Running extra 5V equipment of the OTGW

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Running extra 5V equipment of the OTGW

Postby adijk » Wed Dec 31, 2014 11:28 am

There have been some tries on this and some have succeeded. But for the sake of documentation I would like to sort this out.

I want to run extra equipment off the 5V power supply of the OTGW to power, for example, a USR-TCP232 or EZL-70, or some other equipment that can do stuff. It seems that if you draw extra power off the 7805, the 7805 gets very hot and unstable.

Some have used external 5V power supplies but that somehow doesn't cut it, because you then have 2 power sources running into the OTGW. Or maybe the solution is to use a bigger print transformer, like a 2.8VA-model and an extra 7805 to power it? Or use a suitable DC/DC-converter like the OKI-78SR-5/1.5-W36 to replace the 7805 to cope with the extra power? Or maybe some extra circuitry is needed to filter off some noise or something. I'm aiming for an extra 250-300mA of 5V.

I'm not into circuit design and all, so maybe someone can answer or help with this or post their experience?
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Re: Running extra 5V equipment off the OTGW

Postby hvxl » Thu Jan 01, 2015 10:28 pm

A 7805 uses a very unsophisticated method to create 5V from its input voltage: It just turns the excess into heat. The gateway has a transformer that creates around 24V DC after rectification and smoothing because that voltage is needed for other purposes. Using such a high voltage as the input for the 7805 means the 7805 has to get rid of about 20V.

I could get away with that because the 5V part doesn't draw much current, generally less than 50mA. The formula to calculate power is P=U*I, so that amounts to 20V*0.05A=1W. However, if you want to draw an additional 300mA at 5V, the 7805 will have to dissipate another 20V*0.3A=6W, or 7W in total. The device is not that big, so that would heat it up rather quickly.

There's no technical objection to have two power sources running into the gateway. But if you connect an external 5V power supply, I suggest to run all 5V parts off that power source. So remove the 7805 and power the PIC and MAX232 from that external power supply.

Getting a bigger transformer and a second 7805 isn't really going to help much. That second 7805 will still have to dissipate 6W. Unless you get a transformer that produces two different voltages. But those are quite difficult to find.

Probably the best solution is to replace the 7805 with a DC/DC converter. An alternative to the one you mentioned is the TSR 1-2450. There should be enough space on the print to simply put it in place of the 7805.
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Re: Running extra 5V equipment of the OTGW

Postby adijk » Fri Jan 02, 2015 10:10 am

So the 15V out off the transformer is converted to 24V in the circuit? Or something else that's probably not within my electronic knowledge of the used circuitry? :D

Can the transformer on the standard part list handle the extra power that is needed because it is only rated at 1.2VA. Someone else in this forum already replaced the 7805 with a DC/DC-converter but he only mentioned the replacement of the 7805, not which parts he used. Examining his pictures of his final setup however revealed the same transformer only rated at 2.3VA.

Dropping in the mentioned DC/DC-converter would be a nice solution and basically any DC/DC-converter rated 24V to 5V would do the trick?
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Re: Running extra 5V equipment of the OTGW

Postby adijk » Fri Jan 02, 2015 11:23 am

My colleague has enlightened me about AC/DC-conversion and transformers. So never mind my first question about the 15V/24V conversion. But regarding the voltage of AC/DC, if you convert 15V AC to DC, you get around 20V DC (15 * (square root 2)) = 21V - (AC/DC conversion loss of around 1V) = 20V.

The transformer can supply 80mA. The max power at DC would be 15 x 0,08 x (square root 2) = 1.6W. If the DC/DC-converter would be 100% efficient you would get 1.6W / 5v = 320mA. But because the DC/DC-converters are at best 90% efficient at 24V, it would be safe to assume at 20V you would get 85% efficiency. That would leave 320mA x 0,85 = 270mA. Subtracting the 50mA the current circuitry would use that leaves 220mA for the rest. Driving 5 LED's, the DS1820 temperature probe leaves no room to spare for the 250-300mA needed for something else. If you want to be on the safe side, you would need 1.8w of power on the and of your chain, which is around 12,5% more power than the current circuitry can provide. So you would need a transformer that can supply at least 90mA. In that case, the EI 30/12,5 would be a suitable new transformer. That transformer can supply 100mA, which is 20% more power, which would be enough. And to cut costs, you could use a the Recom R-78E5.0-0.5 24 V/DC 5 V/DC 500 mA because I don't need the 5V 1A output, and 5V 0,5A is enough.

Does this looks right or am I making some strange or wrong calculations anywhere?
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Re: Running extra 5V equipment of the OTGW

Postby hvxl » Sat Jan 03, 2015 1:34 pm

You're right, I neglected to check the transformer specifications. To handle additional circuitry on 5V, you'll need a bigger one.

Your calculations for the DC voltage produced by a 15V transformer would also be correct - if a 15V transformer actually produced 15V AC. It generally produces more. Based on similar calculations, I originally designed the gateway with an 18V AC transformer. But in practice that resulted in a DC voltage close to 30V.
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Re: Running extra 5V equipment of the OTGW

Postby nlrb » Sat Jan 03, 2015 4:16 pm

The parts I used (Conrad) are:
  • 154974 DC-DC-omzetter R-78B5.0-1.0
  • 710262 Print transformator VB 2,3/1/15
It runs very stable with a USR-TCP232-E.
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Re: Running extra 5V equipment of the OTGW

Postby adijk » Sat Jan 03, 2015 5:30 pm

Thanks for all the info guys!

I guess I'll need to order two replacement components then. And nice to have this figured out for the archive and other people. 8)
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Re: Running extra 5V equipment of the OTGW

Postby ThinkPad » Tue Nov 03, 2015 9:42 am

I had the same question. The module i ordered: USR-TCP232-2 needs 200mA at 5V at max.

I'm going to order this transformer: EI 30/15,5 printtransformator VB 2 VA Primair: 230 V Secundair: 15 V 133 mA 2 VA VB 2,0/1/15 Block (710564-89) because the one mentioned before isn't available anymore.
Together with this DC-DC converter: DC/DC-converter, print Recom International R-78E5.0-0.5 24 V/DC 5 V/DC 500 mA Aantal uitgangen: 1 x (157954-89)

Maybe it is a bit overkill, but i rather prefer being on the safe side, than having weird crashes because the stuff is running almost maxed out.
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Re: Running extra 5V equipment of the OTGW

Postby Templar » Thu Nov 05, 2015 9:52 pm

How about using a 24v AC to DC switching power supply like this one: conrad.nl/nl/hn-power-hnp12-240-c-stekk ... ;WT.srch=1. Much safer then mains electricity on the OTGW PCB.
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Re: Running extra 5V equipment of the OTGW

Postby ThinkPad » Sun Nov 08, 2015 12:28 pm

What temperatures are people seeing with a different than advised transformer?

I just completed the build of the OTGW, i used the https://www.conrad.nl/nl/ei-30155-print ... 10564.html 2VA transformer. The transformer measures 58 degrees when i measured it after it was on for at least a week. Which i think is warm, but not extreme. Measured with an IR-thermometer btw.

The whole OTGW uses 3W (measured with Everspring Z-Wave plug) from mains. This is with a different transformer, and DC/DC-converter instead of 7805. And the OTGW is feeding a USR-TCP232 Serial to Ethernet converter.
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Re: Running extra 5V equipment off the OTGW

Postby Max_nl » Thu Apr 21, 2016 3:13 pm

hvxl wrote:But if you connect an external 5V power supply, I suggest to run all 5V parts off that power source. So remove the 7805 and power the PIC and MAX232 from that external power supply.

I was wondering how stable power needs to be when borrowing power from another board.
Can one simply leave out 7805, C1, C2 and connect 24V and 5V directly?
Or would extra components be needed, e.g. (decoupling) capacitors near the ICs?

In particular I am interested in the possibility of using this $ 6.50 ESP8266 wifi board which has a 24V DC jack connection and exposes both VIN and 5V on headers: http://www.wemos.cc/Products/d1_r2.html
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