Putting two PTC fuses in parallel - Page 1
Forum section: electronics / beginners

author: Medo
author_rank: Regular Contributor
timestamp: June 27, 2021, 12:23:09 am
content: I need 200 mA 60V fuse and I would like to have it in 1206 package (since I want to change it potentially for smaller and 1206 is what I have stock off). Unfortunately, there is no 1206 PTC fuse with 60V rating as far as I can see. However, in theory I could put two 100 mA fuses in parallel and, again theoretically, they should share current and blow in the absolute synchronization once current goes above 200 mA.Well, I know that fuses don't "blow" at the exact current rating but well above it and I am not worried about that. And I know that current sharing is not perfect and one will go kaboom sooner than the other. And I assume each should have a separate footprint so that heat from one doesn't have (much) impact on the other.Has anybody tried putting two fuses in parallel for low voltage application and is the resulting rating at least reasonably close?

author: bob91343
author_rank: Super Contributor
timestamp: June 27, 2021, 05:16:23 am
content: Fuses have such low resistance that paralleling probably should be accompanied by a couple of equalizing resistors.

author: boB
author_rank: Frequent Contributor
timestamp: June 27, 2021, 07:00:17 am
content: I use PTCs all the time. I don't think that paralleling these PTCs for higher current should be a problem.Just don't apply more voltage across them than they are rated for. I wouldn't stack one on top of another but next to each other with at least a slight bit of spacing should work well.I use the 1206 200 mA (lower than the holding current I think) 16V PTCs and some others.Anything smaller for around that same current rating and their ON resistance gets too high for my liking.We also use 250V PTCs at around 1/2 amp.boB

author: GlennSprigg
author_rank: Super Contributor
timestamp: June 28, 2021, 11:25:16 am
content: FUSES can be funny things!, and parallel Circuits in 'general' equally so...There are numerous designs/types of fuses, that blow very quickly, through to very slowly! Slow blowing types for example,are used for say Motor-Starting, as start-up current is momentarily high. Even a 'typical' fuse, say rated at 1-Amp, will NOTblow if the current is say 1.2-Amps, for quite some time, if at all. It is usually as a result of the ratio of current against time.For example, in the above case, 2-amps may take 2 or 3 seconds. And say 10-amps, may take 1/2 a second!! etc.Parallel circuits have an additional factor though... and that's the relative resistance of the parallel paths!! Ohms Law isOhms Law, and even if the resistance of ONE path is say only .001 Ohms, and the OTHER path is say only .002 Ohms, thatis twice the resistance, so only has HALF the current!! ONE 'fuse' MAY have a current of say 800-mA through it, and the otherfuse has say 1,600-mA through it! (etc etc). So that one will blow first, with the other fuse following quite rapidly.

author: Berni
author_rank: Super Contributor
timestamp: June 28, 2021, 11:45:24 am
content: Yeah current sharing tends to be a bit wonky when just paralleling.However fuses (both regular and PTC) do have quite a bit of internal resistance themselves. This resistance is what gets the fuse hot enough to blow at large currents. The temperature coefficient is also positive, so if one gets hotter, it will increase in resistance and reduce its share of current. So as long as the two fuses are the exact same part number it should be fine since they will have a somewhat similar internal resistance. But mixing two different part numbers of fuse is a bad idea. Also to get a true doubling of the rated current on a PTC fuse you need to also have some spacing between the fuses. If they are placed right next to each other then they help heat each other (especially trough the copper trace running between them).Yes there are tolerances in the internal resistance, so current sharing is still not perfect, but the tripping point of individual fuses is pretty inaccurate in general. They do not blow at very precise points. They are not designed to do that. They are there to blow when a large overload of something like >200% happens.

author: tszaboo
author_rank: Super Contributor
timestamp: June 28, 2021, 11:53:04 am
content: Cold resistance is not precisely controlled. It's is a probabilistic device, where the internal structure expands, flexible parts inside which breaks current paths, which you have dozens of it inside. You have a sandwich like layers on top of each other connected to either pins.So no, placing two PTCs in parralel is not going to give you 200% the current rating.https://www.mouser.com/pdfDocs/eaton-ptc-resettable-fuse-application-guidelines.pdfEaton said 160-180%.Get a PTC fuse with larger rating.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2021, 04:05:49 pm
content: PTC fuses should be self ballasting because of their considerable positive temperature coefficient. If one is lower resistance, then it will heat up until the current is equalized between the fuses. What would fail is PTC fuses in series; do not do this.