Re: reverse polarity protection

On 19 Apr 2006 09:08:48 -0700, "Jenalee K." <jenaleek@xxxxxxxxx>
wrote:

Hi,

I tried to make a MOSFET reverse polarity protection for a 24V power
supply. The MOSFET I had available has a Vgs max of +/-20V so I came up
with the design below (I only had a 7V5 zener). The OK LED serves as
power indicator, the other LEDs are test load.

When the power supply is connected correctly it works as expected
(measured Vgs of -7V4). But when I reverse the polarity the red LED
goes on and I measure a Vgs of -3V6. How is this possible?

Thanks,
Jenalee K


IRF6215
.------o------+^+------.
| | ||| |
| z === |
| 7V5 A | |
| | | |
| o--------' .--o--.
| | | |
| OK V -> green V -> - red
/+\ - led - ^ -> led
24V ( ) | | |
\-/ | '--o--'
| | |
| .-. .-.
| | |1k5 | |2k2
| | | | |
| '-' '-'
| | |
'------o---------------'

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

See "OverAndReverseVoltageProtection.pdf" and
"PerfectDiodeForChargerIsolation.pdf" on the S.E.D/Schematics page of
my website.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

Old Latin teachers never die...they just decline
.