# A special combination of MOV used in LED driver

#### PY HRM

Joined Jan 1, 2022
28
I have seen a special kind of arrangement of MOV used in an LED driver. I cant find how to calculate how much surge protection it might provide. Using normal method on internet it results Max surge voltage of 2KV, but the manufacturer claims 2.5 KV surge protection.

here is the image and schematic

#### Marley

Joined Apr 4, 2016
501
F1 is a fusible resistor. It has 10 ohms resistance so with ZV2 it will attenuate voltage spikes entering the equipment.
Very large over-voltages will fuse the resistor. The fusible resistor also protects against fire if there is a fault in the equipment (say the bridge rectifier develops an internal short).
I think the second MOV (ZV2) is redundant - not sure why its there!

#### AnalogKid

Joined Aug 1, 2013
10,557
F1 is a fusible resistor. It has 10 ohms resistance so with ZV2
That should be ZV1.

Very large over-voltages will fuse the resistor.
A large voltage surge or spike will cause F1 to blow like a fuse and become an open circuit. When the voltage is greater than the clamping voltage of ZV1, ZV1 changes very rapidly from a high resistance to a low resistance. This low resistance draws enough current through F1 to cause it to blow open. That is, blow electrically; it does not actually break apart.

ZV2 protects the optocoupler secondary (its output transistor or circuit) from transients produced by the LED power regulator circuit. This implies that the regulator is a switcher of some kind, but we can't tell without the rest of the circuit.

ak

#### seanstevens

Joined Sep 22, 2009
213
That should be ZV1.

ZV2 protects the optocoupler secondary (its output transistor or circuit) from transients produced by the LED power regulator circuit. This implies that the regulator is a switcher of some kind, but we can't tell without the rest of the circuit.

ak
The 4 pin component DB1 is a bridge rectifier as outlined in the TS circuit. I suspect the large brown cap above them all, is a dropper cap power supply configuration. So no transformers, no switch mode PSU, just a fusible resistor, two MOVs a bridge and a large-cap as the AC dropper cap PSU configuration.
If that is the case/configuration, you need to be very very careful handling a live circuit, it will not be isolated from the mains and I would certainly not poke it with a (mains powered) scope unless you want to blow that up.

#### PY HRM

Joined Jan 1, 2022
28
That should be ZV1.

A large voltage surge or spike will cause F1 to blow like a fuse and become an open circuit. When the voltage is greater than the clamping voltage of ZV1, ZV1 changes very rapidly from a high resistance to a low resistance. This low resistance draws enough current through F1 to cause it to blow open. That is, blow electrically; it does not actually break apart.

ZV2 protects the optocoupler secondary (its output transistor or circuit) from transients produced by the LED power regulator circuit. This implies that the regulator is a switcher of some kind, but we can't tell without the rest of the circuit.

ak
Rest of the circuit is a constant current buck APFC led driver. I'm just confused about the configuration of MOVs used with the bridge rectifier at input.

#### PY HRM

Joined Jan 1, 2022
28
The 4 pin component DB1 is a bridge rectifier as outlined in the TS circuit. I suspect the large brown cap above them all, is a dropper cap power supply configuration. So no transformers, no switch mode PSU, just a fusible resistor, two MOVs a bridge and a large-cap as the AC dropper cap PSU configuration.
If that is the case/configuration, you need to be very very careful handling a live circuit, it will not be isolated from the mains and I would certainly not poke it with a (mains powered) scope unless you want to blow that up.
Certainly. I'm having all precautions. Plz tell me about the purpose of MOV used at both sides of the rectifier, i know about the first one but can't figure out about the second one which is located after the rectifier. Its a constant current APFC buck led driver.

#### seanstevens

Joined Sep 22, 2009
213
I am guessing its just more safety in numbers. 2 MOVs have a better chance then 1. I have seen this double MOV configuration in the Chinese mains LED board.

#### PY HRM

Joined Jan 1, 2022
28
I am guessing its just more safety in numbers. 2 MOVs have a better chance then 1. I have seen this double MOV configuration in the Chinese mains LED board.
Yes but that should be in parallel to increase the maximum surge current capacity. This is not something like parallel.

#### seanstevens

Joined Sep 22, 2009
213
One catches the AC side one the DC side...

#### AnalogKid

Joined Aug 1, 2013
10,557
One catches the AC side one the DC side...
Nope. An AC line transient will not couple through to the output. In detail, a very small amount of energy might couple though capacitively, but the coupler's internal design specifically reduces this to an almost trivial level.

no switch mode PSU,
I wasn't saying the circuit as shown is a switcher, only that the un-shown pile of stuff to the right probably is. And, guess what -

Rest of the circuit is a constant current buck APFC led driver.
ak

#### AnalogKid

Joined Aug 1, 2013
10,557
I am guessing its just more safety in numbers.
Nope.

2 MOVs have a better chance then 1.
Not the way you mean. The two MOV's are suppressing two different kinds of transients from two different sources. Neither one can help out the other in any way.

ak

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#### PY HRM

Joined Jan 1, 2022
28
Nope.

Not the way you mean. The two MOV's are suppressing two different kinds of transients from two different sources. Neither one can help out the other in any way.

ak
Is it possible that the buck converter itself can generate any surge sometime? That might be the only thing can explain the need of second MOV.