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From: john on 18 Feb 2010 16:23
SO, the function generator has output of 50 ohm impedance. My be it
can drive the bridge or a single diode.

From: George Herold on 18 Feb 2010 16:35
On Feb 18, 4:15 pm, john <conphil...(a)hotmail.com> wrote:
> Hi,
> Yes the function generator is feeding the bridge. I increased the
> input voltage frequency upto 5000Hz and 50khz and amplitude to 24
> volts peak to peak. And also took the full bridge out and replace with
> one diode and connect a 20kohm as a load and 220uF 25 volts capacitor
> in parallel with the load. I can see the output of 9 volts (volt meter
> mesurement)
>
>  So, now I have half bridge rectifier output but still I am not
> getting the 5 volts output. As soon as I connect the output of the
> single diode bridge to the DC to DC converter the  output of the
> single diode bridge falls from 9 volts to 3.5 volts and the out put of
> the DC to DC converter is 3.5 volts not 5 volts. The ground of the
> fumction generator is connected to ground of the dc to dc converter
>
> John

Hmm, Maybe you are loading down the signal generator? Does the output
change when you attach the DC-DC converter? How much current is the
DC-DC drawing.

George H.
From: pimpom on 18 Feb 2010 16:43
john wrote:
> Hi,
>
> I am using the following function generator to produce a 60Hz ,
> 16Volts peak to peak output voltage.
>
> http://www.globaltestsupply.com/datasheets/4011a.pdf
>
> I am using following DC to DC converter and Bridge Rectifier
>
> http://cds.linear.com/docs/Datasheet/3970f.pdf
>
> http://www.diodes.com/datasheets/ds17002.pdf
>
> The bridge rectifier is hooked up to the function generator and
> the
> output of the bridge rectifier is going to the DC to DC
> Converter's
> pin 3. I am using the ciruit as given on the first page of the
> data
> sheet except I used two capacitors in parallel instead of 2.2uF
> , 50
> volts 10uF and 1uf 50v capacitor at the input input Vin.
>
> So, if the input voltage is 16 volts peak to peak , then after
> the
> bridge I get nearly 6 volts (measured by scope) at the output
> of the
> bridge and I get 3.6 volts (measured by voltmeter) at the
> output of
> the DC to DC converter.
>
> The DC TO DC converter suppose to output 5 volts. but it is
> outputting
> 3.7 volts. I am unable to understand what is the problem. I
> hooked up
> the DC to DC converter to the power supply and it produced 5
> volts but
> with bridge rectifier, it is not producing the appropiate
> voltage. Am
> I using the wrong capacitor values? Please advise!
>
> John

Let's take this step by step. A couple of questions before we
attempt to solve the 'mystery': Did you measure the output
voltage from the function generator before or after you connect
the bridge rectifier and the DC-DC converter to it? Did you
connect any load at the output of the DC-DC converter?


From: Dave M on 18 Feb 2010 17:44
john wrote:
> Hi,
> Yes the function generator is feeding the bridge. I increased the
> input voltage frequency upto 5000Hz and 50khz and amplitude to 24
> volts peak to peak. And also took the full bridge out and replace with
> one diode and connect a 20kohm as a load and 220uF 25 volts capacitor
> in parallel with the load. I can see the output of 9 volts (volt meter
> mesurement)
>
> So, now I have half bridge rectifier output but still I am not
> getting the 5 volts output. As soon as I connect the output of the
> single diode bridge to the DC to DC converter the output of the
> single diode bridge falls from 9 volts to 3.5 volts and the out put of
> the DC to DC converter is 3.5 volts not 5 volts. The ground of the
> fumction generator is connected to ground of the dc to dc converter
>
> John

Your posts are confusing to me. If you have the generator ground connected
to the DC-DC ground along with the negative terminal of the rectifier
bridge, that's wrong... the generator ground should be isolated from the
rectifier negative, otherwise, you have a half-wave rectifier. Your hookup
should look like the diagram below. Is this what you have?

Please view in a fixed-width font such as Courier.


Generator

+------+ DC-DC Conv
| | +-----------+
| | FWBridge | |
| | +----+ | |
| |Out AC| |+ | |
| +-----------+ +-----+-------------|Vin |
| | | | | | |
| | | | | | |
| | | | |+ | |
| | | | -+- | |
| | | | -+- | |
| | | | | | |
| | | | | | |
| |Gnd AC| |- | | |
| +-----------+ +-----+ | |
| | | | | | |
| | +----+ | | |
| | | | |
| | | | |
+------+ | | |
| | |
| | Gnd |
| +--+--------+
| |
+----------------+
|
|
---+---
---
-


--
Dave M
dgminala at mediacombb dot net


From: John Fields on 18 Feb 2010 20:28
On Thu, 18 Feb 2010 13:15:37 -0800 (PST), john <conphiloso(a)hotmail.com>
wrote:

>
>
>Hi,
>Yes the function generator is feeding the bridge. I increased the
>input voltage frequency upto 5000Hz and 50khz and amplitude to 24
>volts peak to peak. And also took the full bridge out and replace with
>one diode and connect a 20kohm as a load and 220uF 25 volts capacitor
>in parallel with the load. I can see the output of 9 volts (volt meter
>mesurement)

---
First off, your function generator has an output impedance of 50 ohms,
which means that it looks like a voltage source with a 50 ohm resistor
connected in series with it, the unconnected end of the resistor being
presented to the outside world as the output of the generator, like
this: (View in Courier)

E1
/
+---[50R]----->E2
|
[E]
|
+------------->GND


With a load connected , the circuit becomes a voltage divider, so we
have:

E1
/ R1
+---[50R]--+--->E2
| |
[E] [xR]
| |
+----------+--->GND


Now, with the circuit you've described, a half-wave rectifier followed
by a reservoir capacitor and a load,:


E1 E2
/ R1 /
+---[50R]-+-[DIODE>]--+------+
| |C1 |R2
[E] [220�F] [20k]
| | |
+---------------------+------+
\
GND

We'd expect, to a first approximation, that:


(E1 * R2) 12V * 20kR
E2 = ----------- = ------------ = 11.97 volts
R1 + R2 50R + 20kR


Subtracting about 0.7 volts for the drop across the diode would make E2
equal to about 11.3 volts, but I suspect that the reverse capacitance of
the rectifier at the frequency you're driving it at is sucking charge
out of the cap, making its terminal voltage fall.

Drop the frequency down to around 60Hz and you should see the voltage
across the cap rise to around 11 volts.
---

> So, now I have half bridge rectifier output but still I am not
>getting the 5 volts output. As soon as I connect the output of the
>single diode bridge to the DC to DC converter the output of the
>single diode bridge falls from 9 volts to 3.5 volts and the out put of
>the DC to DC converter is 3.5 volts not 5 volts. The ground of the
>fumction generator is connected to ground of the dc to dc converter

---
Remembering that there's a 50 ohm resistor in series with the output of
your function generator, whatever you load it with will cause its output
voltage to drop and, eventually, to starve your load if the current your
load draws causes the voltage dropped across the generator's output
resistance to increase to the point where there's not enough left to
supply the load with what it needs.


JF
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