3A and then over and over again, with NPN or MOSFET his happens much faster that it is easy to stabilize, with a relay.. not really.

--- End quote ---
Sorry but you dont understand how flipflop works, if load draw more than I set its turn on relay and disconects output, and ONLY ONE WAY to connect it is to reset flipflop and turn relay off. Its acts like a RCD(Residual-current device).

--- End quote ---
That's great, but that does not change anything I said, so you are going to need to reset the relay, you want other loop \ micro controller to do that? that is just slower.
if you want your power supply to limit the current to any value then 0A you are going to need a feedback that maintenes this current, usually by an op amp controlling the the pass transistor, but if you have a relay that takes a couple of ms to switch you cc will be really, really slow.

So lets say that you want to maintain an CC of 1.5A, you relay will have to turn off and on very very very fast, or else you are going to have a very slow current limit, because until the relay switch on or off the current shoot up and the poor op amp cant do nothing because a slow relay is limting the speed

Best way is to simulate it or build your schematic and see the result. There is a reason no power supply ever in a world uses a relay to pull down or turn off the output. (its also much expensive then a MOSFET or NPN).


Maobuff:

--- Quote from: ZeTeX on May 22, 2016, 06:29:29 pm ---So lets say that you want to maintain an CC of 1.5A, you relay will have to turn off and on very very very fast, or else you are going to have a very slow current limit, because until the relay switch on or off the current shoot up and the poor op amp cant do nothing because a slow relay is limting the speedr schematic and see the result. There is a reason no power supply ever in a world uses a relay to pull down or turn off the output. (its also much expensive then a MOSFET or NPN).

--- End quote ---
But i dont need a CC in my PSU, just overcurrent protection.


--- Quote from: ZeTeX on May 22, 2016, 06:29:29 pm ---Best way is to simulate it or build your schematic and see the result. There is a reason no power supply ever in a world uses a relay to pull down or turn off the output. (its also much expensive then a MOSFET or NPN).

--- End quote ---
Relay make thinks little bit easy. And im currently using power supply with sama overcurrent protection.

ZeTeX:

--- Quote from: Maobuff on May 22, 2016, 06:39:53 pm ---
--- Quote from: ZeTeX on May 22, 2016, 06:29:29 pm ---So lets say that you want to maintain an CC of 1.5A, you relay will have to turn off and on very very very fast, or else you are going to have a very slow current limit, because until the relay switch on or off the current shoot up and the poor op amp cant do nothing because a slow relay is limting the speedr schematic and see the result. There is a reason no power supply ever in a world uses a relay to pull down or turn off the output. (its also much expensive then a MOSFET or NPN).

--- End quote ---
But i dont need a CC in my PSU, just overcurrent protection.


--- Quote from: ZeTeX on May 22, 2016, 06:29:29 pm ---Best way is to simulate it or build your schematic and see the result. There is a reason no power supply ever in a world uses a relay to pull down or turn off the output. (its also much expensive then a MOSFET or NPN).

--- End quote ---
Relay make thinks little bit easy. And im currently using power supply with sama overcurrent protection.

--- End quote ---
if you only need overcurrent and not constant current then relay is fine, for a bench power supply I was pretty sure you are going to use constant current. anyway relay is fine then.

Liv:

--- Quote from: Maobuff on May 22, 2016, 03:24:32 pm ---Using one LM317 for CC and CV is not a good solution
--- End quote ---

Using LM317 beyond its standard features is a bad idea. I recently made a PSU adjustable 0..±15 V and 0..+5 V based on LM317/337. The number of auxiliary parts was larger. In addition, it was necessary to add the supervisor scheme, since the power is turned off at the output there was a voltage surge. If you need a power supply with CC and CV modes, properly do it on discrete components. Here is a good example of a simple power supply with the processor control.

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