14497	2018-01-09 07:54:46	Ravi Miranda	Reverse polarity protection Diode
Message	Date	From	Subject
Hi, What would be the best way to select an optimal reverse polarity protection diode? This is for use in a VHF RF amplifier, with 13,8v as the input voltage and the current drawn should be 10A. I was considering STPS30M80CG-TR Dual SMT which may be too much for the present circuit. Alternatively, there is VBT3080S-E3. Would 1N5408 x 2 be good enough? or should I consider a FET Si4838DY or FQP47P06? Would something like the FQP or equivalent require the 10v zener from Source to Ground? Source: https://www.youtube.com/watch?v=IrB-FPcv1Dc Thank you for your time. Best 73, Ravi -- I'm here to add more value to the world than I'm using up.
14498	2018-01-09 08:22:28	Roelof Bakker	Re: Reverse polarity protection Diode
Hello Ravi, Have you considered using a P-Channel Mosfet? There is no voltage drop as when using a diode. 73, Roelof Bakker, pa0rdt
14499	2018-01-09 08:57:26	Ravi Miranda	Re: Reverse polarity protection Diode
Hi Roelof, Yes, I have listed those in the original post, (Si4838DY or FQP47P06), but after looking at RSonline (as a an example), I chose the parameters Maximum Continuous Drain Current to be between 10A and 15A, should this be higher? and Maximum Drain Source Resistance between 0.01 ohm and 0.02 ohm is this ok? or should I go higher to 0.3 ohm? trying to keep the price point low. Even if I did choose a FET that was >10A and RDS < 0.3 ohm, would I still need the 6 - 12 v zener from Source to Ground? Sorry about the large amount of questions. Kind regards, Ravi
14500	2018-01-09 09:05:09	Jim Strohm	Re: Reverse polarity protection Diode
Since the protection diode needs to flow only enough current to pop the fuse, the single-cycle maximum current rating for the diode should be sufficient. Hence, if you read your datasheet the right way, a 1N4148 would be more than adequate, assuming a fast blow fuse rated at a few amps. With an input voltage of 13.8VDC, a 1N34 could be adequate.
14501	2018-01-09 09:42:41	John Marshall	Re: Reverse polarity protection Diode
Ravi, Let’s use the information in the video you referenced to evaluate the parts you listed. STPS30M80CG-TR Dual SMT - each diode handles 15A forward current so no problem there. Forward voltage drop at 10A looks like about 0.57V so dissipation will be 5.7 Watts, less depending on your amp’s duty cycle. That’s heat and lost power. The video warns about reverse leakage current in Shottky diodes. This one is spec’d at 40 uA max at 25 deg C, which is fine, but it’s 25 mA at 125 deg C. You would have to run the amp hard enough to raise the diode’s temperature, then reverse the polarity while it was still hot and even then 25 mA might not be a problem. VBT3080S-E3 - Handles 30A and dissipation similar to the STPS30M80CG, heat and lost power. Reverse leakage is good at 25 C but 45 mA at 125 C. 1N5408 x 2 - Maximum forward current for these is 3 A per diode so you would need at least 4 in parallel. Forward voltage drop is 1.2 V so dissipation is more than twice that of the Shottkys. Also, diode’s V/I characteristics aren’t identical so you can’t be sure they will share current equally. FQP47P06 - P-channel MOSFET, can handle 33A. On resistance is 0.026 ohm so it only dissipates 0.26 W at 10 A. Gate-Source voltage is plus/minus 25 so no problem there. This looks very capable. Si4838DY - N-channel MOSFET. Since this is an N-channel device it won’t work in the circuit shown in the video. Regarding the 10V zener, you shouldn’t need it with the FQP47P06 because of its Vgs spec. If you were to use a device with a lower spec, connect the zener between gate and source, as shown in the video. AD5X has published another simple way to protect your amp. It uses a transient suppressor diode and fuses to provide reverse polarity and over voltage protection: < http://www.ad5x.com/images/Articles/Vprotect.pdf >. Hope this helps, John, KU4AF Pittsboro, NC >
14505	2018-01-09 22:11:41	Raj	Re: Reverse polarity protection Diode
Ravi, For 10A use a POLARIZED relay to switch the power on/off. You will need only a low current switch for the relay and the relay will not operate when the polarity is reversed. That protects the VHF amp. This method is used in many commercial rigs! Cheers & HNY Raj At 09/01/2018, you wrote: > > >Hi, > >What would be the best way to select an optimal reverse polarity >protection diode? This is for use in a VHF RF amplifier, with 13,8v >as the input voltage and the current drawn should be 10A. > >I was considering STPS30M80CG-TR Dual SMT which may be too much for >the present circuit. Alternatively, there is VBT3080S-E3. Would 1N5408 >x 2 be good enough? or should I consider a FET Si4838DY or FQP47P06? >Would something like the FQP or equivalent require the 10v zener from >Source to Ground? > >Source: https://www.youtube.com/watch?v=IrB-FPcv1Dc > >Thank you for your time. > >Best 73, >Ravi
14511	2018-01-17 09:16:01	Frank Dinger	Re: Reverse polarity protection Diode
Good advice using a P-Channel MOSFET for polarity protection . I use a P-Channel MOSFET to switch-on an ARDF receiver , when plugging-in the headphones. Ref ARDF rx by DL4CU. Frank , GM0CSZ / KN6WH
14512	2018-01-17 09:16:48	Frank Dinger	Re: Reverse polarity protection Diode
Good advice using a P-Channel MOSFET for polarity protection . I use a P-Channel MOSFET to switch-on an ARDF receiver , by plugging-in the headphones. Ref ARDF rx from DL4CU Frank , GM0CSZ / KN6WH
14517	2018-01-17 20:32:14	mosaicmerc	Re: Reverse polarity protection Diode
Have a look at my project here: https://hackaday.io/project/28114-power-conditioner-for-the-ft817-power-pole-adapter It is designed for a lower current RF device but the principles are the same to scale up to handle more power. The PFETs would need an upgrade.
14524	2018-01-23 14:15:23	Ravi Miranda	Re: Reverse polarity protection Diode
Thank you all for your suggestions. I have incorporated the FET + zener + resistor model for this design. I hope to receive the boards within the next 4 weeks. Will test and let you all know the results. Ravi