pls help, what value I need for power supply component

[Logic1020]

Sorry this post is so long. I have no formal electronics training and have actually surprised myself how far I have gotten in this project.

I have designed a fairly complex circuit that switches ac as well as having DC and logic components to it. It needs to fit in a small area so I do not want to use a transformer on the board, and since it also needs to switch ac, I do not want to run ac and dc to the board, so I needed a more unusual way to create the different voltages I need.

I designed a power supply circuit that used used a resistor voltage dividing circuit to get ~24V @ 0.6Ato run a standard sprinkler valve, a second to provide ~10v @ 1mA to operate the FET that controls the water valve and a third to provide ~18v @ 0.5A to supply a standard 5V regulator for the logic supply.

All of these voltage divider circuits were using the output of a standard 1.5A rectifier bridge with the standard 120v AC input so all resistor values were calculated with 168v expected output of the rectifier bridge (minus a volt or two from the diodes in the rectifier). After ordering all the parts and assembling the first prototype circuit, my output from the rectifier bridge is only 106V on the prototype board and 113v on a spare rectifier bridge. This huge change from what was expected caused problems in the circuit and needed to be addressed, but I was also afraid that if I start making bunches of these I will run out of bridge rectifiers and have to get replacements that will actually put out the ~168V I originally expected.

I have decided to re-design the power supply section to replace the resistor voltage divider circuits with a single resistor and Zeener diode arrangement to provide a set 24V or 30V signal that then uses one FET or BiPolar transistor to provide a 24V output that I can then use for all the other circuits, same principle as the first cicuit in this post except I don't need an exact voltage and I haven't been able to find voltage regulators that can handle ~170V input so I would use a resistor & zeener instead of a voltage regulator. This will greatly simplify the circuit and save money also since just the most expensive resistor was $12.00 by its self.

I tested the theory with this FET and it worked ok at low voltages with a 100mA load but when I increased the voltage the FET started getting very hot and I don't think it could handle the 1.1A @ 24V I need. I'm not sure if my problem is the voltage/current (specificlly the high voltage from dropping from 106V (or 168V) down to 24V) is out of the safe operating area or if the power rating is being exceeded because of the 24V @ 100mA

What I need is help understanding what minimum values I need in order to select the correct FET or BiPolar transistor for this supply circuit. My output requirements are 24V @ 1.1A with an input of up anywhere from 100V to 170V. I could also use two of them broken down to one 24V 0.6A supply and one 10-15V 0.5A supply.

I looked at this part I think would work at a 106v input, but it looks like it would be out of spec for a 170V input.

Thanks in advance for the help!!!

[pebe]

Apart from saying that you want to switch AC, you have not said what your are aiming to do. You are currently dissipating over 10W in heat in the resistors just to drop the supply down to the voltages you want and there are probably much better ways to do that in a small space.

Could you post your circuit and outline what your project is, then we can probably suggest something.

[I_Daniel]

I agree with Pebe.

Apart herefrom you are playing very dangerously by placing a voltage divider across the mains. The safest is to use a transformer which will isolate the mains and give you the desired voltage(s).

To place the voltage divider across the mains will give you the required voltage, but this is at virtually no load since the VOM (Volt/Ohm/Amp) meter could have a 10 to 50 Meg. Ohm input impedance which is thus drawing micro amps. i.e the no load condition. You will therefore have to adjust the first resistor in the chain by calculating the current draw of the load.
Since such a system is wasteful of electrical energy and does hold a fire hazard it is not recommended.

You must check the specs for the FET to make sure it can handle the load. You will, if it is a Power FET, have to mount it on a heatsink.

[Logic1020]

I'm sorry, apparently I was not clear.

I don't want to use the resistor divider I was using.

I want to use a FET or a Bi-Polar Transistor to drop the voltage but only after the bridge rectifier. the only thing attached to the mains is the bridge rectifier and 2 IC chips that are designed to work with mains voltage.

I am looking for a component that will allow only as much voltage to pass as I need based on the voltage present at the Gate or Base please see attached schem....

All I need help on is how to pick the FET or transistor. do I need to look at the voltage drop from 168 to 24 for the voltage? or just the 24V it is outputting at the 1.1A?

[pebe]

You do not 'allow voltage to pass', you allow current to pass.

Regarding your circuit.
1. Your full wave rectifier is incorrectly drawn.
2. You cannot ground the output of a bridge rectifier fed from mains supply.
3. By dropping 100 volts at 1.1A your transistor would be dissipating 110watts !

This is a dangerous and impractical way to reduce voltage.

[I_Daniel]

As Pebe says this is a dangerous and impractical design.
I personally think you should scrap the idea but will nevertheless give the following comment.

First the errors:
1. Your two diodes on the left hand side of the bridge rectifiers are drawn the wrong way round.
2. You don't have a smoothing capacitor after the bridge so you cannot get the peak value of about 170 volts.
3. Using a 30 Volt Zener will give you a power supply of about 29.4 Volt and not 24 Volt.

In a discrete component design I always recommend the use a 63 Volt capacitor across the Zener diode. This makes the power supply a slow start supply - no excessive surge thumps - and it also gives extra electronic smoothing. ( i.e MFD x Gain of FET or transistor)

The FET must be able to handle at least 200 Volts.
The power handling capability will at a minimum be the voltage drop across the FET times the required Amps. That is The FET or Transistor must drop almost 140 Volt across it so it is a case of 140 Volt x 1.1 Amps = 154 Watt minimum that the FET must handle and a large heatsink will thus also be required.

There are many small transformers commercially available of a reasonably small size that will give you 1 amp at 12 or 24 volts. Actually a 20-24 volt transformer will be ideal for the available DC voltage will be about 28 - 34 Volt which is ideal for using a 7824 IC Voltage regulator. The combined cost of the IC Regulator and the transformer will in all probability cost less than the very high power FET or Transistor that you will need.

[Logic1020]

I_Daniel,

Thank you for the help!! I would have never thought of a cap across the Zener...

I am space limited to a 4" circle and I have about 80% board coverage now, by removing the large resistors I can get almost 50% of my 4 inch board but that is not near enough room for any 120 to 24 volt @ 1 amp transformers I can find. If I have missed one, please point it out, and better yet, if there is a better way to drop over 100V please let me know.

As far as the power handling requirements of the FET, the one I linked to in my original post (http://www.fairchildsemi.com/ds/FD%2FFDA38N30.pdf) is rated at 300v, 38A, and 312W. It's kind of hard to read but Figure 9. Maximum Safe Operating Area looks like 140V @1.1A is well within the DC region. They are $3.83 each from Mouser and I already have 4 from another project.

Thanks!

[I_Daniel]

If you have a broken down PC power supply then you can use one of the caps from it for smoothing. They are usually 200 Volt and 220 Mfd.

Please remember not to earth your circuit. I assume the earth shown on you schematic is actually the negative line and not an earth connection.

I have seen circuits such as yours especially if you consider a PC power supply, printers and laptop power supplies. Your circuit should work with the FET you have mentioned. Also remember the Heatsink is a must.
I am too much of a scary cat to use such a circuit but switch mode power supplies appear to have taken over in most equipment that used transformers. Of course these power supplies do give isolation from the mains. Oh and remember to put a fuse in the circuit.

From the 110 Volt mains I see you are in America. In South Africa, where I am, we have a much more dangerous voltage 230 Volts. This also means I cannot help you much with where to buy a small transformer. However I see people are many times referring others to Radio Shack for transformer and spares.

[pebe]

I think your safest and best bet is to use a switch mode supply.

This link shows the size of a 12V 2A plug-in unit. If you look around on Ebay you will probably find a 24V one of equivalent size. If you remove the components and the PCB from inside they would probably fit into your available space.

http://www.ebay.co.uk/itm/Zoostorm-Free ... 1e6307e32a

[I_Daniel]

@ Pebe - Now that is the best advice and again much safer.

@ Logic1020:
I know you have all the components for your project but if it blows up then consider Pebe's advice, because as he previously said it is a hazardous/dangerous design to use a voltage regulator direct on the mains.
Looking at the link I see that the switchmode power supply is built into the mains plug. If it is too large to fit into your available space then I suggest you just snip of the DC leads of the power supply and connect them to your 24 volt DC control circuit.

If this circuit is switching relays or triacs or thyristors on and off then a 12 volt control system would probably be in order.

[I_Daniel]

@ Pebe

Just one question - are such blatant free advertising as the foregoing permitted on this forum ? He merely quotes the original poster as if it is his own and then sends you to his business link.

[pebe]

@I_Daniel,

The posting was made during the night (at least, it was night in the UK), and your reply was also before I could do anything about it. But I have now removed the posting as I do with all postings that are trivia with a blatant hidden advert.

Geminideal and others, please take note!

[I_Daniel]

@ Pebe

Sorry I appeared so hasty. I forgot the time difference - that is that 09:00 in South Africa is 07:00 in Scotland.