Voltage rating for resistors

Thread Starter

fastbike

Joined Dec 29, 2020
85
I'm looking (again) at a dimming circuit based around the OnSemi FL5150/FL5160 dimmer on a chip IC.

The datasheet shows example circuits with three resistors connected to the mains supply load/neutral: two to provide over current sensing and one to provide zero cross detection.
1666838887833.png

Given that the "GND" reference voltage for the IC is tied to the mid point (common source) of the back-to-back switching mosfets, what voltage would there be across these three resistors ?
I'm thinking when the mosfets are off, there will be a max of 120V * 1.4 between "GND" and Line Hot. And when the switch is on the "GND" reference point will float 17v (the internally provided Vs) below Line Hot.

In this case resistors will need to be rated for 170V (or 325V for 230V ac operation) plus any required safety allowances.

Does this sound reasonable ?
 

ronsimpson

Joined Oct 7, 2019
2,540
I think it is good to assume that all three 1M ohm resistors will see (line voltage x 1.414).
By line voltage I assume the lowest voltage to the highest voltage normally on the power line.
Then I like to add some voltage for spikes and other nasty events.
I often use two 470k resistors or three 330k because small resistors often are not rated very high.
 

Ian0

Joined Aug 7, 2020
6,713
That ground connection seems very dubious to me.
If you wanted to sell it in Europe or Britain, it would have to pass a 1500V flash test between live and ground.
 

Thread Starter

fastbike

Joined Dec 29, 2020
85
I think it is good to assume that all three 1M ohm resistors will see (line voltage x 1.414).
By line voltage I assume the lowest voltage to the highest voltage normally on the power line.
Then I like to add some voltage for spikes and other nasty events.
I often use two 470k resistors or three 330k because small resistors often are not rated very high.
Thanks Ron. I've found some 400v rated resistors (Rohm KTR10EZPF1004) so two in series will get me well over a 2x safety factor (will be making this for a 230V ac supply).
Is my original thinking correct ?
 

Thread Starter

fastbike

Joined Dec 29, 2020
85
That ground connection seems very dubious to me.
If you wanted to sell it in Europe or Britain, it would have to pass a 1500V flash test between live and ground.
It's not a connection to ground - it is a zero volt reference point referenced to the centre point of the mosfet pair. It will follow line when the mosfets are conducting and be closer to neutral when the mosfets are off. There is no connection to either protective earth, or the GND of the digital control circuit.
All isolated digital connections will be 2.5kV rated.
 

WBahn

Joined Mar 31, 2012
27,903
Don’t you think that On Semi should have known better than to use an earth symbol?
I don't see why they should have known any better than Texas Instruments or just about every other manufacturer who have long histories of using the earth symbol to mean circuit common.

For example, from the TI TL082 Datasheet published just last year:

1666862669185.png

It's just one of those things that is what it is. People (and companies, or divisions of companies) that seldom, if ever, need to distinguish between a reference common, a chassis connection, or an earth ground tend to use the earth-ground symbol for everything. It's a bad habit that they were indoctrinated with in school because nearly all circuit textbooks do the exact same thing.
 

Ian0

Joined Aug 7, 2020
6,713
There’s quite a fundamental difference: in the examples you cite, the connections shown by the earth symbol could be connected to earth, and in the case of audio circuitry probably really are connected to earth. For the OnSemi circuit, it connects to the negative supply for the IC, but the one place it must not be connected to is earth.
 

Janis59

Joined Aug 21, 2017
1,537
If use the "golden age" massive resistors BC-0.125 they are able withstand the 200 V, BC-0.25 the 350V, BC-1 - 500 and BC-2 about 0.75 kV. All the art is to stack them enough much in series. The same art is about SMD only their characterial voltages stand between 50V and 100V. See the datasheet and stack em .
 

Ian0

Joined Aug 7, 2020
6,713
A standard 1206 resistor is good for 200V. A 2512 will withstand 230V mains. I tend to use two 1206 in series as we have them in stock.
 

MaxHeadRoom

Joined Jul 18, 2013
26,291
I don't see why they should have known any better than Texas Instruments or just about every other manufacturer who have long histories of using the earth symbol to mean circuit common.
Agree, Why set a standard if no-one obeys it, The gross miss-use makes it meaningless.
 
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