Thread: How hard is it to figure out the size of cap and bridge rectifier for a power supply

1. How hard is it to figure out the size of cap and bridge rectifier for a power supply

I have a good sized transformer that is about 60 volts. I was told my caps are to small and I need a larger ones. I will be powering stepper motors and would like to use one cap if possible. What do I need to know to find out the correct model I need or do I just buy based on the size of the vdc?

2. Originally Posted by journeyonline
I have a good sized transformer that is about 60 volts. I was told my caps are to small and I need a larger ones. I will be powering stepper motors and would like to use one cap if possible. What do I need to know to find out the correct model I need or do I just buy based on the size of the vdc?
What were you told was too small about the caps and what size are they? If you have a 60Vrms transformer, I doubt you want to use it to make a supply for most steppers.

There are some rules of thumb, but they all use real numbers....none of which you supplied. Basically, you choose the cap based on the max current you will draw and how much ripple you will tolerate. Of course, the cap has to be rated for the voltage you put to it, or you are gonna remember next time.

3. A 60 VAC transformer will produce approximately 84 VDC when connected to a bridge rectifier. The voltage rating for the capacitors should be 100 VDC. This will give a fair percentage of leeway for voltage spikes.

The capacitance rating for the capacitor(s) is 2000 micro-farad per amp of DC voltage. If your transformer can provider 10 amps, then the capacitor(s) should be 10*2000 or 20,000 micro-farad. This is the minimum.

The larger the capacitance rating, the smoother the DC voltage will be.

The full wave bridge rectifier is rated at PIV or Peak Inverse Voltage. The amperage rating must be higher than the amps rating of your transformer. Many of these rectifiers are rated at 600PIV and 35 amps. You can find 1000 PIV and 50 amps at many surplus shops.

I would invest in a large rectifier to prevent any issues of voltage spikes or excess current draw ruining the rectifier. Mount this device on a heatsink and use a fan inside the powersupply case to keep it all cool.

Hope this helps.

RipperSoftware.

4. There is 2 caps and they are 55 vdc 66 peak. Here is a picture of the power supply if that helps. I want to buy a nice kit but a spare 2k is not sitting around currently for my hobby. Any suggestions or links would be appreciated. I had help from a friend who is much better with electronics and he told me I'm only using one leg and the power supply is currently putting out 30 volts plus.

5. I've saved a copy of the link ger21 provided. It's very good information.

I noticed in your picture the transformer is taged as 22v ct?

Jim

6. I believe the reason you see the 22Vct is the person I got the power supply tested it with a 10 amp load and that is how much voltage is was producing with a 10 amp load. I had another electrical repair friend help me wire up the unit and he said it pushes an easy 30+ volts on one leg. He said it will push double 60+ with both legs connected.

My 3 motors are Vexta 6volt 2 amp nema 34 models.

7. Originally Posted by rippersoft
A 60 VAC transformer will produce approximately 84 VDC when connected to a bridge rectifier. The voltage rating for the capacitors should be 100 VDC. This will give a fair percentage of leeway for voltage spikes.

The capacitance rating for the capacitor(s) is 2000 micro-farad per amp of DC voltage. If your transformer can provider 10 amps, then the capacitor(s) should be 10*2000 or 20,000 micro-farad. This is the minimum.

The larger the capacitance rating, the smoother the DC voltage will be.

The full wave bridge rectifier is rated at PIV or Peak Inverse Voltage. The amperage rating must be higher than the amps rating of your transformer. Many of these rectifiers are rated at 600PIV and 35 amps. You can find 1000 PIV and 50 amps at many surplus shops.

I would invest in a large rectifier to prevent any issues of voltage spikes or excess current draw ruining the rectifier. Mount this device on a heatsink and use a fan inside the powersupply case to keep it all cool.

Hope this helps.

RipperSoftware.

I think 60Vrms is too high for most common steppers. I don't think this is what he has, but hard to tell from the info supplied.

2,000 uF per amp load is the rule of thumb and I would consider it a minimum. It doesn't matter what the transformer can supply as long as it is adequate.

8. My 3 motors are Vexta 6volt 2 amp nema 34 models.

9. I was told I can get another matching 30 volt power supply and run them in series to get my 60+ volts? Is this correct? The only difference is I will have 2x the current or amp?

Thank-you

10. Your current capacity will be the same as one PS, If you series the AC up you will have to make sure of the phasing, but you only need one set bridge & caps, if you series the DC side, you don't need to worry about phasing, but 2 x bridge & caps.
Al.