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#1
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unregulated or regulated power supply
Hi I'm going to be running uirobot stepper drivers with 4 amp motors, the problem is I cant go over 40 volts and my Supply power fluctuates between 230v and 247v, so if i get a toroidal for the higher voltage and it then goes down to 230v my rectified dc will probably be too low, so would I be best using a switching supply and would 9.7 amp be sufficient....Thanks
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#2
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Andrew, you're describing a fluctuation of less than 7.4%, or less than 3 volts at the secondary. I think you'll be fine with a supply of 36v to 39v. (or 32v to 35v for that matter). Go unregulated.
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#3
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With an AC transformer rated at 28 VAC, you would get 39.5VDC when your power was at 247VAC and you would get 36.8VDC when your power dropped to 230VAC.
That would work just fine. I often run my power supply as much as 25% less than the rated value and still get very good speed and torque. |
#4
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Hi Andrew, I went to my local Toroidal Transformer man and explained to him what I want. To make sure I don't exceed the voltage of the UIM Robot drivers he made me a transformer.
I cannot remember the exact terminology but het explained that usually when one asked for a 28 volt transformer, they make it a bit higer voltage to compensate for the drop in voltage once you draw current (so that when it is fully loaded, you still get 28 volt). This one he made me is 28 volt , but does not have the extra to compensate for the voltage drop under load. I took this transformer, put it through a bridge rectifier and then 3 x 68000 uF caps. Works like a dream for me. |
#5
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power supplies
Thanks for the replies, its just this that concerned me...........If using an unregulated power supply, check peak voltage across output before connecting Driver50 board. A 40V unregulated supply can have 56V peak unloaded output.
Last edited by andrewuk; Tue 18 January 2011 at 12:42.. |
#6
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Andrew,
There may be a misunderstanding about power supplies. AC voltage is measured differently than DC voltage. Here in the United States, we have 120VAC, which is really about 170 Volts peak to peak (half sine wave) or 340 Volts (full sine wave). AC voltage is measured using the RMS (Root Mean Square) value of a half sine wave. DC voltage is measured at it's highest sustained point (peak voltage). An unregulated DC power supply with an adequate capacitor gives the equivalent of the AC voltage's half-sine wave peak to peak voltage. We use the equation: SQRT(2) * RMS voltage of the AC transformer's output = the DC voltage. A 28VAC power supply * SQRT(2) = 39.5 VDC. The quality of transformer might alter that figure a little (number of turns). The input rating of the transformer must also be accounted for. If the transformer is rated for 240VAC input, it will deliver slightly less voltage than a transformer that is rated for 230VAC input. The main point is that you match the input voltage of the transformer as close as you can to your power source. Then you multiply the desired DC voltage by 1 / SQRT(2) to find what the transformer's AC output voltage should be. For example: 40 Volts DC X (1 / SQRT(2)) = 40 Volts DC X 0.7071 = 28.28 VAC. ------ Remember that there are three main parts to an unregulated power supply: 1. The transformer which changes the level of the AC voltage. 2. The bridge diode which turns a full sine wave into a half sine wave that has 2X the frequency of the full sine wave (120 cycles per second instead of 60 cycles per second here in the United States). 3. A filter capacitor which stores power between the half-wave sine wave's cycles so that there is almost no AC ripple. That's how AC power becomes DC power. A properly sized filter capacitor will keep the output voltage at +/- 5% of that desired voltage. |
#7
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Mike, I think you are missing the point made by Kobus and Andrew; the output voltage of a transformer is influenced by the load on the transformer. There is a small voltage drop as the load increases. The suppliers of transformers typical spec their transformers at full load....which means that their output voltage at no load is somewhat higher than what they state in their spec.
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#8
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That's exactly what two suppliers have just said Gerald, and they want three times the price of a switching supply. They wouldn't be a problem if I had gone for higher voltage drivers, although I can't complain about the price of uirobot if they perform well.
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#9
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Andrew, I am running 5 x UIMRobot 4 Amp drivers.
My simple Power Supply is a toroidal transformer, bridge rectifier and 3 x 68000uF caps. I am sure it is cheaper than a switching P/S I have a Ammeter in the circuit. With all the motors working I draw max 5 Amp. Maybe you can get away with a smaller cheaper transformer. |
#10
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I just ran an experiment on my test bench to see what voltage effect there would be between no load and a standard four-motor heavy load.
I set the voltage to 40VDC with 4 PK299 size motors, rated at 4.5A each, running at various loads and speeds (voltage did not fluctuate more than 1 v whether the motors were at standstill, running slowing, or running at maximum speed). Then, I disconnected the power supply from all loads and measured the voltage. It was 42VDC. That's all, just a 2V difference. The transformer is a 500VA 35VAC toroidal unit. The bridge and capacitors are furnished by a PMDX-8050. I used a Variac in front of the transformer to adjust the voltage to 40VDC. That power supply typical of an unregulated toroidal supply. (Keep in mind that salesmen often spout nonsense if they can make a few extra dollars by selling you a high-priced switching power supply.) |
#11
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Thanks for running the check Mike. That 500VA transformer wasn't near full load with those 4 motors, my guess is they were drawing less than half of the 500VA capability. If you had used a 300VA transformer for the same test, you may have seen 3-4 Volt drop/rise, but even that isn't a reason to use a regulated supply.
(I remember this story, with a much smaller transformer) |
#12
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I just re-ran the test with a 20V 250VA transformer that normally puts out 28VDC.
Under load, with those four PK299 size motors, I was able to turn up the voltage to 39VDC by cranking the Variac up all the way. With the motors disconnected, the voltage was 42.5 VDC. If we think of voltage as pressure and current as volume and compare it to a water hose, it's easy to see that higher pressure at low volume isn't inherently bad and low pressure and higher volume isn't bad, but high pressure and high volume can cause problems. The fire department uses a pumper truck to boost the pressure as high as possible while shooting out as much volume as the hose can handle. In electronics Voltage X Amps = Watts. 44 V at 0.25 A = 11 Watts. 38 V at 5 A = 190 Watts. A lightly loaded transformer may have higher voltage, but with little current, it isn't going to produce much heat. Most electronic components are rated at a particular voltage because the designer knows that they will pull a specific amount of current. Volt X Amp = Heat. Heat melts delicate junctions and components fail. In a stepper driver, the components are rated to handle full current at full load (usually with the use of a heat sink). If the voltage goes higher and the current remains at it's maximum, too much heat is produced and the stepper driver fails. However, in an unregulated power supply, as the current increases, the voltage drops and the heat remains within tolerance. Let's remember the discussion about running PK296A2A-SG7.2 motors at more than 39VDC. Those motors are rated at 1.5mH and 3A by Oriental Motor. Using Mariss's formula, that means that the maximum power supply that should be used is 39VDC; however, at 39VDC, those motors are still running cool. Some users have run those motors at 45VDC and even higher before reaching the magical 80 degrees C. The reason is that those motors are really rated at 4.5A by Oriental Motor, but de-rated to 3A because of the gearboxes used. 39V X 3A = 117W and 39V X 4.5A = 175W. Turning it around, 175W / 3A = 58VDC. So, in theory, you could use a 58VDC power supply with those motors running at 3A and not produce any more heat than you would get at 39A when the motors were pulling 4.5A. |
#13
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Thanks everyone for the advice,and thank you mike for doing the tests.Ill go for a 27volt toroidal
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