Application of resettable fuse

燕园技术宅

Application of resettable fuse [Copy link]

 

I have a small question for you guys: I want to add a PTC resettable fuse to protect a 220V AC to 13.6V DC (1.45A) transformer before it goes through a bridge rectifier and enters the 7812. After the bridge rectifier, the maximum voltage becomes 13.6 * square root of 2, or 19.23V. So for the maximum voltage of the resettable fuse, should I choose the 16V or 24V in the typical value?

maychang

"For the maximum voltage of a resettable fuse, should I choose the 16V or 24V typical value?"

Both 16V and 24V can be used.

qwqwqw2088

After being rectified by a rectifier bridge, I want to add a PTC before entering 7812.

Since the self-recovery fuse is added, it is recommended to put it in front of the rectifier bridge

maychang

In fact, in the case of a power frequency transformer like the original poster, it is better to add insurance to the primary (220V) of the transformer than to add insurance to the secondary of the transformer. Moreover, it is best to use a non-resettable fuse for the primary (220V) of the transformer.

曹伟1993

The function of the resettable fuse is to cut off the input of the entire circuit when the circuit is abnormal, so it should be placed at the input to protect the circuit. Otherwise, if only one part is cut off, the other parts will still be damaged.

燕园技术宅

I have made a preliminary sketch. I have just started working with linear power supplies and I am not very familiar with the usage and parameter selection of safety capacitors. The PTC and TVS diodes are also used to protect the high-power 7812 and subsequent circuits. They prevent overload and short circuit.

燕园技术宅

maychang posted on 2019-12-16 14:37 "The maximum voltage of the resettable fuse, should I choose 16V or 24V in the typical value?" Both 16V and 24V can be used.

Does the voltage value of the resettable fuse have little effect? Do we only need to focus on the current value?

燕园技术宅

qwqwqw2088 posted on 2019-12-16 15:44 After a rectifier bridge rectifier, before entering 7812, I want to add a PTC,,,,, Since the self-recovery insurance is added, it is recommended to put it...

I uploaded a sketch on the sixth floor. The fuse is placed in front of the rectifier bridge to protect all the circuits after the secondary coil, right?

燕园技术宅

maychang posted on 2019-12-16 16:15 In fact, in the case of a power frequency transformer like the original poster, it is better to add insurance to the primary (220V) of the transformer than to add insurance to the secondary of the transformer. Moreover, the primary...

The primary coil has a non-recoverable fuse, and the secondary coil also wants to add an overload protection

燕园技术宅

Cao Wei 1993 posted on 2019-12-16 16:26 The function of the resettable fuse is to cut off the input of the entire circuit when the circuit is abnormal, so it should be placed at the input to protect the circuit...

There is also a fuse on the primary coil end

maychang

Yanyuan Technology House published on 2019-12-16 20:42 Does the voltage value of the resettable fuse have little effect? Do we only need to focus on the current value?

"The voltage value of the resettable fuse has little effect, right?"

That's not what I meant.

The voltage value that a resettable fuse can cut off is limited, and your worry is not wrong. But if you place a 16V resettable fuse before the rectifier bridge (that is, on one of the two secondary lines of the transformer), there will be no problem at all. Moreover, if you place it here, even damage to the rectifier bridge can be protected.

maychang

Yanyuan Technology House published on 2019-12-16 20:49 The primary coil has a non-recoverable fuse, and the secondary also wants to add an overload protection

"The primary coil has a non-resettable fuse, and I want to add an overload protection to the secondary as well."

Want double protection?

It is still recommended to place the secondary self-recovery fuse between the transformer secondary and the rectifier bridge.

燕园技术宅

maychang posted on 2019-12-16 20:55 "The primary coil has a non-resettable fuse, and the secondary coil also wants to add an overload protection." Want double protection? The secondary coil is self-...

OK, thanks. I want to ask, is it necessary to add safety capacitors to linear power supplies? Do linear power supplies have no EMI noise?

燕园技术宅

maychang posted on 2019-12-16 20:53 "The voltage value of the resettable fuse has little effect, right?" That's not what I mean. The voltage that the resettable fuse can cut off...

Got it, I'll move the fuse.

maychang

Yanyuan Technology House published on 2019-12-16 21:00 OK, thank you. I want to ask, is it necessary to add safety capacitors to linear power supplies? Linear power supplies have no EMI noise, right?

"Is it true that there is no need to add safety capacitors to linear power supplies? Do linear power supplies have no EMI noise?"

Linear power supplies do not work as switches, so there is no EMI noise and no need to add safety capacitors. However, linear power supplies must have large and heavy power frequency transformers.

燕园技术宅

maychang published on 2019-12-17 07:49 "Is it not necessary to add safety capacitors to linear power supplies? Linear power supplies have no EMI noise, right?" Linear power supplies do not work as switches, so there is no E ...

Thanks for your guidance

qwqwqw2088

Yanyuan Technology House posted on 2019-12-16 20:48 I uploaded a sketch on the sixth floor. The fuse is placed in front of the rectifier bridge to protect all the circuits behind the secondary coil, right?

PTC is a self-recovery insurance, mainly for overcurrent protection

You can put it in front.

There is a TVS behind it.

What is the load of this circuit? Is the protection component not overused?

燕园技术宅

qwqwqw2088 posted on 2019-12-17 08:35 PTC is a self-resetting insurance, mainly for overcurrent protection, which can be placed in the front, and there is a TVS behind this circuit load...

The load is an FPGA core board plus 6 sensors, and the total maximum current should be around 1.8A

maychang

Yanyuan Technology House published on 2019-12-17 14:52 The load is an FPGA core board plus 6-way sensors. The total maximum current should be around 1.8A

The DC output is 1.8A, so a 7812 chip may not be enough. Moreover, even if the 7812 outputs half of 0.9A, let alone 1.8A, the heat generated is 2.7W (the chip voltage drop is calculated based on 3V), so a heat sink must be added.

maychang

Yanyuan Technology House published on 2019-12-17 14:52 The load is an FPGA core board plus 6-way sensors. The total maximum current should be around 1.8A

Your transformer secondary rated voltage is 13.6V, rated current is 1.45A, I am afraid that when the output current is 1.8A after voltage regulation, according to your 6th figure, the capacitor is only 1000uF, and your transformer secondary after rectification cannot output voltage above 15V. Moreover, you have to consider that the mains power is allowed to drop by 10%.