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post #1 of 15 (permalink) Old 12-15-2018, 06:48 AM Thread Starter
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Any Electrical Engineers ??

I have an older Snap-On wheel balancer that uses a plug in class 2 transformer for it's power supply. It has the specifications: Input 120V 60HZ 20VA and Output 8.5VAC 1.5 AMP. This "power block" has failed and I am having trouble finding an exact replacement unit. Does anyone here know if it would be ok to use one similar, but not exact? I have found two that are close. One has output of 10VAC and 2.4AMP, the other 9VAC and 1.1AMP Thanks!

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post #2 of 15 (permalink) Old 12-15-2018, 05:12 PM
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post #3 of 15 (permalink) Old 12-15-2018, 07:41 PM
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I read the first post and I decided not to reply because I don't know how critical is the voltage requirement of the equipment. The 10V 2.4 amp has surplus current capacity but I don't know if the extra voltage could cause it to operate incorrectly.

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post #4 of 15 (permalink) Old 12-15-2018, 09:17 PM
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post #5 of 15 (permalink) Old 12-16-2018, 04:11 AM Thread Starter
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@aflp , @alfaparticle , a few images.
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post #6 of 15 (permalink) Old 12-16-2018, 05:53 AM
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@papajam has extensive background in electrical wiring. He's wired many, many different pieces of equipment at the powder coating facility he once worked at. IT would not surprise me if he wired the entire building.

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post #7 of 15 (permalink) Old 12-16-2018, 07:00 AM
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Can you post a photo of the other side of the circuit board? Most equipment like this will have a rectifying and voltage regulating circuit.
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post #8 of 15 (permalink) Old 12-16-2018, 07:26 AM
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Quote:
Originally Posted by Chas H View Post
Can you post a photo of the other side of the circuit board? Most equipment like this will have a rectifying and voltage regulating circuit.
Agree. The point Chas H is leading toward is that if you use a transformer that puts out slightly more voltage, the voltage regulating circuit (assuming there is one) will limit it to the correct level. So the 10VAC and 2.4AMP transformer might work OK. That's why it would be useful to see what components are installed on the other side of the board - we can try to determine if they look like a voltage regulator.

The 9VAC and 1.1AMP transformer might work as well. Again, the regulator circuit (assuming there is one) would limit the voltage reaching the logic circuits, and that "missing" .4 amps might not cause a problem. But the larger-capacity transformer is probably the better bet.

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Originally Posted by andlarry
This "power block" has failed
How do you know it's the transformer that has failed? I assume you aren't measuring any AC voltage at the output connector. Are you sure the wiring to the transformer's output connector or to the AC-to-DC rectifier (photo below) isn't bad? Transformers themselves are usually pretty reliable - the wiring & connectors are probably more prone to failure.


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post #9 of 15 (permalink) Old 12-16-2018, 07:35 AM
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Have you tried contacting Precision Components, Inc or Snap-On for a replacement or equivalent?

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post #10 of 15 (permalink) Old 12-16-2018, 08:12 AM Thread Starter
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@Alfajay , you know what happens when you assume... I bared the wires closer to the cube, no output. I checked my VOM; VAC range on another VAC power cube. VOM works. @papajam , I was told that Snap-On no longer supports this older unit. I did email PCI on Friday. No reply yet.
But, after much searching, I found this: Output - 9VAC 1.5AMP I can cut and splice the proprietary end. Do you think that this is a suitable replacement?
https://www.jameco.com/z/ADU090150-9...r_2227444.html

ALFA ANDY
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post #11 of 15 (permalink) Old 12-16-2018, 08:34 AM
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Originally Posted by andylarry View Post
But, after much searching, I found this: Output - 9VAC 1.5AMP I can cut and splice the proprietary end. Do you think that this is a suitable replacement?
https://www.jameco.com/z/ADU090150-9...r_2227444.html
In a word, yes.
The reason is that there is always a bit of leeway in the primary and secondary voltages used. Not all receptacles will be exactly 120VAC. Could be 115 for example. This would lower the unregulated power supplies output (secondary voltage) to about 8.6VAC. In addition, I'd think that, as mentioned, there could be voltage regulating circuitry on the PCB if the voltage tolerance was that critical that it be held at exactly 8.5.

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post #12 of 15 (permalink) Old 12-16-2018, 08:49 AM
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Another electrical engineer jumping in here.

As others have said, the board has digital logic on it which probably uses 5VDC.
The board most likely has a bridge rectifier circuit on it to convert AC to DC. Some of those big capacitors shown in the picture sit across this DC voltage to remove AC ripples.

It then goes through a linear regulator to step down the 8-10V DC voltage to 5V DC. If you put a higher voltage into the linear regulator, it puts out a little more heat to step down the voltage but it is probably designed to handle 12V max.

So, you should be fine with your 10VAC/2.4AMP transformer. You won't blow anything up trying that out.

And by the way, the 2.4Amp part of the transformer rating is the max current that the transformer can safely produce. If your circuit uses less current, the transformer will only supply that current at the same voltage...

Now back to diagnosing the non working turn flashers on my new 1969 Duetto...
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post #13 of 15 (permalink) Old 12-16-2018, 09:10 AM
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As former electronics engineer, let me mention a couple of observations (before reading ihart's and papajam's comments, both of which I agree with):

1. The circuit board list "Interbalco B930/B940/B945." A user manual (in PDF format) for this type of equipment (B930/B940) is available online here. The user manual mentions on p.3 that "The B930/B940 can be powered by any kind or 12volt battery" and gives instructions for the wiring by mentioning specifically that "either polarity can be used". In other words, any 12V DC power supply capable of supplying 1.5 Amps current will do.

2. The "block" lists 8.5V and 1.5 Amps, which translates to 12.75 Watts power. Since the "block" does not list the output voltage as DC, it must be AC voltage -- and therefore, the "block" is probably a simple transformer, not a power supply. Now, when AC gets converted to DC, the AC voltage needs to be converted from "effective voltage" to "peak-to-peak voltage" by multiplying the effective voltage by the square root of 2 (2^0.5 = 1.4142). Applied to 8.5 V AC, the resulting DC voltage would be 8.5 V AC * 1.4142 = 12.02 Volts DC. This calculation would confirm that (a) the "block" is a transformer, and (b) that a 12 V DC power supply (instead of a 8.5 V AC supply) should work as well.

It appears the input voltage level (8.5 or 9 Volts AC, or 12 Volts DC) is not critical. I'm pretty sure the Jameco 9 V AC power supply would work (but if you have an unused 12 V DC power supply lying around, I'd try that first).

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post #14 of 15 (permalink) Old 12-16-2018, 09:42 AM Thread Starter
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@tubut , thank you for a proper explanation. I would never have thought that I could use a 12VDC power supply when the original is a 8.5VAC.

ALFA ANDY
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post #15 of 15 (permalink) Old 12-17-2018, 01:49 PM Thread Starter
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Thanks to everyone here that helped me solve my problem, especially @tubut . I found an old power cube in my bucket of old power cubes that has an output of 12VDC and 2AMPs. I spliced the cable, and now, it works!
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