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I'm in need of 14 VDC @ 100 amps. Switching power supplies are available but too costly. I have enough supplies left over from various desktops to assemble such a beast it's just how? This is to power a 500 Watt HF amplifier so solid voltage regulation and excellent filtering is a must. I've found projects to convert a PC power supply as a multi-voltage bench supply but nothing found to increase the 12 volt rail to 14 volts and then parallel for greater current.

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  • \$\begingroup\$ I'm not AN EE, but how about this: rectify 230V -> 325V DC. Minus 14V = 311V. Use a 3.11 ohm series resistor to create a 311V drop at 100A. This has to be a power resistor ;-) \$\endgroup\$
    – stevenvh
    Jul 10, 2012 at 13:22
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    \$\begingroup\$ P(watts) = E*I, 311 volts at 100A works out to 31.1 KW, far more than the output of my amp! \$\endgroup\$ Jul 10, 2012 at 15:29
  • \$\begingroup\$ Yeah, I know, I told you you'd need a power resistor :-) (I was just kidding!) \$\endgroup\$
    – stevenvh
    Jul 10, 2012 at 15:30
  • \$\begingroup\$ If you are asking these questions, I don't think it is safe for you to try to power a 500 W HF amp. Apart from you needing a license in most, if not all, countries. \$\endgroup\$ Jul 11, 2012 at 19:13
  • \$\begingroup\$ Data sheet for the amplifier? It should tell you the noise requirement on the power supply, if any. \$\endgroup\$ Jul 11, 2012 at 19:14

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No, you can't generally parallel multiple supplies and get higher voltage. Each supply will regulate to a specific voltage. Even if all are rated the same, some will be a little higher and some a little lower. The ones set a little higher will take most of the load. There may also be oscillations as the multiple supplies try to fight each other. In no case does this have anything to do with making more voltage.

It is possible to parallel some supplies to achieve higher overall current. However, the supplies need to be designed or rated for this. Switchers are often easier to design for paralleling. Some even have a current sharing signal. This signal is connected between the supplies, which is used to level the amount of current each supply produces. Some supplies achieve this instead by having a somewhat soft current limit. Even if one supply is set a little higher than the others, it will hit its current limit and its output voltage will drop a little. This causes other supplies to kick in and start producing current to hold up the bus. This is generally not considered as good as deliberate current sharing, but it can work with the right design.

In any case, you're going to need switchers one way or the other. You want 1.4 kW out, so dealing with waste heat is going to be a major design issue. Even at 90% efficiency you still have 156 W of heat to deal with. That's not trivial. You say switchers are too costly, but what you want to do is not trivial and doing it right will cost. If you think switchers are too expensive, take a look at the total cost of the alternatives!

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It may be possible if you have done this before, otherwise your power cable will become a 1 kW CB noise jammer. I've seen it happen many times from Mainframes with current shared PSU's to towers we made at Unisys in Rancho Bernardo with OEM PSU's that were designed for current sharing with cross feedback of current. The fix implemented was fine tuning of each supplies voltage and 10% of rated power dummy resistor. When you do not have symmetrical rate of change of voltage in a no-load condition, it is non-linear and the PSU's will oscillate at low RF frequencies at full power.

I have never seen a 1.4 kW HF radio designed for 14.0 V but certainly spec'd at 14V with a range of 10~18V or 12~15 etc. I agree switching noise or humm must be avoided, but how low is required?

This is what I recommend;

  1. Test your radio on your vehicle after confirming the voltage is within spec. Cars are typically 14.2V +/- 0.2
  2. Use welder's cable or starter cable and connect to radio.
  3. Test for noise levels on transmit and receive with another radio.
  4. Put in a remote starter for your vehicle and run it when you need to power your radio.
  5. If you really need it 24x7. bring a car battery inside to power the radio and make sure it has low ESR = good specific gravity in each cell. ( very important )
  6. Use a regulated battery charger to maintain voltage at 14V then add essential Ferrite filters for CM choke on twisted pair power cables with low ESR (1 mΩ ) plastic caps ( I prefer Panasonic)

Please report on each step of your test results and specs for power source noise acceptable levels and ensure you use 50 Ω method for measuring noise.

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  • \$\begingroup\$ Don't forget to add a stove type suitable rated fuse or magnetic breaker (household type) and allow for voltage drop on cables, connectors & fuse. 0.1V max if done right ( <1mΩ), >0.4V if not. This requires some effort to regulate, so please get specs. \$\endgroup\$ Jul 10, 2012 at 4:20
  • \$\begingroup\$ Well 1-3 have been done in the past in a Dodge 2500 diesel, check my profile for my website to read about that install. I no longer own the Dodge but do the amp. The rig is an Icom 7000 powered by a separate supply. 4 is simply too far away to work. You did give me an idea, how about using one of those very large capacitors normally used on high powered car audio amplifiers? The main mode is single sideband, so the amp can draw up to 75 amps on voice peaks. \$\endgroup\$ Jul 10, 2012 at 4:46
  • \$\begingroup\$ SuperCap or even better UltraCap's can help a bit on AC ESR but battery may have lower ESR. Long cable inductance from front to rear of car is reason for SuperCaps help audio amps. But Battery connected close to supply such as 1000 CCA battery means ~ 5mΩ ESR. If you get acquainted with the ESR of each component and report back, I can tell you what will help the most. V vs I with ∂V/∂I = ESR. Car batteries should have Spec. Grav.= 1.26g/cc to 1.29g/cc and matched within +/-1.5% same for voltage per cell. Also Battery open rested voltage = (S.G. + 0.84 )*6 otherwise not charged or bad \$\endgroup\$ Jul 10, 2012 at 5:50
  • \$\begingroup\$ I just checked Super CAP-XX specs... 150F at 14V, and an ESR of 4.5 milliohms, the CAP-XX module offers the best power density available today ( Those are expensive 150 Farad Caps not the ones you are thinking of for audio systems) so not much hope there. \$\endgroup\$ Jul 10, 2012 at 5:53
  • \$\begingroup\$ BTW... 1000 CCA means current at 7.5V @ -18'C from fully charged battery. i.e. from 13 to 7.5 = 5.5V so ∂√=5.5V and if ∂I=1000A then ESR is 5.5mΩ but ESR can be measured at much lower currents too. \$\endgroup\$ Jul 10, 2012 at 6:03
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Yes, is possible to increase voltage up to 14.5V for a PSU and to parallel two or more to increase oputput current (power). Some server PSU have a pin (RETURN) that alow to increase otutput voltage to about 14.5V; regarding paralleling, need to have care that each PSU share about same current as each others.

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