Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Ac step down

Status
Not open for further replies.
It doesn't sound very crucial then, the normal way is to use a higher voltage transformer (say 9V) and a 5V regulator IC such as a 7805 to give a regulated 5V output. In your case it might be a good idea to feed the amplifier from before the regulator to get more power from it, depending on what it is?.
 
hi fahime,
Sorry if my remarks about safety offended you, that was not my intention.
I am sure that when you give someone advice about a subject, you will feel
its your duty to caution them about the safety hazards. You understand,
I do not know your level of experience when I give you advice.

Anyway back to business.

As I understand, you want to generate +4 volts dc, with a current output
maximum of 100mA from a 220v 50hZ mains supply.

Your idea of a bridge rectifier and smoothing capacitor are fine, but if you
require a regulated +4V, you need a voltage regulator. I would suggest
a LM317K the 1 amp version, this an adjustable voltage regulator from about
0v thru +25volts. Get the datasheet and example circuits.

The problem is most voltage regulators require their input voltage, [ from the bridge] to be at least +3 volts higher than the output voltage.

So for a +4volts regulated output from the LM317 you will require at least 7volts input from your bridge. This means that your transformer needs to
be at least 9v ac. If you cannot get a 9v transformer [its a standard voltage]
you can use a 12v transformer. A transformer rated at 6VA will be more than enough for your project.

Regards
EricG
 
Which amplifier IC are you going to use?
The LM386 has a minimum supply voltage of 4.0V. Its output to an 8 ohm speaker is only 60mW, about like an earphone played loudly. Its max supply current is only 25mA.

With a 6V supply, an LM386 has an output of 200mW to an 8 ohm speaker. The max supply current is 67mA.

With a 9V supply, an LM386 has an output of 450mW. The max supply current is 100mA.
 
hi,
please post a sketch of your circuit and I will try to assist.

There could be a number reasons why the voltage is changing.
Also indicate on your sketch at which point the voltage is changing.

Regards
EricG
 
The MC34119 doesn't work well with a speaker impedance as low as 8 ohms. With a 4V supply its max output is 150mW and its dissipation is 400mW. So its max battery current is 138mA. With a 32 ohm speaker its max output is also 150mW but its dissipation is reduced to 120mW. Its max battery current is 68mA.
 
I don't understand why a melody generator should require an AC voltage.

Use a 6V power brick AC adaptor and an LM317 to get 4VDC, there is no reason why it should be any more complicated than that.
 
ericgibbs said:
hi,
please post a sketch of your circuit and I will try to assist.
I tried to send the schematic, but it failed. Anyway, after the transformer, I have a diod bridge, a capacitor and then it goes through the LM317. There is a 1Kohms between ADJ and output and a 2.2Kohms from Adj to ground. the output is connected to a melody generator and the signal goes to an amplifier and then to a 8ohms speaker. That's it.
 
The LM317 needs a minimum of a 10mA load or a 120 ohm resistor between ADJ and output or its output voltage will rise on some of them. Then the resistor from ADJ to ground will need re-calculation.
 
audioguru said:
The LM317 needs a minimum of a 10mA load or a 120 ohm resistor between ADJ and output or its output voltage will rise on some of them. Then the resistor from ADJ to ground will need re-calculation.
Thanks, The problem was that.
 
audioguru said:
The LM317 needs a minimum of a 10mA load or a 120 ohm resistor between ADJ and output or its output voltage will rise on some of them. Then the resistor from ADJ to ground will need re-calculation.
Dear Audioguru,
I read the datasheet of LM317. But I can't figure out how you undrestood the resistor between Adj and output should be 120ohm. Could you please explain it for me? It would be a good experience for me.
 
Fahime said:
Dear Audioguru,
I read the datasheet of LM317. But I can't figure out how you undrestood the resistor between Adj and output should be 120ohm. Could you please explain it for me? It would be a good experience for me.
The datasheet specifies "a minimum load current". Without a load then the current in R1 and R2 sets the minimum load current.

Many people see the schematic on the first page of the datasheet with the 240 ohm resistor for R1 but don't notice that the IC is the more expensive LM117 with half the minimum load current of the less expensive LM317.

The minimum load current is spec'd with a Vin - Vout voltage of 40V and is less with less voltage but it doesn't cost any more to make R1 always 120 ohms with an LM317.
 

Attachments

  • LM317.PNG
    LM317.PNG
    37.8 KB · Views: 211
audioguru said:
The datasheet specifies "a minimum load current". Without a load then the current in R1 and R2 sets the minimum load current.

Many people see the schematic on the first page of the datasheet with the 240 ohm resistor for R1 but don't notice that the IC is the more expensive LM117 with half the minimum load current of the less expensive LM317.

The minimum load current is spec'd with a Vin - Vout voltage of 40V and is less with less voltage but it doesn't cost any more to make R1 always 120 ohms with an LM317.
Thanks alot. It was really helpful.
 
audioguru said:
Many people see the schematic on the first page of the datasheet with the 240 ohm resistor for R1 but don't notice that the IC is the more expensive LM117 with half the minimum load current of the less expensive LM317.
Which really annoys me, they shoud change the datasheet to show a 120:eek:hm: resistor!
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top