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Reverse and Overvoltage protection

R

Richard Hosking

Jan 1, 1970
0
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard
 
C

Circuit Breaker

Jan 1, 1970
0
Richard said:
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

If the voltage drop from a diode is a problem, perhaps consider using a
larger (voltage) transformer with a readily available 12-v voltage
regulator like a 7812 chip. The regulator chip comes in several
varieties, and I'm not sure, but I think the 7812 that radio slap sells
can handle an amp an a half. This will keep the voltage at 12v pretty
steady, but you'll need a 15-volts-when-loaded transformer to feed it
properly.

They also have other voltage regulator modules that would provide a high
enough voltage that a few .6 volt drops would be acceptible. May not look
very pretty, but once it's in the case, nobody will know.

You can also do what I just did... I thought I fried my 7812 regulator,
and I found that it hadn't been regulating at all (because the supply
voltage was at 12v when under load, not enough for the chip to regulate),
so in my soldering, I got it backward once, and it got really hot. I went
and bought an LM317 from Radio Stank and have been using it. Now I can
vary my voltage from my new 26 volt transformer from 1.2 up to 35v (35
under no load). Only bad thing here is my box is too small to fit the
larger transformer and 3-inch-square heatsink I need ;-/ But with a large
enough box, it should do quite nicely.

By the way, at the RS, the 7812 regulator is $1.49, and the LM317T is
$1.99. Both are TO-220 cases.

Just my thoughts...
 
S

scada

Jan 1, 1970
0
Richard Hosking said:
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

If you use a "Crowbar circuit" for overvoltage protection, that should take
care of your reverse voltage problem as well. A crowbar is usualy an scr
connected in paralell to the output and is trigered by a comparator circuit
that will output to the scr gate when a reference voltage is exceeded. That
can be as simple as a zener diode! The supply output is shorted resulting in
a tripped breaker or blown fuse! Now if you were to encounter a backfeed in
excess of that crowbar voltage, the supply would trip. If the supply itself
were to exceed the reference it would also trip the supply!
 
M

Mac

Jan 1, 1970
0
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

I saw a really cool reverse polarity protection device in a Bob Pease
streaming video from National's website. I think it was just an N-channel
MOSFET hooked up in series with the ground return, but with the drain
grounded, and the source connected to the negative side of the load. The
gate was connected to the positive terminal of the supply. Like this: (use
courier or similar font)


V+ -----------------------+-------------------|
| |
| |
---- +--+
drain | |source | | load
V- --------------------+ +-----------+ | |
| | |
| +--+
| |
+------+

So when the polarity is correct, the MOSFET is on, and the source-drain
diode is forward-biased. But when the polarity is reversed, the MOSFET is
off and the source-drain diode is reverse biased, so no current flows.

I believe I have it right, although it seems strange to have the current
flowing from source to drain under normal operation. But if you do it the
other way, it won't work because the diode will be forward-biased under
reverse voltage. And that's not what we want.

By the way, this technique is apparently patented.

Mac
 
V

Vincent Himpe

Jan 1, 1970
0
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

Well since you need both : Use a Zener diode.
ASCII schematic ( use fixed width font like a courier to see correctly )


in ----[=====]-----+-------- out
Fuse |
/---
/ \
---
|
gnd ---------------+-------- gnd


Reverse polarity : The zener works like a regular diode. Blows the fuse to
smithereens : load protected

Overvoltage : the zener will conduct creating a short := fuse blows to
smithereens : load protected.

Get a zener diode that can handle 3 to 5 watts. If you need to protect a
load of 12 volts use a 12.1 volts zener. Your load will never seem more
then about 12.3 volts ( there is some tolerance before the zener really
goes 'HARD' into on state. )


Other solution
________
--->|---------| |-------------
diode | 7812 |
|________|
|
|
-------------------+-----------------

This'll get you in the ballpart. The 7812 will create 12 volts at its
output. The diode at the input protects against reverse polarity
drawback you'll need around 15 volts at the input in order for it to work
ok.
if you cool the 7812 you can drain 1.5 amps out of it ( provided it doesn't
need to sink too much voltage : check max power dissipation )

vinnie
 
J

JeffM

Jan 1, 1970
0
I want ...protection for reverse and overvoltage.
I guess a diode would give reverse voltage protection
but the 0.6V drop is a problem.
Richard Hosking

A crowbar-type clamp gives reverse protection with zero insertion loss.

+V +--------+
---|| ||--+------
+--------+ |
|
___|__
/ \
/___\ D1
|
|
___|___
_____
___
 
J

JeffM

Jan 1, 1970
0
I saw a really cool reverse polarity protection device
in a Bob Pease streaming video

V+ -------------+----------+
| |
| +--+
| | | load
V | |
--------- +--+
drain | |source |
V- -------+-|<|-+---------+

when the polarity is correct, the MOSFET is on,
and the source-drain diode is forward-biased.
But when the polarity is reversed, the MOSFET is off
and the source-drain diode is reverse biased, so no current flows.
Mac

Still has a 1-diode drop. FET is more expensive than a diode.
 
R

Richard Hosking

Jan 1, 1970
0
I think the point is that when the supply is correctly connected, the
voltage drop across the FET is across Rds, which should be pretty low
for a decent sized FET (say 0.1 ohms). The drop across this would only
be 100mV at 1A.

Richard
 
M

Mac

Jan 1, 1970
0
Still has a 1-diode drop. FET is more expensive than a diode.

I shouldn't have even mentioned the diode in the forward direction case.
In fact, it might not be forward biased because the drop from source to
drain should be very low. The resistance may potentially be only a few
milliohms for some FET's.

Also, please be careful when "quoting" people. You completely re-drew the
circuit. And though you used indents indicating quoting, you didn't
actually attribute my words to anyone.

best regards,
Mac
 
A

Allan Herriman

Jan 1, 1970
0
On Sun, 11 Jan 2004 20:31:34 +0800, Richard Hosking wrote:

V+ -----------------------+-------------------|
| |
| |
---- +--+
drain | |source | | load
V- --------------------+ +-----------+ | |
| | |
| +--+
| |
+------+

The Mosfet may be destroyed by the first transient that comes along.
Abs max Vgs is usually only +/-20V.

Protection may be as simple as a large value resistor in series with
the gate. (It forms an LPF with the large Cgs.) Use a zener if
paranoia persists.

Also, the OP's "low power rig" probably has a negative earth. A
P-channel FET in the positive lead may be more appropriate than an
N-ch in the negative lead. The adventurous may try an N-ch in the
positive lead with a suitably protected high side driver.

Regards,
Allan.
 
G

GPG

Jan 1, 1970
0
Relay contact

_/
Vin ----------------o-o/ o- Vout
|
|
V
-
|
_|_
Relay |_/_|-
|

0V
 
J

JeffM

Jan 1, 1970
0
and the source-drain diode is reverse biased, so no current flows.
you didn't actually attribute my words to anyone.
Mac

Look again.
 
W

Walter Harley

Jan 1, 1970
0
Richard Hosking said:
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

In addition to the other suggestions: consider using a Schottky diode.
Lower forward drop (around 0.4v or a bit less). Disadvantage is relatively
low reverse voltage tolerance, perhaps 40v depending on which one you
choose. Higher reverse voltage means higher forward drop, and vice versa.
 
M

Mac

Jan 1, 1970
0
Look again.

I don't see why looking again should change anything. You kept my
signature, but not there is no attribution at the top.

Even if keeping the signature constitutes attribution, you still changed
my diagram a little bit without making it clear that you did so.

It's no big deal.

Mac
 
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