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2 power supplies in series?




 
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Author Topic: 2 power supplies in series?  (Read 1312 times)
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wa2tak
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« on: May 24, 2023, 07:29:25 AM »

I have 2 vintage work bench power supplies.
Each gives 400VDC @ 150 Ma.

Can I 'wire' them in series to get 800 VDC?
Any issues?
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K8DI
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« Reply #1 on: May 24, 2023, 09:02:07 AM »

I have 2 vintage work bench power supplies.
Each gives 400VDC @ 150 Ma.

Can I 'wire' them in series to get 800 VDC?
Any issues?
As usual,  “it depends”
Are they a floating design? (Heathkit variable bench supply comes to mind, I have two)
If so, no issue.  If I remember correctly, the Heathkit ones cover this in their manual.
If, on the other hand, the negative is connected to the chassis/safety ground, not so much.

If you do this, remember that in a faulted condition (blown fuse, etc.) it is not fully off…there’s still 400v someplace..

Ed
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wa2tak
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« Reply #2 on: May 24, 2023, 11:40:42 AM »

Heathkit IP-17 & IP-32
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K8DI
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« Reply #3 on: May 25, 2023, 07:53:55 AM »

Heathkit IP-17 & IP-32

Per their schematics, both of these completely float.  Before doing anything, I’d thoroughly check that they were assembled that way and that there’s not been any mods made…but if they match their schematic, there should not be issues at these voltages.  There will be more voltage from components to chassis of the top supply if the chassis are tied together/grounded and the bottom supply negative is also grounded via test leads, coax shield, etc. but we are talking 800V,  not 3-5 kV; something would have to be pretty marginal to break down from that. As these are variable supplies, I’d recommend adjusting them to be roughly equal, say 250 and 250, vs one at max and the other at some low amount to get 500v…

Note that this is my opinion.  It’s also my opinion that the best option is to build a kV-level psu and stick it on a variac, but, I’ve spent plenty of time and had mostly success doing less than best because that was what was available…..and it was good enough.


Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
wa2tak
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« Reply #4 on: May 25, 2023, 08:55:32 AM »

Thank you very much for your time and help, Ed.

Please excuse me ignorance...on your mention of "grounds".

I would "think" that I need simply to connect a negative from one PS..to the positive of the other PS.?

Thus, putting them in series.

Each PS would get the AC from a common outlet.

Yes?
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K8DI
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« Reply #5 on: May 25, 2023, 09:29:25 AM »

Thank you very much for your time and help, Ed.

Please excuse me ignorance...on your mention of "grounds".

I would "think" that I need simply to connect a negative from one PS..to the positive of the other PS.?

Thus, putting them in series.

Each PS would get the AC from a common outlet.

Yes?

These are old enough that they probably came with two wire power cords.  If you updated to three wire, or if you connect the chassis together (the chassis ground terminal, NOT the common/negative terminal) or to station ground (effectively tying them together), AND you connect the negative/common to the ground directly or through your circuit/RF thing you are testing, then the top supply will have parts that are 800V above that chassis.

Yes, you'll tie the negative/common of the top supply to the positive of the bottom supply.  No, the negative/common is NOT chassis or ground. That's the point of a floating supply -- neither positive nor negative output is connected in any way to the chassis or ground.  This is handy if for some reason your negative line in the circuit under test is NOT at ground. Also handy to connect them in series, as otherwise it wouldn't work...

If the differences between negative, common, ground, and chassis are confusing, please be very careful to know what you are connecting.

Ground is ground is the safety green wire and the ground rod at your antenna and the ground bus behind your radios.

Chassis is the metal box the PSU is in, and would be connected to ground through the green wire in a three wire power cord if it has one, or by a wire to the station ground if you connect one.

Negative is the supply output that is negative compared to the other supply output. It may or may not be negative compared to anything else; its being called negative is ONLY in reference to that supply's positive output. Neither are negative or positive to anything else under the sun. Neither are grounded. Neither are connected to the chassis. They FLOAT.

Once you connect negative of one to positive of the other, that connection, which is the "negative" of the one/upper supply, is still 400V positive of the the lower supply output. At that point, there is still no connection from any of either supply's outputs to the chassis or station ground, UNLESS one of the supplies is defective/built wrong, or YOU connect the chassis to the lower negative and ground the chassis one way or another.  If this doesn't make sense, please stop and figure it out before connecting anything.  I don't mean to sound harsh, but I also don't want you lighting up like a Christmas tree ;-) !!



Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
wa2tak
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« Reply #6 on: May 25, 2023, 10:08:28 AM »

Welllllllllllllllllll...I'm was only half-ignorant.
I know the different "grounds"...just had not been 'clear' on the details.
With your very generous explanation...I'm now 3/4-ignorant Smiley
Got to get all this in my head.

As I don't need to join the PS's....will put the IDEA to rest.

Tnx a bunch,
Steve
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WBear2GCR
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« Reply #7 on: May 26, 2023, 10:49:38 AM »

One needs to be quite careful when doing this with voltages above say 50 volts!!

IF the upper PS has its negative lead connected to the positive lead of the lower supply,
the center point is now at the upper supply's voltage. 400volts in your example.

IF the negative terminal of the upper supply is connected to the CHASSIS,
and that power supply has a two wire plug, and therefore is not grounded to the
AC MAINS "safety ground" (the "third pin") THEN the chassis (the box) that you will
touch will be AT 400 volts!!

No good.

So, the supply actually floats (and those supplies typically have 3 binding posts for outputs, negative, GROUND, and positive) you are okay...

                        _-_-

PS. in the case of building such a supply inside a chassis, for a piece of homebrew gear, not a problem.
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« Reply #8 on: May 26, 2023, 02:56:20 PM »

I have a Peter Dahl 6A 3 kv ac out xformer.  That gives 4.2kv at 6A CCS on a customers 2 x 3cx3000A.

He was slightly low on pep drive so I said the easiest fix is to uo the gain by upping the voltage.

Control xformer, 120/208-240 to 480.  480 is hitting a doubler giving a bit over 1.3kv.

Together, at 5kv under load, it hit the spot. Hes happy.  Been in operation for 3 years.

I don't know that I'd try it with commercial supplies until I had opened them up and verified that the - lead was utterly isolated from the chassis.

--Shane
WP2ASS / ex KD6VXI
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Patrick J. / KD5OEI
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« Reply #9 on: May 28, 2023, 11:17:50 AM »

Several vintage tube type 0-400V commercial bench supplies here which are floating in all respects, B+, filament, bias, whatever they supply. They have chassis-grounding power cords. Have put two in series for up to 800V many times. Agree on the remark about less than 1000V total.

I would not necessarily do it with solid state units, who knows what can happen if one fails it could take the other one with it because of issues with the load and transients.
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