Blown Milk frother resistor

Thread Starter

finnegale

Joined May 14, 2019
8
I am a novice and tried to replace a burned resistor on a starbucks milk frother. It burned instantly. It was hard to read the resistence of the blown one, so I'm posting a pic and hopefully someone with more experience can take a look. It is the two watt resistor you see on upper right of photo by the turquoise capacitor.
If there's any other info about the device you need let me know! Thanks!
 

Attachments

jpanhalt

Joined Jan 18, 2008
10,482
I am a novice and tried to replace a burned resistor ...
It then blew immediately.
You say you replaced what you thought was the correct resistor. What value resistor did you replace it with? As an empiricist (sometimes), just increase the resistance and see what happens. Since the first value band appears to be brown, you might use 10X what you thought it was and then work backward until it works. Two watt resistors are expensive. You might consider a bunch of cheaper 1/2 watt resistors in parallel (4x 1/2 W = 2W) for testing purposes. Or, stick to the 2 W version, if you can find a cheap source.

Alternatively, forget about frothing your milk. I preheat my half-and-half in a microwave and get a little scalding when I add my Mr. Coffee hot coffee to it.
 

AlbertHall

Joined Jun 4, 2014
10,538
It is likely that some other fault blew the resistor, so any replacement is going to have a very short life.
Without some knowledge of the circuit it is difficult to speculate.
You could check other components, especially power semiconductors and capacitors.
 

Thread Starter

finnegale

Joined May 14, 2019
8
Ok, Here are two pics of PCB. I read from the burned one that it was a 39 ohm resistor, so i got one in the 2 watt and 3 watt but it burned up both wattages, so I don't know if it's another component like someone said or too burned up to read its color bands.



IMG_20200929_085749.jpgIMG_20200929_085759.jpg
 

AlbertHall

Joined Jun 4, 2014
10,538
As you are looking at it in that picture, the right hand terminal is the gate.
With your meter on the diode check range, you should get a low reading between the gate and the left hand terminal with the leads either way round. You should get a high reading between the two left hand terminals with the leads either way round.
 

Thread Starter

finnegale

Joined May 14, 2019
8
the rectifier "D8" reads the same resistance in both directions while the same component next to it, also a US1 J, only reads resistance one direction. Does that mean "d8" is blown? the other black box diodes read resistance in one direction only. the Triac reads normal. :)
 

Thread Starter

finnegale

Joined May 14, 2019
8
It then blew immediately.
You say you replaced what you thought was the correct resistor. What value resistor did you replace it with? As an empiricist (sometimes), just increase the resistance and see what happens. Since the first value band appears to be brown, you might use 10X what you thought it was and then work backward until it works. Two watt resistors are expensive. You might consider a bunch of cheaper 1/2 watt resistors in parallel (4x 1/2 W = 2W) for testing purposes. Or, stick to the 2 W version, if you can find a cheap source.

Alternatively, forget about frothing your milk. I preheat my half-and-half in a microwave and get a little scalding when I add my Mr. Coffee hot coffee to it.
When you say work backwards from 10* amount i thought it was, which was 37 ohms, what increments do you recommend? I got a kit of 1/2 watt resistors to wire in parallel as you advised
 

Dodgydave

Joined Jun 22, 2012
9,642
Put the Dvm on diode test, and check D8 it should only read in one direction, if it reads in both, then ideally you need to lift one end up to prove its blown.
 

jpanhalt

Joined Jan 18, 2008
10,482
If 37 Ω blew immediately, then I would try at least 100 Ω -- maybe 200 Ω to start.

Here are a few caveats:
1) Don't touch anything while the device is plugged into the mains, even if that is just to see if it is hot.
2) That resistor may just have been the weakest link when something else failed in the sircuit (e.g., a bad diode as has been mentioned). In other words, it was a protective fuse of sorts. Finding a resistor that doesn't blow may not fix the device. In fact, since resistors are pretty robust, I suspect the real problem is somewhere else. However a resistor that doesn't blow may allow you to find the real culprit if it then overheats.
3) Resistors blow when over heated. That heat is (watts dissipation) = I^2 x R. Of course, E = I x R (E = voltage ). Thus, doubling the resistance halves the current so I^2 is reduced by 1/4. Since R is double, the power (W) is decreased by half when the resistance is doubled, if all else stays the same.
4) The effective resistance of resistors in parallel is calculated as the reciprocal of the sum of the reciprocal resistances. That is, 4, 400 Ω resistors in parallel will have an equivalent resistance of 100 Ω. If your resistors are all the same value, then the effective resistance is just that value divided by the number in parallel.
5) The resistors in your kit have fixed values based on a standard progression, e.,g E12 or E24. You will have to work with those resistances. There is probably no 400 Ω resistor, so you should go a little higher, such as 430 Ω = 430/4 = 108 Ω. For 200 Ω, four 820 Ω resistors in parallel will be close. For convenience, here is the E24 table:

1602413459103.png
Source: https://www.electronics-notes.com/a...tor-values-e-series-e3-e6-e12-e24-e48-e96.php
That souce shows the other tables too.
 
Top