Hello all! This is my first post. Thanks in advance for any help. I'm pretty new to electronics but work as a software programmer, so hopefully my technical mind will help me out on this little project

My goal is to take a 10V/32ma signal from a lighting control system to flip a SPDT relay to turn on a 24v peristaltic liquid pump. I was hoping I could do this with a standard 12v automotive relay switch, but either the voltage or current wasn't high enough to flip the relay, or I'm doing something else wrong.

Here is the relay I got. Unfortunately I can't find specs for it on the minimum switching voltage or current. I know 15 volts works from testing, but putting three batteries of 9v, 1.5v and 1.5v in series doesn't trigger the relay, which I find very confusing. Maybe those batteries aren't generating enough current to energize the coil? Maybe I need a low current relay? But those seem to be digital whereas this is an analog application.

In short, what is the simplest approach for a novice to turn on a 24v pump with a 10V/32ma analog signal?

 

In automotive current is not important. It is likely that the relay will not trip with only 30mA. It might take more current than a 9V battery can deliver.

How much current to run the pump?

I just picked a little 12V relay. The coil needs 16mA and the contacts are rated for 2A.

Next I looked at some 10A relays and the coil needs 15mA to 35mA depending on many factors. Some relays have simple contacts and some have many contacts.

I could not find your relay but 20A relays have coil current in the 85 to 200mA range.

Next; auto 12V is more like 14.5V when the car is running.
So I think you need a much smaller (simple) relay.

 

A 12VDC signal relay should pickup at ~75% of rated voltage which would be 9VDC. Cutting it kind of close. I don't know about using 10V on a 9V rated coil... A small signal relay doesn't use much coil current and contacts should be good for ~50VDC or 120VAC @1A. Look at what Digikey has and check the product specifications for small signal relays.

 

Welcome to the collection of folks here.
I applaud you for rather completely stating both your "problem" AND a whole lot of relevant information about what is surrounding it, and also very specifically what your intended result is. So far you have done a great deal towards giving the information that we need to come up with an applicable answer. One missing thing is at what voltage do you want the pump to witch on, and then at what lower voltage do you want it to switch off? I am guessing that the 10volt/32ma control signal is an analog control signal that is normally used to control lamp brightness in standard applications, rather than an ON/OFF sort of control.
At this point I am going to guess that the pump should switch on when the control voltage reaches 3.0 volts, and switch off when the voltage drops below two volts. I picked those two values for convenience and because they seem reasonable to me.
Since the pump has a 24 volt motor I will choose a 24 volt automotive style relay with a coil that is rated to draw 200ma. That is similar to a lot of truck relays used for constant duty service such as lights, so it will have contacts able to handle the motor current of the pump. The relay coile will be connected between the 24 volt + and the collector of an NPN silicon transistor. The emitter connection of that transistor will connect to the "common", the negative side of that 24 volt source. That transistor will be chosen to have a collector maximum current rating of at least one amp, and a max collector voltage of at least 50 volts. It must also have a minimum gain of 20, at a 200 ma collector current.
That 10 volt/32ma output will connect to the base of that power transistor and the common for that output will connect to the 24 volt common, where also is connected the 24 volts negative connection. With this set of connections and that one power transistor the relay will switch on somewhere around 1 volt out, and switch off below the 1 volt level, I think.

 

You might be mis-reading the pump documentation. You might have what's called an open collector output with those maximum limitations?

 

Many thanks for the speedy responses! The 12V pump instructions say it consumes less than 3 watts, so that's 250ma?

I can control the volts in the signal from 0 to 10 in 1V increments. For simplicity of the programming, ideally the pump would turn on when the signal is around 10V and turn off when it's around 0V. From my rudimentary testing, the signals tends to be within 0.1V of the target.

@MisterBill2 thank you for the specs on incorporating an NPN transistor with a 24v relay. If I want to flip the relay when the control hits 10V and flip it back when it hits 0V, do the specs of the transistor change? Do you mind sending me a link to your favorite place to buy transistors online? Then I can poke around.

@KeepItSimpleStupid, you mean the pump or the relay is open collector output? Can you explain your concern further?

 

Sorry, my bad, the pump is 12V not 24V!

 

Did I miss something? His signal is 10VDC @ 35mA to control a relay. The relays contacts are for a 12V load @ 250mA. The question is the pullup and dropout voltages of the relay Vs. what his requirements are for ON/OFF control of the pump.

 

Did I miss something? His signal is 10VDC @ 35mA to control a relay.

Ever seen a 10 V, 35 mA relay? Well, SSR relays can work in this range.

A link to the lighting system manual or datasheet I think is in order or at least measure the voltage.

If it's really 10V up to 32 mA and a 24 VDC pump, these http://www.crydom.com/en/products/panel-mount/perfect-fit/dc-output/d06d-series/ could fit the bill. They need to be heatsinked.

 

Ever seen a 10 V, 35 mA relay?

A 6 or 6.3 volt relay are very common. There are some very small 5V relays. Even some solid state relays that work at 1.5V and low current. So many options.

 

Here is a link to the device that has the pump inside. I took the device apart to get a part number/details on the pump, but it doesn't show anything I could see.

@ronsimpson sorry for being dense, but can you point me to a place or brand you trust for relays? I'm happy to look myself if you name a good supplier, it's just there are a lot of options. Also, I understand that "6v relay" means that it's for activating a 6v device, not for receiving a 6v signal? I thought I must have a 12v relay since my pump is 12v?

 

Here is a link to the controlling device but it's long and the section on DIY lighting is terse. I can say that I measured the voltage of the dimming output and it does work in 1V increments that I can control from 0V to 10V at about 32ma.

@KeepItSimpleStupid, thank you for suggesting the Crydom pumps. They are outside my budget, unfortunately. Maybe that dooms my project! But I was hoping there'd be some transistor magic that could make a cheapo relay work.

 

Ever seen a 10 V, 35 mA relay?

See post #3.

 

You have 24 Vdc available, right?

You can use something like this: http://www.farnell.com/datasheets/1911511.pdf with 24 Vdc logic. I've seen screw terminal only versions.

It can interface to your 10V signal and drive a traditional 24 Vdc relay.

If you want to engineer your own, look up OPTOMOS relay. It's an IC that usually needs about 1 mA for the LED to operate.

Here http://www.crydom.com/en/products/digital-i-o-modules/dr-series-output-modules/ is another.

The ULN2004 may work for you, but it's going to require work. it's a relay driver and not isolated.
http://www.ti.com/product/ULN2004A/technicaldocuments

or based on this:

https://www.digikey.com/products/en?keywords=CPC1511Y

 

brand you trust for relays?

Relays have several numbers to look for.
Input or coil voltage and current.
output or contact numbers. 100 volts 2A
But it is never that simple.
DigiKey.com search for relays DigiKey has truck loads of parts. The link goes to digikey.com with search for set to "relay".
The first options is Power relays greeter than 2A contact. Signal relays 2A and below. Solid state relays (no moving parts). Automotive. And more.
Right in the middle of the page there is a button "More Filters" This will give you many options. By filling out some information you can find what you want. Coil voltage = 5dc, and 6dc, 6.3dc not 6ac. Contact current =2A etc.

What do the numbers mean. Coil voltage is the input voltage. "12V" in the case of Automotive. (use dc not ac) More detail. It might turn on at 6V but slowly and a little unreliable. It might hold on until 2V it you don't shake it.
Contact voltage and current have some options. Almost any relay will work at your 12 or 24V pump voltage so you are more interested in current.

 

Coil voltage, A to pull coil in, pullup voltage, dropout voltage, this is just for the coil. Contact voltage and amps, both AC and DC ratings. All have to be with the parameters of your criteria. That is just for mechanical relays. Whole new ballgame for solid-state relays but pretty much similar. Then latching or not, if latching then unlatching requirements, are back-emf suppression diodes needed, etc.

 

Woah this Digi-Key is a whole universe. If my expected coil current is ~32ma, what is the tolerance I can look for? I'm guessing a coil current of 26 or 37 wouldn't work?

 

Many thanks for the speedy responses! The 12V pump instructions say it consumes less than 3 watts, so that's 250ma?

I can control the volts in the signal from 0 to 10 in 1V increments. For simplicity of the programming, ideally the pump would turn on when the signal is around 10V and turn off when it's around 0V. From my rudimentary testing, the signals tends to be within 0.1V of the target.

@MisterBill2 thank you for the specs on incorporating an NPN transistor with a 24v relay. If I want to flip the relay when the control hits 10V and flip it back when it hits 0V, do the specs of the transistor change? Do you mind sending me a link to your favorite place to buy transistors online? Then I can poke around.

@KeepItSimpleStupid, you mean the pump or the relay is open collector output? Can you explain your concern further?

The big change would be adding a resistor in series with the base connection to the transistor, in order to limit the base current. And it occurs to me that also there should be a resistor between the base and the emitter of the transistor to assure that it switches off when the control signal drops to zero, since I have no clue about the sourcing impedance of that control voltage. A suitable transistor would be a TIP29 or a TIP31, or some equivalent type. They should work just as well at 12 volts as at 24 volts. You will need to look in a transistor data sheet to find the connections and the base current needed for saturation in the on condition, so that you can correctly size the series base resistor. Also include the current through the 1000 ohm (1K) base pull-down resistor.
So there you have the circuit, total parts cost should be less than $2 usd. DigiKey is a reliable source of parts, but they may charge for such a small order. Probably any local electronics part store should be able to sell the parts, although they may need to cross to a transistor type that they have in stock. You probably will not need to use a heat sink with that transistor type, but beware that the tab is usually common with the center lead.

 

The big change would be adding a resistor in series with the base connection to the transistor, in order to limit the base current. And it occurs to me that also there should be a resistor between the base and the emitter of the transistor to assure that it switches off when the control signal drops to zero, since I have no clue about the sourcing impedance of that control voltage. A suitable transistor would be a TIP29 or a TIP31, or some equivalent type. They should work just as well at 12 volts as at 24 volts. You will need to look in a transistor data sheet to find the connections and the base current needed for saturation in the on condition, so that you can correctly size the series base resistor. Also include the current through the 1000 ohm (1K) base pull-down resistor.
So there you have the circuit, total parts cost should be less than $2 usd. DigiKey is a reliable source of parts, but they may charge for such a small order. Probably any local electronics part store should be able to sell the parts, although they may need to cross to a transistor type that they have in stock. You probably will not need to use a heat sink with that transistor type, but beware that the tab is usually common with the center lead.

Ok I'm digesting this and think I get it. In this design the coil connects to the 12v power source on one side and the transistor on the other. I think that means when the SPDT relay has a 0v signal, the NC side of the relay won't be powered. Is that right? I failed to mention that the reason I'm using an SPDT relay is because there are actually two pumps. One should be on and the other off when the 10V/35ma signal is off, and vice versa when the signal is on. Apologies for not mentioning this earlier; I thought I could extrapolate to this scenario on my own ;-)

 

Ok I'm digesting this and think I get it. In this design the coil connects to the 12v power source on one side and the transistor on the other. I think that means when the SPDT relay has a 0v signal, the NC side of the relay won't be powered. Is that right? I failed to mention that the reason I'm using an SPDT relay is because there are actually two pumps. One should be on and the other off when the 10V/35ma signal is off, and vice versa when the signal is on. Apologies for not mentioning this earlier; I thought I could extrapolate to this scenario on my own ;-)

The intention of the circuit is that when the control signal is zero the relay will not be energized, and when the control signal is at 10 volts the transistor will be turned on and the relay will operate. which pump connects to which contact, NO and NC, will be up to you.
It sounds like an interesting aplication, I don't know if you can share it with us, or not.