I needed another power supply. A quick google search tells me I do not want to pay the kinda of money required for such a high voltage high amperage power supply. New solution discovered! I could build a voltage inverter that gives me -12 V from my extra 12 V line. The max draw on the 12 V lines from the PSU is 380W. That should be enough right? New solution = new problem. The voltage inverter calls for a fast switching mechanism to charge a capacitor with a negative voltage. The problem is that it cannot charge it fast enough. The instantaneous power requirements for these pinball solenoids are too much. So you would get maybe a flipper to work once then wait ages for the capacitor to charge enough for another go. I could have built a massive capacitor bank but that’s impractical, not to mention the extremely slow start up time.
There is one solution left to my power issues that does not involve buying a new power supply. That is to build one. For an unregulated power supply I needed: a transformer, some big capacitors, and a rectified bridge. I purchased the transformer for $68 from http://myworld.ebay.ca/johnango. Very good deal. It’s a 24 + 24 VAC 600VA transformer. More than enough for my needs and later on I can use it for other projects. The rectifier bridge is a set of diodes that turn AC into DC power.
There are a few things you need to figure out for a power supply besides what voltage and power you need. The rectified power is not quite DC power. The voltage is not a sinusoid but is an absolute sinusoid. IE. It does not go from positive to negative voltage but instead stays positive going from 0 to 24V and back. This isn’t even the actual voltage you’ll see come out of your power supply. What comes out is the root mean squared voltage which is √2*V. In my case that would be about 34 VDC. This is actually a good thing for me as I looked up the voltage requirements again for the solenoids. The 24 VDC requirement was for the older generation of pinball machines made in the 80’s and early 90’s. I, of course, had the newer generation that required 50 VDC. Hopefully they’ll still work. Should have done my research better.
The power coming from the rectifier bridge needs to be smoothed out to a proper DC power. For this you need some capacitors in parallel to your main power lines. The voltage drop while draining the capacitors between peaks in that absolute sinusoid is called voltage ripple. You’ll never have a perfectly smooth DC voltage but you can get close. Based on a 10A guesstimated constant average current draw and acceptable limit of a 4 V ripple voltage, I calculated that I needed 22 000 uF. Those are some big capacitors. Luckily, there happens to be an electrical engineering section in the building I’m working in that I happen to have friends in. So I manage to get myself an old amp. This is what is left of it after taking some of the parts:
Another thing that you have to take into consideration is inrush current. When the power is first turned on there is a huge inrush current. This is calculated using the equivalent resistance of the transformer. The one I got was had such a low resistance that the inrush current was 422A. You need a rectifier bridge that can handle such a large inrush current. The one I have can handle 400A. Good thing that 422A is a worse case scenario. It’s calculated assuming the resistance stays the same as more current comes in. The magnetic field created at such high currents would limit it below 400A and probably doesn’t even come close. Phew! I couldn’t find a rectifier that could handle any more than that.
IT WORKS! My first mini-electronics project complete! I have hope yet it seems. This power supply pumps out 32 V. The 2 V drop is due to the rectifier bridge. It has 4 diodes in it that take 0.5-0.7 V each. I have some resistors hooked up across the capacitors to discharge them when not in use. Foolishly, I just hooked up the resistors I had a bunch of, 470 Ohm. These started to smoke quite quickly. They were only ¼ watt and were pulling more like 4 W. I then calculate what I needed to keep it under ¼ W and that was 4100 Ohm. In parallel I had two resistors and they needed to be equivalent to the 4100 Ohm needed. So using 1/R1 + 1/R2 = 1/ Rtot, I needed at least 8200 Ohm resistors. Safety factor of 2 and what I had lying around caused me to use 22 K Ohm resistors. Works perfectly. Now I just need to add a heat sink and fan to cool the rectifier bridge when max current is going through it.
Tested the power supply with on of my smaller solenoids and blew the fuse! Misread the multimeter as it was 0.4 ohm and not 4 ohm like my larger solenoids. This resulted in a tiny little solenoid trying to pull 80 A! Good thing I had a fuse. Had an awful time trying to find new slowblow fuses but now I have a couple 5A slowblows which lets me run just below the max current for the transformer which is about 5.2 A on the 110V side. As for fuses, put them on the high voltage side, because it is much easier (and cheaper!) to get fuses for low amperages for high voltages than the other way around.
Time for power supply test number 2! I hooked up my larger solenoid this time and it works. It’s quite strong and pulls about 5.5 A when cool. As it warms up the resistance increases so it pulls less and less current. So for 5.5A and 32V that makes the solenoids have a power of 176 W! So that’s 76% more power than your average 100W light bulb. Not too bad.
There is one solution left to my power issues that does not involve buying a new power supply. That is to build one. For an unregulated power supply I needed: a transformer, some big capacitors, and a rectified bridge. I purchased the transformer for $68 from http://myworld.ebay.ca/johnango. Very good deal. It’s a 24 + 24 VAC 600VA transformer. More than enough for my needs and later on I can use it for other projects. The rectifier bridge is a set of diodes that turn AC into DC power.
Basic Unregulated Power Supply Circuit
There are a few things you need to figure out for a power supply besides what voltage and power you need. The rectified power is not quite DC power. The voltage is not a sinusoid but is an absolute sinusoid. IE. It does not go from positive to negative voltage but instead stays positive going from 0 to 24V and back. This isn’t even the actual voltage you’ll see come out of your power supply. What comes out is the root mean squared voltage which is √2*V. In my case that would be about 34 VDC. This is actually a good thing for me as I looked up the voltage requirements again for the solenoids. The 24 VDC requirement was for the older generation of pinball machines made in the 80’s and early 90’s. I, of course, had the newer generation that required 50 VDC. Hopefully they’ll still work. Should have done my research better.
Rectified Voltage vs Smoothed Ripple Voltage
Ripple Voltage Equation where 'Vpp' is peak to peak voltage, 'I' is the max current requirement of your system or the current you are designing to, 'f' is the frequency of the AC power (usually 60 Hz or 50 Hz) and 'C' is the capacitance of your resevoir capacitor.
The power coming from the rectifier bridge needs to be smoothed out to a proper DC power. For this you need some capacitors in parallel to your main power lines. The voltage drop while draining the capacitors between peaks in that absolute sinusoid is called voltage ripple. You’ll never have a perfectly smooth DC voltage but you can get close. Based on a 10A guesstimated constant average current draw and acceptable limit of a 4 V ripple voltage, I calculated that I needed 22 000 uF. Those are some big capacitors. Luckily, there happens to be an electrical engineering section in the building I’m working in that I happen to have friends in. So I manage to get myself an old amp. This is what is left of it after taking some of the parts:
A former, very old audio amp.
One last quick thing.
You need to use the right size of wire so it can handle all the
current. My max current draw is 25 A for a 24 V 600VA transformer so
I need 15 AWG wire. Remember to use the 'chassis wiring' requirements, not power transmission. The 15 AWG requirement for my use is still over the top because the power I require is instantaneous and not constant. Also, there is a large safety factor put into the AWG requirements. I'm using 18 AWG, mainly because that's what I've salvaged from those broken PC power supplies.
Everything has been
calculated and I’ve got all my parts. I just need to build it.
My first homemade power supply and electronics project.
IT WORKS! My first mini-electronics project complete! I have hope yet it seems. This power supply pumps out 32 V. The 2 V drop is due to the rectifier bridge. It has 4 diodes in it that take 0.5-0.7 V each. I have some resistors hooked up across the capacitors to discharge them when not in use. Foolishly, I just hooked up the resistors I had a bunch of, 470 Ohm. These started to smoke quite quickly. They were only ¼ watt and were pulling more like 4 W. I then calculate what I needed to keep it under ¼ W and that was 4100 Ohm. In parallel I had two resistors and they needed to be equivalent to the 4100 Ohm needed. So using 1/R1 + 1/R2 = 1/ Rtot, I needed at least 8200 Ohm resistors. Safety factor of 2 and what I had lying around caused me to use 22 K Ohm resistors. Works perfectly. Now I just need to add a heat sink and fan to cool the rectifier bridge when max current is going through it.
Tested the power supply with on of my smaller solenoids and blew the fuse! Misread the multimeter as it was 0.4 ohm and not 4 ohm like my larger solenoids. This resulted in a tiny little solenoid trying to pull 80 A! Good thing I had a fuse. Had an awful time trying to find new slowblow fuses but now I have a couple 5A slowblows which lets me run just below the max current for the transformer which is about 5.2 A on the 110V side. As for fuses, put them on the high voltage side, because it is much easier (and cheaper!) to get fuses for low amperages for high voltages than the other way around.
Time for power supply test number 2! I hooked up my larger solenoid this time and it works. It’s quite strong and pulls about 5.5 A when cool. As it warms up the resistance increases so it pulls less and less current. So for 5.5A and 32V that makes the solenoids have a power of 176 W! So that’s 76% more power than your average 100W light bulb. Not too bad.
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