Bridge rectifiers

[patty3]

I want to build a low rpm PMG, but first I am building a transformer to become a 12 volt DC power source. I bought some 1000 volt, 50 amp bridge rectifiers for the PMG and the transformer. For my power source I want to take 120 volt AC and go down to 12 volt DC at 10 amps. Does it matter what size rectifiers are used for amps wanted? In other words, does it matter what voltage or amperage the bridge rectifiers are rated at, whether I want to end up with 1 amp or 50 amps at 1 volt or 1000 volts? Also how much voltage/amperage drop would there be between the secondary transformer coil and the DC out put after the rectifiers?  

[commanda]

The ratings of diodes (of which a bridge rectifier has four) are maximum ratings. A 50 amp bridge will work quite happily at 1 amp. You normally need to leave some headroom when specifying components. Don't run your 50 amp bridge at 50 amps continuously. The voltage rating is also a maximum rating. Your peak voltage is the rms voltage multiplied by the squareroot of 2 (1.414). Again, you need to leave headroom. So your 1000 volt bridge is only good for 700 volts rms.

The voltage drop of diodes is normally quoted as 0.6 volts (giving 1.2 for your bridge). However, in reality, it does vary as a function of the current. If you get the part number of your bridge, and do a google for "partnumber datasheet" (without the quotes) you should find the datasheet. It will have a curve of voltage drop vs current.

And more than several amps, you will need a heatsink.

HTH.

Amanda

[Flux]

Patty3

Amanda has covered the rectifier bit. You don't say what you want the power supply for. As it stands the output will have a lot of ripple which may or may not matter.

This may show itself as a volt drop, depending on your application. The peak volts will be about 17 and on load it will fall to a mean of about 10v.  The drop in the transformer depends on its rating, but if it is properly rated it will be of the order of 10%.

If you are using it to replace a battery, it will depend on the type of load whether it will work without smoothing.

Flux

[patty3]

I want to power a 12 volt pump for a pressurized water system. The pump is rated at about 8 amps. The pump should not be running more than 60 seconds at any given time and in most cases it will run less than 30 seconds. My transformer is not very big, but I think I can get 10 amps out of it. I don't think this could be continuouse output and so I am wondering if this could be calculated for duty cycle? My understanding of electricty is not great, but is getting better since I started comming to this site. I think I understand amps, but was wondering about volts. When powering an electric motor, which is worse; Too low amount of voltage or too much voltage and is there a percentage + or - that is OK. Thanks for all the great information!  Pat!

[TomW]

Patty3;

If it was me I would put some capacitors on the output from the rectifiers [diode]. This will have several effects. The main one I am thinking of is increased startup current capacity for the pump. It will also help hold the voltage up at all times.

I think, in general, you can say the speed of a dc motor is a function of the voltage and the torque is a function of the current. Of course they both need to be there in order to get any useful work from the motor. How much depends on the motor and its load.

When you first put voltage to a stopped motor it will draw a lot of current compared to after its spinning well.

Cheers.

TomW

[patty3]

Thanks TomW; What size capacitors would I need and exactly how should they be installed?

[nack]

Regarding the transformer:  Transformers are generally rated in VA capacity, VA is Volt-Amperes, and is derived from secondary voltage times secondary current (in amps).  When transformers have multiple secondaries it is the sum of VA of each secondary.  If you are looking to get 12V at 10A, you are looking for a 120VA transformer.  Many times a seller will not spec VA, but rather give primary V, and V and A for each secondary (ie 115:12@8A) - either system will tell you what you need to know about the tranny.  If you are dealing with an unknown tranny and want to get a rough idea of what it could do, try weighing it, and finding a similiar weight in a catalog to guesstimate the VA rating then be conservative, as your unknown might not be quite as efficient as a newer part.  The one you want seems to be 120VA, a quick glance at an 96VA (12V@8A) surplus part shows a weight of 2.5 lbs, so if you have less than that you may run into trouble (too much voltage sag when loaded and/or melted windings).  Also, poke around on hammondmanufacturing.com for a hammond_selection_guide document that shows many different rectified power supply arrangements and what you can expect the circuits to do to the available V and A.

Regarding the rectifiers, follow commanda'a advice to the letter.

Hope this helps.

[TomW]

Patty3;

Gee the size, hmmm,  in my case its usually what I have on hand. At least rated for something like 20 volts, polarized and within reason as many microfarads as you have.

They get wired + on cap to + on bridges, - to -.

If you need some exact values I could do some math to find ideal values but basically filter capacitors from most any dead piece of electronic gear would suffice. Generally known as electrolytic capacitors. The values are not critical although too much capacitance may cause problems on initial startup when they charge up and may draw too much current for your supply transformer. Too little capacitance would be ineffective. I would start with a couple thousand microfarads in capacitors which should do fine and if you have access to more add them in too. My little 12 volt charger I run off my inverter for keeping my "B" bank charged up to run my wireless internet uses a 1 farad cap. This is overkill but I have the cap so I use it. 1 farad caps are pretty expensive in 20 volt range but its a part I had so I use it. 1 Farad is equal to 1 million Microfarads, BTW. or .000001 farads = 1 microfarad. Just so you don't get confused by that as it has happened before here.

Maybe someone with more recent power supply building experience will chime in with some values.

I am pretty comfortable just trying stuff and if it blows I try something different. You may not feel comfortable doing this.

Cheers.

TomW

[Flux]

For 8 Amps you will need at least 10,000 microfarad to have much useful effect.

If you have the capacitors use them, but if you haven't, try without.

Unless the transformer is very under rated there should be no problem starting the pump, its not like using solar panels where the current is limited.

If your transformer is rated for 5A or more it should be fine on this short time rating.

Flux