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Oven Control PCB developed by China Vendor

jacklam2048

Feb 22, 2021
33
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Feb 22, 2021
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Today I went to the PCB developer who is designing an electronic control board for the oven of our company. I have a few questions:

1. I found that at the position of 230V~ 50 Hz voltage entrance, they just put a MOV (Metal Oxide Varistor) across the L and N line but no slow blow fuse. The PCB developer told me that there was no need to have the slow blow fuse, because when big voltage comes, the varistor will let the inrush current passing through. The slow blow fuse is redundant. Do you think he is correct ?

2. The PCB uses the old rectifier for converting AC to DC (this is to provide power to the oven control PCB), but not the Switching power circuit. Thus, you can see a big transformer on the PCB. The developer says that the Old rectifier type AC-DC converter is more reliable than the Switching power and less likely to get damage. Do you agree to this comment ?

3. There is no Common Mode Choke to filter out Common Mode noise. The PCB developer told me that this PCB did not carry high frequency signal, so Common Mode Choke is not required. But what about out coming high frequency signal ? Do you think he is correct ?

4. There is only one capacitor for the EMI filter circuit. Again, the PCB developer claims that this PCB does not generate high frequency signal, so just one capacitor for EMI filter is sufficient. Do you think he is right ?

5. At last, there is no bleeding resistor across the capacitor. The PCB developer says that for PCB design, the simplier the better, so he skips the bleeding resistor. I doubt about this. Is he correct ?

If anyone can give me some comments to the above, will be very appreciated !

Thanks.

Jack Lam
 

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bidrohini

Feb 1, 2023
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Before contacting the PCB developer, did you make any prototype of your design and test that?
 

kellys_eye

Jun 25, 2010
6,225
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Analogue PSUs are easier to design, less susceptible to faults, less noisy (electrically) and can be cheaper under some circumstances (low power applications). They are also, imho, a safer solution.

A fuse protects the line chord, not the equipment. No fuse in existence will save an electronic component from damage as normal fuses simply don't act fast enough. If the equipment uses a domestic power plug that houses a fuse then no onboard fuse is necessary.

The 'bleed resistor' is the primary of the transformer.

Common mode chokes aren't a requirement of analogue PSUs either and the use of a single (presumably X-class) filter capacitor is common practise - just make sure they spec a quality manufacturer....

Most equipment design is cost-based. If you are happy to pay more for a chosen approach then tell them and let them spend your money, otherwise, assuming they have the right credentials and reputation, let the specialists do what you're paying them to do.
 

Delta Prime

Jul 29, 2020
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You need 3 types of suppression. The clamping properties of a MOV for voltage transients. As well as a a slow blow fuse for they have greater I2*t capabilities.Power is consumed in the form of heat due to the I2*R heating effect. For Current transients.
I would incorporate a thermistor as well.
That's the holy Trinity in transient protection.
Take a look at the schematic here.
 
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jacklam2048

Feb 22, 2021
33
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Feb 22, 2021
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Before contacting the PCB developer, did you make any prototype of your design and test that?
No, this is the first time that this developer shows me the power supply unit he is going to give me.
 

jacklam2048

Feb 22, 2021
33
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Feb 22, 2021
Messages
33
You need 3 types of suppression. The clamping properties of a MOV for voltage transients. As well as a a slow blow fuse for they have greater I2*t capabilities.Power is consumed in the form of heat due to the I2*R heating effect. For Current transients.
I would incorporate a thermistor as well.
That's the holy Trinity in transient protection.
Take a look at the schematic here.
But what about if we skip the fuse ? when inrush current comes, does the the inrush current just pass through the MOV (because of extraordinary high voltage across it) and hence the MOV protects the circuit after it; hence no need for the fuse. This is the point makes me confuse.
 

Delta Prime

Jul 29, 2020
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(Negative Temperature Coefficient)NTC
thermistor is used as a power thermistor to effectively limit an inrush current, with which the input and output capacitors are charged at the time of powering on.
At the time of powering on, the device is charged with an inrush current peak that is several times larger than the "steady state current"to charge the DC capacitor.This inrush current may have a harmful effect on the service life of a DC capacitor because
there's no bleeder resistor for the capacitors the capacitors will be fully charged at turn on.Therefore this protection is needed with the schematic shown. Or simply add a resistor to bleed the capacitors.
The fuses used if there is a short circuit on the secondary side of the transformer it will save the circuit breaker from tripping. The MOV is in parallel with the supply rails and will limit voltage and the thermistor for in Rush current limiting protection which are connected in series with 230V mains to which these voltage and current surges originate.
Edit: these are just my opinions I'm sure the circuit would work fine. Your concerns have merit my remarks were out of an overabundance of caution. :)
 
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jacklam2048

Feb 22, 2021
33
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Feb 22, 2021
Messages
33
Thanks alot
(Negative Temperature Coefficient)NTC
thermistor is used as a power thermistor to effectively limit an inrush current, with which the input and output capacitors are charged at the time of powering on.
At the time of powering on, the device is charged with an inrush current peak that is several times larger than the "steady state current"to charge the DC capacitor.This inrush current may have a harmful effect on the service life of a DC capacitor because
there's no bleeder resistor for the capacitors the capacitors will be fully charged at turn on.Therefore this protection is needed with the schematic shown. Or simply add a resistor to bleed the capacitors.
The fuses used if there is a short circuit on the secondary side of the transformer it will save the circuit breaker from tripping. The MOV is in parallel with the supply rails and will limit voltage and the thermistor for in Rush current limiting protection which are connected in series with 230V mains to which these voltage and current surges originate.
Edit: these are just my opinions I'm sure the circuit would work fine. Your concerns have merit my remarks were out of an overabundance of caution. :)
Thanks a lot ! I think I understand more now.
 
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