# 27 VAC to 32 VAC?

• posted

Hello all,

I am a lurker novice with a few questions. I have a transformer (specifically out of a pinball machine) that outputs 27 VAC, and it is fused for 8 amps in the circuits it supplies. I would like to change the output to 32 VAC, for a circuit fused at 0.25 amps. Is this technically feasible, or is my novice understanding currently embarrassing me (without me knowing :-)? I have "cut and pasted" the material below from a previous post. Is this technology relevant to my question? If so, would I incorporate this chip into a homemade circuit board with other devices to plug in line to the

27 VAC output? I am capable of the tech/soldering/schematic reading, but I am way short on the engineering.

>National Semiconductor have a handy online tool to help you use their

>>ICs
• posted

Hi, Dan. The National Semiconductor ICs are for converting a DC voltage to another DC voltage. You've got AC, so your problem is different.

If your transformer is rated for 27VAC at an 8 amp load, the voltage will be quite a bit higher if it's unloaded or lightly loaded (1/4A is just 3% of the transformer rated load). You may already be close enough -- you should try it and see.

If it's not high enough, though, you'll have to do something else. The easiest way to vary the secondary voltage in a transformer is to vary the primary voltage. That would mean you have to bump the primary voltage up to 119% of nominal (about 142VAC for 120VAC nominal primary). That's too high -- the transformer isn't meant to work at that high of a primary voltage. However, as noted above, the secondary will almost certainly be high for your light load, and you might be able to get away with 10% overvoltage or less. That would be acceptable. You can do this with an autotransformer.

If this isn't acceptable, you can add the secondary voltages of two transformer secondaries. You'd need to find a second transformer with a secondary voltage around 5VAC, and connect the two secondaries together to get your 32VAC, like this (view in fixed font or M\$ Notepad):

| | L1 _ 27VAC _ | o----o---o_/ \\o--. ,---------o_/ \\o-----o | | FU1 )|( FU3 | L2 | )|( | o-o--)-----------' '--. | | | T1 | 32VAC | | | _ | | | '---o_/ \\o--. ,--' | | FU2 )|( | | )|( | '--------------' '--------------------o | T2 | 5VAC | (created by AACircuit v1.28.6 beta 04/19/05

Be sure to measure the output voltage. If your output voltage is more like 22VAC than 32VAC, it's bucking instead of boosting -- reverse the leads of the second transformer.

I hope this has been of help. Feel free to post again if you have any other questions.

Good luck Chris

• posted

I agree with chris in that your needs as far as what you are running, can still operate on the 27 vac. What is the circuit you intend to run that is needing 32vac?

• posted

Hello James,

thank you for your attention to my queries. I am wishing to build a test device for pinball machine circuit boards, for a particular series of electronic pinballs. Within this series there were a number of iterations of score displays, used in the games. The transformer I have does not provide the necessary voltage for the newer type of score display I would wish to test. The new type of display in question requires 32 VAC; this voltage is subsequently (on the score display pcb) rectified and "boosted/bucked" (I hope I have those terms correct) to -45 VDC and -15 VDC respectively. These voltages are then used by the digit driver ICs on the pcb. I am sorry for the long-winded answer; I felt some background was necessary. You may be correct, the 27 VAC may be enough; I just haven't built the thing yet to test. What do you think? I will also add more material in response to Chris's post above.

Regards, Dan

• posted

Hello Chris,

thank you for your thoughtful and informative message. It is appreciated. After reading it I went back to look at the schematic for the transformer I own. As you may or may not know an electronic pinball machine transformer has a number of secondary windings; these provide the different but necessary voltages for a pinball to function. For the project I am contemplating (outlined in my message to James) it turns out there will be both a 3 and 6 VAC windings I will not need. Would it be possible for me to "stack" one of these windings on the existing 27 VAC winding, (should it be necessary) and hence make my life easier? If so, could you draw me one of those cool schematics as you did before, if it would be different? I have the first one saved as an M\$ Notepad file.

Regards, Dan

• posted

Hi, Dan. There are several variables to take into consideration. Here's the drill:

Get a 100 ohm 10 watt resistor. Hook up your 27VAC transformer to the line, and measure the output voltage. Then briefly attach the 100 ohm resistor, and measure the output voltage again.

If both of thee readings are over 30V (they probably will be), then you should be OK using the transformer as-is. All electrical equipment is made to tolerate variances of at least +/-10% in line voltage. If you're within that, you're OK for a repair-type quick&dirty test fixture.

To be safe, you may want to take variations in your line voltage into consideration. As I said, if you use a Variac or autotransformer, you should be able to crank input voltage up to 132VAC or so without causing the transformer any grief. That way, you'll be able to provide precisely 32VAC to your test fixture every time. That's what I'd do.

Good luck Chris

• posted

Hi, Dan. Assuming your 3VAC secondary isn't connected to the 27VAC secondary winding, you should be able to boost with the additional 3VAC like this (view in fixed font or M\$ Notepad):

| | L1 _ 27VAC _ | o----o---o_/ \\o--. ,---------o_/ \\o-----o | FU1 )|( FU2 | L2 )|( | o----. )|'--. | | )| | 30VAC | | )| | | | )|,--' | | )|( | | )|( | '-----------' '--------------------o | 3VAC | | (created by AACircuit v1.28.6 beta 04/19/05

This assumes you have two separate leads for the 3VAC that aren't connected to the 27VAC. Use an ohmmeter to check there's no continuity. After you hook it up, be sure to check that you are boosting the voltage. If it looks more like 24VAC, try reversing the leads on the second transformer.

This should be close enough for a quick&dirty test with no problem.

If you're interested in details on a particular pinball machine, try rec.games.pinball.

Good luck Chris

• posted

Hi Chris,

thank you again for your messages. Although I am pretty sure the 3VAC and the 27 VAC are all separate, I will check with the ohmmeter to confirm. After studying your diagrams I have to ask: What do the FU1, FU2, and FU3 icons/symbols signify?

I will also do your 100 ohm, 10 watt resistor test, as well. I certainly do not have to make this any more complicated than it already is.

Actually, I am quite familiar with rec.games.pinball (RGP). I lurk and post regularly over there. My interest is Gottlieb System 80 games; pretty much a black sheep in the eyes of most RGP contributors. I am interested in making a test fixture for these games, just as an extension of my hobby. I will keep my day job :-).

Regards, Dan

• posted

Ok, FU equals fuse. Duh. Time for bed...

Regards, Dan

• posted

Hi, Dan. FU means fuse. You can use a circuit breaker instead, if you want. Be safe.

The Gottlieb El Dorado was a favorite in the days of my misspent youth. I believe that was about 5 years before the System 80 series started, though. I kind of liked Black Hole, although it was a little to busy for me. I guess it wouldn't matter much to a pinball wizard, but I started losing interest as the displays started getting busier. Sensory overload, I guess.

Cheers Chris

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