randallsussex
Posts: 13
Joined: Thu Oct 10, 2013 5:13 pm

Measured Relay Specs

Mon Oct 21, 2013 5:01 pm

Hello everybody,

I'm building a solar powered pi-based system and using RasPiConnect App for remote control.

One of the things I do is turn on and off a fan from my iPad (with the app above) in addition to graphs and meters (pic below).

Since it is a solar powered system, I'm very concerned about current draw and have put INA219s through out the system so I know just where the power is going.

Here's some information:

Raspberry Pi Model A - 200ma typical. Adds about 100ma when the WiFi is in used
5V computer fan - takes about 80ma when it is on.

I was very concerned about current draw from the relays so I bought a selection and measured the currents. Here are the results:

Name / Measured Current Draw with Relay On / Specification


SainSmart 2 Signal Relay / 30ma / 8.7ma
SainSmart 2 Relay / 70ma / 20ma
Evil Mad Scientist Relay Shield / 70ma / 72ma

I'm using the SainSmart 2 Signal Relay as it saves me 40ma. Note the huge difference in spec to measured on the SainSmart 2 Relay and the big difference between the spec and measured for the SainSmart 2 Signal Relay. 30ma is better than 70ma however.

Here's the picture of the control panel:

IMG_0172 copy.PNG
IMG_0172 copy.PNG (63.79 KiB) Viewed 1632 times
Best,

Randall

pjc123
Posts: 913
Joined: Thu Mar 29, 2012 3:37 pm
Contact: Website

Re: Measured Relay Specs

Mon Oct 21, 2013 6:09 pm

As has been mentioned many times before on this forum, the specs for the Sainsmart relays that most people are using is incorrect. I don't know which one you have (for the SainSmart 2 Relay / 70ma / 20ma that you mention in particular), but If you are using one of the ones that states a 15 -20ma operating current, I don't know on what planet they got those numbers from (unless Sainsmart forgot the decimal point). They take about 1.5ma to 2.0ma to trigger the relay and about 65ma to operate the coil, so a total of 66.5ma more or less......per relay, which is indeed what you are seeing.
My Raspberry Pi Project Page:
https://www.flaminghellmet.com/launch/

randallsussex
Posts: 13
Joined: Thu Oct 10, 2013 5:13 pm

Re: Measured Relay Specs

Mon Oct 21, 2013 9:49 pm

pjc123,

Thanks for the comments. I should have checked the forum before buying the Sainsmart 2-Channel 5V Relay Module. That was a waste of money!

To clarify exactly what devices I measured:

SainSmart 2 Signal Relay - SainSmart 2 channel Signal Relay Module Board
SainSmart 2 Relay - Sainsmart 2-Channel 5V Relay Module

randall

pjc123
Posts: 913
Joined: Thu Mar 29, 2012 3:37 pm
Contact: Website

Re: Measured Relay Specs

Mon Oct 21, 2013 10:22 pm

randallsussex wrote:pjc123,

Thanks for the comments. I should have checked the forum before buying the Sainsmart 2-Channel 5V Relay Module. That was a waste of money!

To clarify exactly what devices I measured:

SainSmart 2 Signal Relay - SainSmart 2 channel Signal Relay Module Board
SainSmart 2 Relay - Sainsmart 2-Channel 5V Relay Module

randall
Well, you would hope when you buy something at least the specs are correct. The second relay you list is the one I am talking about. I never came across that first relay....I either didn't notice it or it is something new. Be aware that the second relay will not work reliably as delivered if you plan on controlling it from the pi's GPIO ports (And from the reviews on the Sainsmart site, neither will the first relay). You either have to lower the value of the input resistors, or like I do, use transistors (or you can use a darlington pair IC) between the GPIO pins and the relay inputs. The second relay is active LOW (Which is fixed by adding the transistor), but not sure about the first as I have never used it. There are now relay boards from other manufacturers that are both active HIGH and work directly from the GPIO port (I believe I saw one on Ebay not too long ago).

If you need more information about the second relay, look at some of the threads where I have made posts. Type the following into Google:

site:raspberrypi.org sainsmart pjc123
My Raspberry Pi Project Page:
https://www.flaminghellmet.com/launch/

techpaul
Posts: 1512
Joined: Sat Jul 14, 2012 6:40 pm
Location: Reading, UK
Contact: Website

Re: Measured Relay Specs

Mon Oct 21, 2013 11:55 pm

What exactly are you switching with the relay?

Volts and current wise.

I have put together in past banks of realys (over 800 in two racks), where 12 at a time were on at same time, the combined load for USB serial to CPLD, 2 x 7 seg LEDs, control bus with decodes and 12 relays, all powered from USB at less than 120mA TOTAL.

If this range http://www.pickeringrelays.com/dropdown/101series.html will do, and you only need a few I can probably source some of my spares, these are 5V coil, built in diode, less than 4mA coil current. Should do a test for if direct drive from Pi will work sufficiently.

Photos are of assembled racks of boards and relay cards, you do not want to see the back with 1500 high temperature hand soldered wires
Attachments
sRelay Switcher-all.jpg
Realy cards before final assembly
sRelay Switcher-all.jpg (54.83 KiB) Viewed 1571 times
sfrontrelayrack.jpg
Assembled Racks of relay cards
sfrontrelayrack.jpg (62.24 KiB) Viewed 1571 times
Just another techie on the net - For GPIO boards see http:///www.facebook.com/pcservicesreading
or http://www.pcserviceselectronics.co.uk/pi/

User avatar
Tage
Posts: 287
Joined: Fri May 24, 2013 2:29 am
Location: St Thomas, Ontario Canada

Re: Measured Relay Specs

Tue Oct 22, 2013 2:19 am

if you are very concerned about power usage, a relay is a very bad idea. it is better to try to implement the function with a MOSFET. a MOSFET does not require any power at all on the control input. all it takes is to apply a voltage to the gate.
if you have to use a conventional relay, you can take advantage of the fact that a relay has a holding current that is much lower than the pull-in current. that means that if you have a 5V relay you probably only need to apply 1V to keep it closed once it is activated. that saves a lot of power. the relay data sheet tells you what the minimum holding current is.
if the relay does not have to be turned on and off often, you can choose to use a bistable relay. this type of relay does not require any power to stay in the open or closed position. all it takes is to give a current pulse in the positive direction to close the contacts, and a current pulse in the negative direction to open the contacts. in between, the relay will keep its position. usually, a permanent magnet is used to keep the contacts closed.
read up on the subject online.

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