LPL 4500 power supply: how to measure output voltage

I do not, yet, have an LPL 4500 enlarger but I have two power supplies that may or not be good.
So as the check the output voltages which of the ,four pin, outlet plug terminals are for 120volt and which for 82 volts?
The ones I have show a permanent 120volts across two terminals and a switchble 120 on the others!!

I'm sure I've recommended him before, but in case it helps:
Kevin Brown at KHB - the Canadian distributor of LPL.
http://www.khbphotografix.com/LPL/LPL4x5.htm

I hope you don't mind, but I changed the thread title to make it more specific.

The bottom right terminal is neutral , the bottom left is ground.
Which upper terminal is 120volts and which is 82 volts.
I may be able to have a local electronics repair person fix it.
Looking at the inside it is old school transistors potentiometers and resistors and a relay, no IC's , not rocket science.

koraks said:

I hope you don't mind, but I changed the thread title to make it more specific.

Click to expand...

Any help welcome.
Thanks.

I also measured 120 across both pairs, with the upper right unswitched and the upper left switched. I have read, IIRC, that 82V is what you get from a simple 1/2 wave rectifier (single diode) applied to 120VAC. I think it's clear that the switched pin is for the lamp and the unswitched is for the fan. Is it possible that the rectifier is in the head? Or maybe you need to measure the lamp voltage while under load? I haven't looked inside mine, the transistors and pots might be some kind of adjustable voltage regulator?

I did not think to measure VDC, I had the meter set to VAC. But it sounds like my good P/S is measuring the same as yours, maybe that helps.

mrosenlof said:

I also measured a constant 120 across both pairs, with the upper right unswitched and the upper left switched. that 82V is what you get from a simple 1/2 wave rectifier (single diode) applied to 120VAC. I think it's clear that the switched pin is for the lamp and the unswitched is for the fan. Is it possible that the rectifier is in the head? Or maybe you need to measure the lamp voltage while under load? I haven't looked inside mine, the transistors and pots might be some kind of adjustable voltage regulator?

Click to expand...

mrosenlof said:

I also measured 120 across both pairs, with the upper right unswitched and the upper left switched that 82V is what you get from a simple 1/2 wave rectifier (single diode) applied to 120VAC. I think it's clear that the switched pin is for the lamp and the unswitched is for the fan. Is it possible that the rectifier is in the head? Or maybe you need to measure the lamp voltage while under load? I haven't looked inside mine, the transistors and pots might be some kind of adjustable voltage regulator?

I did not think to measure VDC, I had the meter set to VAC. But it sounds like my good P/S is measuring the same as yours, maybe that helps.

Click to expand...

. I have read, IIRC,
Sorry I do not understand.
As for the meter , my meter would differentiate between AC and DC.
You could be correct that lamp load could well effect the output, more load , less voltage , no load high voltage!!

My situation is that on a whim i purchased two power supplies for an as yet! not purchased 4500 GXL with no power supply.
Should the power supplies be good I will have an enlarger that should/maybe/ hopefully replace the ones in my darkroom that individually dont meet my requirements.
Perhaps there are no issues with the power supplies?

Does the fan come on when the power supply is turned on or when an exposure is being made?

ya, looks like I had some editing issues. What I meant to say is that I have read that 82V is what you get with a half wave rectifier on 120VAC.

When you switch on the power supply, the fan in the enlarger head runs full time. The lamp current is controlled on/off by the external timer.

My main point is that my power supply is good, and sounds like it measures the same as you have described.

oh, and my meter would also distinguish DC and AC, I just wasn't looking for that, and am not in the same place as my darkroom now. I'm also not sure I understand why the lamp would (need to) be powered by DC current.

mrosenlof said:

What I meant to say is that I have read that 82V is what you get with a half wave rectifier on 120VAC.

Click to expand...

I don't see how that would happen.

120VAC means the since wave has an RMS voltage after rectification of 120V. The total swing (peak-peak amplitude) is in fact around 120 * 2 * SQRT(Pi) = ca. 336V.

Running 120VAC into a half wave rectifier and no subsequent load will give an output voltage of 120VDC.
Running 120VAC into a full wave rectifier and no subsequent load will give an output voltage of 120VDC.

Following the rectifier with a filter capacitor and still no load (also assuming no leakage in the capacitor) will result in a stable DC voltage of 120 * SQRT(Pi) = ca. 168VDC.

There's just no way 120VAC through any kind of rectifier and no additional voltage regulation circuitry yielding 82V. Hence, the transistors and stuff in there are actually doing something

Since I don't have the schematic for this unit, I can't really guess what it is you're measuring and if it's as it should be.

Just found this old post from Large format .

Join DateJul 1999Posts24

Power supply for LPL 4500​

Folks:

It seems that if the output from the Power Supply is tested with a voltmeter it shows 110v. But when a 82v lamp is put and switched on it comes down to 82. While the supply for the fan remains at 110v. The electrician I went to showed me this using an ordinary bulb. He says that AC circuits work this way when they are thru a regulator (which is built into the power supply). So I hope my problem is solved. But I am still going to test it out on another enlarger lamp to be doubly sure before I put my LPL to use.

The other issue was the timer which is also Indian and 220v. This he sorted out by changing the "relay" (?) and it seems to work fine. I didnt understand how he did it but now the timer takes its main input from a regular 220v supply and also has a second input of 110v from the Saunders LPL supply and then an output back to the LPL supply.

Sounds complicated to me. But I've asked the gent to make me a circuit diag for the timer change which I am willing to pass on to anyone who needs it.

Click to expand...

koraks said:

I don't see how that would happen.

120VAC means the since wave has an RMS voltage after rectification of 120V. The total swing (peak-peak amplitude) is in fact around 120 * 2 * SQRT(Pi) = ca. 336V.

Running 120VAC into a half wave rectifier and no subsequent load will give an output voltage of 120VDC.
Running 120VAC into a full wave rectifier and no subsequent load will give an output voltage of 120VDC.

Following the rectifier with a filter capacitor and still no load (also assuming no leakage in the capacitor) will result in a stable DC voltage of 120 * SQRT(Pi) = ca. 168VDC.

There's just no way 120VAC through any kind of rectifier and no additional voltage regulation circuitry yielding 82V. Hence, the transistors and stuff in there are actually doing something

Since I don't have the schematic for this unit, I can't really guess what it is you're measuring and if it's as it should be.

Click to expand...

Now I know how to hook it up and know how it operates, I put a 150watt load on the enlarger side and the fan side stays on whilst the enlarger side voltage drops to 62 volts; both power supplies are the same.
If I post a pic of the inside any chance you can point out which part could be at fault?
FWIW, i have 118 volts input.

See post #2 - Mr. Brown is very helpful and his business is the distributor and performs warranty repairs!

Melvin J Bramley said:

Now I know how to hook it up and know how it operates, I put a 150watt load on the enlarger side and the fan side stays on whilst the enlarger side voltage drops to 62 volts; both power supplies are the same.
If I post a pic of the inside any chance you can point out which part could be at fault?
FWIW, i have 118 volts input.

Click to expand...

Sounds like it is working perfectly. With no load the 'leakage current' will cause your meter to show 120v; with a load this leakage current becomes trivial and is no longer seen.

These supplies work by chopping the AC input at the 90/270 degree points. The output voltage then feeds back to the control circuit which varies the chopping point to keep the output constant. Say the input voltage falls to 100VAC then the circuit would chop the waveform at 80 degrees (WAG, no calculation of actual firing point) letting current through for a longer period of time to bring the output back to 82VRMS.

The things are essentially lamp dimmers with a control circuit on the dimming knob that keeps the lamp illumination constant.

Nicholas Lindan said:

The things are essentially lamp dimmers with a control circuit on the dimming knob that keeps the lamp illumination constant.

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Indeed, that's how they generally work. The Durst units from the same era operate in the same way.

Coincidentally, there are now convenient PWM dimming modules on the market that one could use to build something similar together with an Arduino and a handful of discrete components. So in case units like these fail, they are now relatively easy (for the somewhat experienced among us) to replace. I should append the obligatory warning here to not mess with mains voltage circuits unless you know what you're doing!

Melvin J Bramley said:

If I post a pic of the inside any chance you can point out which part could be at fault?

Click to expand...

As Nicholas pointed out, it's probably not at fault. Also, it's generally hard or even impossible to troubleshoot a circuit with only a single measurement and some photos of the components. Photos can still help in determining the overall topology of a circuit and provide a starting point to troubleshooting, but there's more to it.

Thanks for the input.
Indeed it looks like both power supplies are ok.
I convinced myself they were faulty when I found the circuit board in one of them loose and the board standoffs missing! add to that a transistor ! was missing a ground screw.
With a 200watt incandescent 110 volt load I can adjust on of the potentiometers to give 82 volts.
I guess that with a 82 volt bulb this will have to be reduced?
Now I have to find an enlarger to provide power to..

Melvin J Bramley said:

a transistor ! was missing a ground screw.

Click to expand...

Likely not a 'ground' screw but the screw that holds the heatsink in place so that the part is properly cooled. Btw, this can be a transistor, but might also be a triac - the latter seems a bit more likely given the topology of these things. Note that the lug on a typical TO220 part (google for picture; you'll recognize it probably) is usually not ground, and not isolated. There is usually a mica or teflon insulator pad between the TO220 part and the heatsink, as the heatsink is often shared by other parts as well. Hence, the insulator pad is crucial in preventing fireworks.

It sounds to me this unit was probably cannibalized for parts, perhaps to get another unit in perfect shape.

Melvin J Bramley said:

With a 200watt incandescent 110 volt load I can adjust on of the potentiometers to give 82 volts.
I guess that with a 82 volt bulb this will have to be reduced?

Click to expand...

Simple answer: a bulb rated for 82V should be run at that voltage or a little lower. Since color temperature is somewhat critical in an enlarger, don't run it too low. So just dial the potmeter to 82V with the appropriate bulb connected.
It would be a good idea to turn the potmeter back to its lowest setting now, then install the correct bulb, and only then turn it back on. If you now switch out the bulbs without first resetting the potmeter, you risk frying the new bulb since it constitutes a different load than the one you've been testing with.

A triac it is.
Point taken on dialling own the voltage and re adjusting with the correct bulb.

Melvin J Bramley said:

Now I have to find an enlarger to provide power to..

Click to expand...

If you are looking for a Variable Contrast light source, there is one on Seattle Craiglist: https://seattle.craigslist.org/tac/pho/d/tacoma-omega-lpl-vcce-variable-contrast/7573946552.html